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National Research Council (US) Committee on DNA Forensic Science: An Update. The Evaluation of Forensic DNA Evidence. Washington (DC): National Academies Press (US); 1996.

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The Evaluation of Forensic DNA Evidence.

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6DNA Evidence in the Legal System

In the preceding chapters, we have tried to clarify the scientific issues involved in forensic DNA testing. This chapter discusses the legal implications of the committee's conclusions and recommendations. It describes the most important procedural and evidentiary rules that affect the use of forensic DNA evidence, identifies the questions of scientific fact that have been disputed in court, and reviews legal developments.1

All forensic methods for individualization—fingerprints, dental impressions, striations on bullets, hair and fiber comparisons, voice spectrograms, neutron-activation analysis, blood-grouping and serum-protein and enzyme typing, as well as DNA profiling—demand an ability to match samples with reasonable accuracy with respect to characteristics that can help to differentiate one source from another. If such evidence is to be useful in court, scientifically acceptable procedures must permit the reliable measurement and comparison of physical features. Likewise, a scientific basis must exist for concluding that properly performed comparisons can distinguish possible sources.

As to the latter issue—the ability to differentiate between sources—the courts have demanded a more convincing showing of the exact degree of individualization yielded by DNA tests than by any other commonly used forensic technique. Some courts have deemed it necessary for experts not only to demonstrate that DNA profiles usually vary from one person to another, but also to produce uncontroversial, quantitative estimates of how rare the identifying characteristics are within particular groups and subgroups. Whether many other forms of identification-evidence could survive comparable demands is doubtful.2 Jurists and legal scholars have debated whether DNA evidence warrants this special treatment.3 We take no sides in such legal debates, but we do emphasize that the two issues—the scientific acceptability of the laboratory method for comparing samples and the idea that the characteristics studied in the laboratory are probative of identity—are distinct. Consequently, this chapter describes the implications of our conclusions about the state of scientific knowledge both for testimony about the extent to which DNA samples match and for testimony about the probabilities of such matches.

Legal Standards and Procedures

Whether scientific evidence is admissible in criminal cases depends on whether the evidence tends to prove or disprove a fact that, under the applicable law, might matter to the outcome of the case; whether the expert presenting the evidence is qualified; whether the information is derived from scientifically acceptable procedures; and whether the potential for unfair prejudice or time-consumption substantially outweighs the probative value of the information. We discuss those general principles and then consider their application to DNA evidence. We also describe pretrial and trial procedures that might help courts to reach decisions on admissibility and to improve the quality and use of the scientific evidence at trial. We begin with the intertwined procedural issues that arise in connection with a defendant's request for discovery, retesting, or expert assistance.

The Defendant's Right to Discovery

The 1992 National Research Council (NRC) report stated that ''all data and laboratory records generated by analysis of DNA samples should be made freely available to all parties," and it explained that "all relevant information . . . can include original materials, data sheets, software protocols, and information about unpublished databanks" (NRC 1992, p 150, 148). Certainly, there are no strictly scientific justifications for withholding information in the discovery process, and in Chapter 3 we discussed the importance of full, written documentation of all aspects of DNA laboratory operations. Such documentation would facilitate technical review of laboratory work, both within the laboratory and by outside experts.

The rules of discovery determine the circumstances under which a defendant can compel the production of such records. Because many complex technical, scientific, and statistical issues affect the use of DNA evidence, there will be cases in which defendants will contend that without comprehensive and detailed information, they are unable to prepare for trial adequately.4 Although some courts have ordered liberal discovery, providing access to the documentation and information would broaden the scope of discovery in some jurisdictions. Although some courts have ordered liberal discovery with regard to DNA testing,5 other courts have taken a more restrictive approach. 6 In jurisdictions that interpret their discovery rules as applying only to written reports, the defense cannot obtain discovery of laboratory records if the DNA examiner fails to submit a written report or to incorporate a matter into a report, even if the examiner makes an oral report.7 Our recommendation that all aspects of DNA testing be fully documented is most valuable when this documentation is discoverable in advance of trial.


Experts who present and interpret the results of DNA tests must be "qualified by knowledge, skill, experience, training or education" (Fed. R. Evid. 702). There is no well-defined threshold of knowledge or education that a witness must exceed to qualify as an expert. The question is whether the person has enough knowledge "to make it appear that his opinion or inference will aid the trier in the search for truth" (McCormick 1992, § 13, p 54).

Because DNA identification can involve testimony as to laboratory findings, statistical interpretation of these findings, and the underlying principles of molecular biology, expertise in several fields might be required. An expert who is qualified to testify about laboratory techniques might not be qualified to testify about molecular biology, to estimate population frequencies, or to establish that an estimation procedure is valid. Consequently, more than one expert witness might be needed.8

Nevertheless, if previous cases establish that the testing and estimation procedures are legally acceptable and if the computations are essentially mechanical, then highly specialized statistical expertise is not essential. Reasonable estimates of allele frequencies in major population groups can be obtained from standard references, and many quantitatively literate experts could use the appropriate formulas in Chapters 4 and 5 to compute the relevant profile frequencies or probabilities. Limitations in the knowledge of a technician who applies a generally accepted statistical procedure can be explored on cross-examination,9 and, if serious questions arise, more knowledgeable specialists can be called to address those questions.

In addition to hearing testimony from experts called by the parties, a court may appoint experts to report to it, rather than to the parties. 10 Suggestions that court-appointed experts should be used more in science-rich cases have frequently been made (e.g., Fienberg 1989, p 14), but surveys indicate that such appointments are rare (Cecil and Willging 1994, p 529 and n 2). Some issues that arise with regard to DNA testing seem particularly suitable for assistance from a neutral expert. Well-qualified experts could assist a court or jury in understanding basic principles of DNA testing, how such procedures such as RFLP- and PCR-based testing work, and the extent and effect of departures from Hardy-Weinberg (HW) proportions and linkage equilibrium (LE). Court-appointed experts could also provide information about the composition of databases and the scientific literature dealing with specific issues. Some courts have appointed experts to address general questions related to DNA profiling. E.g., United States v Bonds, 12 F.3d 540 (6th Cir. 1993) and United States v Porter, 1994 WL 742297 (DC Super. Ct., Nov. 17, 1994). More controversial is whether a court should appoint its own expert instead of an expert for the defense when there are more specific disputes, such as the precise location of a band on an autoradiograph. A court might conclude that case-specific issues are better resolved with witnesses chosen by and reporting to the parties.11

A court can seek to narrow differences between opposing experts by a variety of techniques. A court could direct experts to address particular issues in their reports or pretrial summaries of testimony. After those have been exchanged, the court could then instruct each side to identify all statements in an opposing expert's opinion that are disputed and to explain the basis for the disagreement. Controverted issues can be further narrowed at a pretrial conference (see Schwarzer 1994). Procedures such as these might, for instance, persuade statistical experts to furnish a best estimate in addition to a range of estimates so that the jury will have a better sense of the degree of disagreement between the two sides. Even if an expert responds that not enough is known as yet to make a statistically valid estimate, the court will have obtained additional information.

Having more information may aid a court in ruling on challenges to the admissibility of expert testimony and may enable it to make more effective plans for how the expert testimony should be handled at trial. In some cases, judges have departed from the traditional order of presenting testimony to enable opposing experts to testify consecutively rather than waiting for the prosecution to conclude its case. In appropriate circumstances, courts have allowed an expert's direct testimony to be presented in written or other recorded form rather than in person.

General Acceptance and Sound Methodology

The technology used to examine VNTRs, STRs, or other loci must satisfy the standard required of scientific evidence. In the United States, two major standards exist for deciding whether scientific findings will be admitted into evidence: the "general-acceptance" test and the "sound-methodology" standard. In addition, some jurisdictions have adopted special statutes that provide for the admissibility of genetic testing in general or of DNA analyses in particular in criminal or civil cases.12 If a timely objection is raised, the judge must determine whether the applicable standard has been met.

The general-acceptance standard was first articulated in an influential 1923 federal case, Frye v United States, 293 F. 1013 (DC Cir. 1923). In jurisdictions that follow Frye, the proponent of the scientific evidence must establish that the underlying theory and methodology are generally accepted within the relevant portions of the scientific community. The biological and technological principles underlying the forensic methods for characterizing DNA variations have generated little controversy in court.13 Indeed, the 1992 NRC report proposed that courts "take judicial notice of [the] scientific underpinnings of DNA typing,"14 and many courts have done so.15 Courtroom debate has revolved instead around the application of those principles to forensic samples and the procedures for declaring a match and interpreting its importance.

The sound-methodology standard is derived from phrases in the Federal Rules of Evidence. In Daubert v Merrell Dow Pharmaceuticals, 113 S.Ct. 2786 (1993), the Supreme Court held that these rules implicitly jettison general acceptance as an absolute prerequisite to the admissibility of scientific evidence. Instead of the Frye test, the court prescribed a broader framework for deciding whether proposed testimony has sufficient scientific validity and reliability to be admitted as relevant "scientific knowledge" that would "assist the trier of fact." In that framework, the lack of general acceptance weighs against admissibility but is not invariably fatal. The court discussed other factors that might be considered. Its nonexhaustive list includes the extent to which the theory and technology have been tested, the existence of a body of peer-reviewed studies, and the known error rates of the procedure.

Before Daubert, many state and federal courts had construed their rules of evidence as not including a rigid requirement of general acceptance. The 1992 NRC report (p 137) described the "helpfulness standard" used in those jurisdictions as encompassing the following factors: "general acceptance of scientific principles," "qualifications of experts testifying about the new scientific principle, the use to which the new technique has been put, the technique's potential for error, the existence of specialized literature discussing the technique, and its novelty." Since Daubert, many state courts have suggested that their "helpfulness standard" was essentially identical with the approach articulated in Daubert; a few have characterized their rules as more permissive.16

Labels like "general acceptance," "sound methodology," and "helpfulness" are just that—labels. Cases decided in each jurisdiction help to define the scientific community in which the degree of scientific acceptance is to be ascertained, the extent of disagreement that can be tolerated, the information that may be used to gauge the extent of consensus, and the specific factors other than general acceptance that bear on relevance and helpfulness.17 The degree of scientific consensus is important to the admissibility of scientific evidence in all jurisdictions, and pretrial hearings in hotly contested cases have lasted months and generated thousands of pages of testimony probing the opinions of experts on various aspects of DNA profiling. The courts have examined affidavits or testimony from scientists selected by the parties, specific papers in scientific periodicals, the writings of science journalists, the body of court opinions, and other scientific and legal literature, including the 1992 NRC report.

Balancing and Weight

Even in jurisdictions where a DNA-identification technology meets the applicable standard of scientific acceptance or validity, the results of particular tests and the manner of their presentation can be subject to challenge. When the dangers of unfair prejudice, time-consumption, and confusion of the issues substantially outweigh the probative value of particular evidence, the trial court should exclude the evidence. E.g., Fed. R. Evid. 403; McCormick 1992, § 185. And even when the court admits expert testimony, the scientific basis and quality of the testimony can be attacked before the trier of fact. Not all expert testimony is equally convincing, and a trier of fact may choose to give admissible evidence little weight in reaching its verdict.

Trends in the Admissibility of DNA Evidence

Application of the standards for admitting scientific evidence to the admissibility of DNA profile evidence has produced divergent results. In the United States, the first wave of criminal cases involving DNA identification began in 1986.18 The focus was on the problems raised in transferring the technology of modern molecular biology from the medical and genetics laboratories, which usually dealt in fresh samples and easily interpretable diallelic probes, to the forensic laboratory, which must handle aged and exposed stains and usually uses more complex, multiallelic genetic systems. Nevertheless, the underlying theory that DNA profiling is capable of helping to identify the source of a DNA sample was never in doubt, expert testimony for the prosecution was rarely countered, and courts readily admitted the findings of commercial laboratories.19 In the wake of those early cases, many experts from several disciplines scrutinized the work of commercial and government laboratories (Kaye 1991, p 357 n 18). The resulting plethora of questions about laboratory procedures and analyses initiated a second wave of cases in which various courts—including the supreme courts of Georgia,20 Massachusetts,21 and Minnesota22—excluded at least some aspects of DNA evidence.23 Nevertheless, in most cases, the courts continued to hold DNA matches and probabilities admissible even in the face of conflicting expert testimony.24

After publication of the 1992 report, commentators pointed to "a third wave of cases . . . crashing down upon this battered legal shoreline" (Kaye 1993, p 103). Those cases focused less on the laboratory methods for characterizing and matching DNA and more on the statistical methods for interpreting the significance of similarities in DNA samples. Many opinions in that period lagged behind the scientific publications, which responded forcefully to early speculations and questionable analyses of the importance of departures from the assumptions of statistical independence of alleles within and among VNTR loci. Indeed, some courts reasoned that the movement of scientific opinion was essentially irrelevant under Frye as long as respected scientists continued to oppose the statistical methods. E.g., People v Wallace, 14 Cal. App. 4th 651, 17 Cal. Rptr. 2d 721(1993).

Even more recently, with the diffusion of PCR-based methods into the forensic realm, a fourth wave of cases has arrived. The newest cases involve attacks on the procedures for ensuring the accuracy of such analyses and questions about the quantitative interpretation of genetic typing. Again, the underlying theory is not seriously questioned, and laboratories' ability (at least in principle) to obtain informative results is not in dispute. As with the later VNTR profiling cases, defendants have questioned whether the protocols used for forensic work are sufficient to prevent false-positive results, and they have challenged the procedures for estimating the frequencies of the genotypes that are detected after PCR amplification. To clarify the legal relevance of our scientific conclusions and recommendations related to typing methods and statistical issues, we turn now to a more detailed review of these issues as they have arisen in the cases and legal commentary.

Typing Methods

VNTR Profiling

Judicial recognition of the scientific acceptance of the foundations of DNA analysis is consistent with our conclusion that the methods of DNA analysis surveyed in this report are firmly grounded in molecular biology. When VNTR profiling is done with due care, the results are highly reproducible, and comparisons at four or more loci are almost certain to exclude the innocent. To the best of our knowledge, no state or federal court has held that VNTR profiling is inadmissible on the grounds that it is not scientifically accepted or sound.25 Some courts have excluded VNTR matches because of misgivings over the statistical interpretation of the similarities in the profiles (we address this below), but there seems little doubt in the courtroom, as in the laboratory, that properly conducted VNTR profiling is a scientifically acceptable procedure to help to identify the origin of particular biological materials.26

The procedures for matching and binning VNTR fragments discussed in Chapter 5 have provoked more dissension. Defendants have argued that the "window" within which an examiner may declare that the electrophoretic bands of VNTRs from two samples of DNA match is too wide.27 The few reported opinions to discuss the size of the match window, however, have simply held that the FBI's window is not so large as to render its analyses of VNTR test results inadmissible. As the explanation in Chapter 5 indicates, because wide windows increase the chance that a match will be declared—and at the same time increase the estimates of the frequency of a matching profile—a broad range of match windows is acceptable.28

Calculations of the population or subpopulation frequency of VNTR profiles that satisfy the statistical criterion for a match require estimates of the allele frequencies in the reference group. We suggested in Chapter 5 that defining these alleles with floating bins is statistically preferable to the fixed-bin approach but requires access to a computerized database. That conclusion does not imply that the use of fixed bins is scientifically unacceptable.29 Fixed bins are more widely used and understood, and when the recommendations in Chapter 5 are followed, they provide a satisfactory approximation to floating bins.

When fixed bins are used, a dispute sometimes arises as to the frequency of a fragment that lies near the border of two bins. In Chapter 5, we noted that summing the frequencies of both bins, as recommended in the 1992 NRC report, will always give an upper bound on the allele frequency. At least one court has concluded that, within the fixed approach, this summing is "the only methodology that can be characterized as being generally accepted" (United States v Porter, 1994 WL 742297 [DC Super. Ct. Nov. 17, 1994]). As we have noted, however, taking the allele frequency from the larger bin provides a better approximation to the more accurate figure obtained from floating bins.

PCR-Based Testing

Courts have had less experience with evidence derived from PCR-based testing. PCR-based test-evidence, however, is being introduced in a substantial number of cases,30 and courts in each jurisdiction must decide whether this new mode of DNA typing satisfies the applicable test for admitting scientific evidence, regardless of whether RFLP-based evidence has been admitted.31 In the reported cases, judges, with the exception of a few dissenters, have held PCR-based techniques sufficiently reliable to establish matches between samples, under both the general-acceptance and the sound-methodology standards.32 As we discuss later, however, the courts have been less hospitable to statistical calculations.

Some opinions differentiate VNTR testing from PCR-based testing. They characterize the former as capable of identifying a suspect but describe PCR-based testing as "answer[ing] the question of whether a suspect can be eliminated as a donor."33 As described in Chapter 4, the individual loci used in current PCR-based tests are less polymorphic than VNTR loci; as a result, the multilocus genotype frequencies from PCR-based tests typically are not as small as those in VNTR typing.34 But that is a quantitative difference rather than a sharp distinction. Furthermore, very small frequencies can be obtained by testing at additional loci.35

In finding PCR evidence admissible, the courts have rejected a variety of objections, some of which rely on language in the 1992 report. 36 The principal concerns are the alleged lack of forensic experience with PCR testing37 and the possibility of contamination.38 Most courts have decided that those criticisms are pertinent to assessing the weight of the evidence but do not warrant the wholesale exclusion of PCR-based tests.39

Laboratory Error

Defendants have challenged the admissibility of DNA results on the grounds that the protocols or procedures followed by the laboratory were inadequate to reduce the risk of error sufficiently, that the laboratory failed to adhere to the stated protocols, or that the laboratory failed to demonstrate its ability to type samples accurately on a series of external, blind proficiency tests. Courts have shown little inclination to exclude evidence on those grounds.40 Although egregious departures from customary practices might well lead a court to exclude the evidence, the possibility of laboratory error ordinarily is said to affect the weight rather than the admissibility of the evidence, e.g., Hopkins v State, 579 N.E. 2d 1297 (Ind. 1991) (departures from protocol). At the same time, some courts, expressing concern over the impact of DNA evidence on jurors, have grafted a procedural safeguard onto the general-acceptance standard. Starting with People v Castro, 144 Misc. 2d 956, 545 N.Y.S. 2d 985 (Sup. Ct. 1989), a minority of courts have treated an inquiry into a laboratory's conforming in a particular case to a generally accepted protocol as an essential part of a pretrial hearing under Frye.41

We emphasized the importance of minimizing laboratory error in Chapter 3, where we called for scrupulous care in sample-handling and laboratory procedures, for regular participation in proficiency tests, and, whenever feasible, for procedures that would offer defendants the opportunity for a second test by an independent laboratory. Those recommendations rest not on a judgment that current error rates are so high that test results are scientifically unacceptable, but on a desire to reduce the incidence of errors to an extremely low value.

It is possible that courts will want to treat compliance with such recommendations as an aspect of admissibility to encourage laboratories to follow them.42 That result is not compelled by Daubert or Frye, but in some jurisdictions a defendant does have the right to examine physical evidence held by the government, and this right has been construed to include the right to test or retest a sample in the government's control.43

A number of issues can arise even when the right to a second test is recognized. Does the prosecution have a right to be present?44 When is the defendant's request timely? How specific a request must the defendant make?45 Does retesting by another laboratory suffice, or must the testing be done under defense supervision?46 Does it matter whether the laboratory is a government, rather than an independent nongovernment, laboratory?47 Will the state pay if the defendant is indigent?

Of course, the right of indigent defendants to expert assistance at state expense extends beyond the right to retest. In some circumstances, the constitution requires that indigent defendants be provided with funds to retain suitable experts. The leading case is Ake v Oklahoma, 470 U.S. 68 (1985). In Ake, the Supreme Court reversed a conviction because the trial court had refused to appoint an expert to assist the indigent defendant, who was relying on an insanity defense. But Ake was a capital case in which the defense sought the assistance of a psychiatrist, and courts have differed in their interpretation of the holding (Harris 1992). Some courts have applied Ake broadly to authorize all types of expertise; others have restricted Ake to its particular facts, focusing on the type of assistance requested and on whether the prosecution was seeking the death penalty (see Harrison v State, 644 N.E.2d 1243 [Ind. 1995]). Furthermore, courts differ in how much of a particularized showing of need and potential prejudice a defendant must make.48 Those variations in the interpretation of Ake have produced conflicting results when indigent defendants have sought expert assistance with regard to DNA testing. Some courts have held that an expert must be provided, 49 and others have found no such need.50

Instead of providing a defendant with an expert, a court might appoint an expert to assist the court. As noted in the earlier discussion of expert witnesses, courts have been more inclined to use this procedure to investigate general scientific issues related to DNA profiling than to resolve controversies related to the particulars of the DNA testing in a given case. However, no rule of law clearly compels such a limitation on court-appointed experts.

Returning to the implications of recognizing a defendant's right to retesting whenever feasible, difficult issues can arise as to informing the jury of the defense's failure to retest or of the results of any retesting. May the prosecution comment on or introduce evidence about the defendant's failure to request retesting or to introduce DNA-testing results?51 May it cross-examine defense experts about a failure to retest?52 May it obtain discovery or testimony from an expert who conducted retesting for the defense but whom the defense does not intend to call as a witness?53 The law with regard to those questions is far from clear.54 Implicated are state and federal constitutional concerns emanating from due process55 and effective-assistance-to-counsel provisions,56 such evidentiary doctrines as the attorney-client57 and work-product privileges,58 and criminal-procedure issues related to discovery.59

Beyond all that, the recommendation to give a defendant the opportunity to retest whenever possible leaves open the question of how to proceed when a sample is too small to permit splitting. Some opinions suggest that if the prosecution consumes the evidence in the course of testing, it will not be constitutionally barred from introducing the results as long as it acted in good faith.60 The Supreme Court has held that even a negligent failure to preserve evidence does not offend due process.61 However, as one commentator notes, a "situation in which lost evidence might be exculpatory differs from one in which inculpatory evidence will be offered. A higher duty of care should be required in the latter situation" (Giannelli 1991, p 820).

One possible response to the problem of testing that legitimately consumes the sample is to give the defendant the right to have an expert present if prosecution testing will consume the available sample. E.g., State v Gaddis, 530 S.W.2d 64, 69 (Tenn, 1975). However, additional steps might have to be taken to make this right meaningful. 62 When later independent testing is not possible and the defendant is not provided an opportunity to have an independent expert observe the testing, or the testing is performed before charges are filed, our recommendation that all stages of the testing process be fully documented becomes particularly important.63 In such cases, experts who report to the defense or directly to the court might be helpful in verifying that there are no ambiguities in the autoradiographs or that any ambiguities are properly accounted for.

Whereas our recommendations are directed at reducing the chance of error and detecting errors that do occur in rare cases, defendants and some legal commentators have contended that the risk of laboratory or handling errors that would falsely incriminate a suspect should be estimated from external, blind proficiency tests, and a few courts have held that a laboratory's record in proficiency tests must accompany its estimate of the probability of a matching profile. E.g., United States v Porter, 1994 WL 742297 (DC Super. Ct., Nov. 17, 1994). We believe that proficiency-testing is a valuable device for reducing errors of all kinds, should be implemented as a matter of social policy, and bears on the weight that should be accorded forensic test results. At the same time, for the reasons given in Chapter 3, we have concluded that it is exceedingly difficult to estimate relevant error rates from either industry-wide or laboratory-specific proficiency-test results.

A question arises as to the admissibility of proficiency-test statistics themselves. The 1992 NRC report stated that the probative value of such statistics, when balanced against their potential to mislead a jury, favored admissibility: ''laboratory error rates must be continually estimated in blind proficiency testing and must be disclosed to juries" (p 89). Inasmuch as the purpose of our report is to determine what aspects of the procedures used in connection with forensic DNA testing are scientifically valid, we attempt no such policy judgment.

Population and Subpopulation Frequencies

As indicated earlier, the concern that has given courts the most pause in admitting DNA evidence involves the methods for characterizing the implications of an observed degree of similarity in DNA types. Impressed with the diversity of the views expressed in the scientific literature and in testimony, many courts concluded that a major scientific controversy is raging over the proper method for ascertaining the frequency of a given profile. Particularly in Frye jurisdictions, those courts have held some forms of testimony about DNA findings inadmissible.64 This section outlines some of the objections to estimating profile frequencies and random-match probabilities that have been heard in court and the implications of our conclusions about them.

Convenience Samples

Estimates of the frequency of matching genotypes depend on estimated allele frequencies. As noted in Chapter 5, databases used to provide allele frequencies come from convenience samples gathered from sources as diverse as FBI agents, university students, blood bank donors, and parties in paternity cases. Some experts discussing DNA evidence in court have questioned the representativeness of convenience samples. 65 Most courts have held that the use of convenience samples does not make computations inadmissible, but a few courts have suggested that a database resulting from a convenience sample provides an unacceptable foundation for the probability or frequency estimates being offered. 66

Nevertheless, the ideal alternative to convenience sampling—some form of random sampling—often is impractical, and convenience sampling can produce reasonable estimates in some circumstances. In Chapter 5, we explained why the allele-frequency estimates from existing databases are suitable for computing genotype frequencies. In other contexts, courts have accepted convenience sampling. For many years, courts in criminal cases and paternity suits have admitted calculations based on allele frequencies derived from convenience samples for genetic markers, such as blood groups, HLA types, and serum proteins and enzymes. Courts regularly admit surveys based on convenience samples in litigation over alleged trademark infringement and deceptive advertising (Jacoby and Handlin 1991). When such samples are drawn from the relevant population and there is no evidence that an important subgroup is underrepresented, sample estimates are widely accepted to prove the likelihood of consumer confusion between products (Diamond 1994, p 238-239).

However, the courts usually view the results of such convenience samples as rough indicators rather than as precise quantitative estimates, and an expert relying on a convenience sample would be well advised to provide evidence that respondents were selected in a manner that was unlikely to introduce bias (Diamond 1994, p 238-239). One widely accepted way to test the potential bias associated with a particular convenience sample is to compare the results obtained from multiple convenience samples selected with substantially different criteria. Thus, in Chapter 5, we suggested that the similarities among DNA samples from a variety of sources indicate that convenience samples used to construct DNA databases are likely to be representative of racial and geographic population groups.

The Disagreement About Substructure

As explained in Chapter 4, the dispute about the "product rule" centers on the degree of population structure and the effect that it could have, in most situations, on the frequency of an incriminating profile in a racial group or, in a few cases, on the frequency within a particular subpopulation.67 In the absence of any effects from population substructure, the product of the frequencies of the alleles (taking into account the factor of two for each heterozygous locus) closely approximates the frequency of the profile in the population.

At the time of the 1992 NRC report, however, little information was available on the extent to which the relative frequencies of VNTR alleles varied among subgroups within the racial groups, and the report described the conflicting views of population geneticists on the validity of simply multiplying allele frequencies. Many courts took the report's description of a "substantial controversy" as proof of a major scientific disagreement.68

Today, the debate is shifting in the direction of accepting the validity of using the assumptions of Hardy-Weinberg proportions and linkage equilibrium to estimate profile frequencies and match probabilities in major racial groups. Courts are beginning to cite this development to support the conclusion that "it is apparent that . . . RFLP DNA profiling has achieved 'a consensus drawn from a typical cross-section of the relevant, qualified scientific community."'69 since the [1992] NRC . . . report have laid to rest any concern over the use of the product rule"); Lindsey v People, 892 P.2d 281 (Colo. 1995) (perceiving a "calming of the DNA waters" and suggesting that "the debate seems to have turned full circle").

Nevertheless, most courts have not had the opportunity to consider the implications of the data uncovered since 1992, and judges continue to express misgivings over the possible impact of population structure on estimated VNTR profile frequencies. In addition, some courts have questioned the scientific acceptance of computations for PCR-based systems.70 Because our conclusions about the limited extent and effect of population structure are derived from studies of many genetic markers, they pertain to the systems that detect DQA, STRs, and other DNA polymorphisms. Although the data on variations among subpopulations are more limited for these systems than for VNTRs, the experience with VNTRs and other polymorphisms indicates that correcting for population structure should make little difference, and the procedures outlined in Chapter 5 can be expected to give fair estimates of the range of uncertainty in population and subpopulation frequency estimates for discrete allele systems.

Ceiling Frequencies in Court

Rather than giving a definitive answer to speculations about population structure, the 1992 NRC report assumed that population structure could be a serious threat to estimates of VNTR profile frequencies within the general population or within subpopulations. To counter the assumed threat, it proposed a procedure for placing an upper bound on the profile frequency—the "interim ceiling principle" discussed in Chapter 5 of the present report. The method, as described in the 1992 report, includes many refinements, and ambiguities or variations in the details have led to the presentation of markedly different values by prosecutors and defendants as the ceiling frequency.71 We believe that combing through VNTR data on many subgroups to find the largest allele frequencies, taking the upper end of a confidence interval for each such frequency, ignoring loci because large samples indicate that alleles for some other locus do not occur in Hardy-Weinberg proportions, and using fixed-bin frequencies with extremely wide bins (e.g., State v Guevara, No. K9-92-1873 [Minn. Dist. Ct., Dakota Cty.. Jan 26, 1993] [Order]; and Weir 1993c) were neither contemplated by the 1992 committee nor reflect reasonable scientific judgments. Although we cannot recommend either the interim or final ceiling methods, in Chapter 5 we identified several guidelines for those who wish to use such methods. These guidelines make such misuses less likely.

In addition to disputes over the details of how a ceiling should be computed, questions as to implications of the recommendation to use a ceiling have surfaced. Are ceiling frequencies sufficiently valid or accepted in the scientific community to be admissible? If so, should they be the exclusive measure of the frequency of an incriminating profile in the reference population, or may they be presented along with other estimates for racial groups or subgroups?72

Shortly after the publication of the 1992 NRC report, appellate courts drew various inferences from the proposal to present ceiling frequencies in court. Some courts reasoned that willingness to advance the proposal undermined the use of population-specific estimates.73 Others intimated that ceiling frequencies might well be admissible and remanded cases to lower courts to decide whether such calculations had achieved sufficient general acceptance to be admissible.74 A few wrote or held that ceiling frequencies already had attained the requisite general acceptance.75

The earliest opinions discussing ceiling estimates did not mention the strident criticisms of the method made by some population geneticists and statisticians. In time, however, the courts began to assimilate this literature. Although a few courts interpreted the criticism as "precluding the admissibility of DNA evidence" under the general-acceptance standard,76 most have recognized that much of the criticism amounted to claims that there was no need for subpopulation studies and ceiling frequencies in the first place or that the recommended procedure for estimating an upper bound was unnecessarily cautious in its details. 77 In jurisdictions that admit scientific evidence on the basis of the sound-methodology standard, ceiling estimates (as well as population-specific estimates) usually have fared well.78

In Chapter 5, we concluded that the ceiling method is unnecessarily conservative. With estimates of the uncertainty in the computed frequencies, population-specific computations of frequencies are scientifically valid. The ceiling procedure is simply one possible method for producing VNTR profile frequency estimates that are expected to be larger than their true values. If, for courtroom use, advocates desire or courts require probable upper bounds on the true value of the frequency, the ceiling approach should yield a very high upper bound. Although we note that there is no convincing scientific reason to insist on such conservative procedures (see Chapter 5), we discuss the legal policies relevant to presentation of frequency estimates or related statistics later in this chapter.

A few courts have required the application of a "ceiling principle" in calculating frequencies or probabilities for a PCR-based test match or have held "unmodified" computations inadmissible.79 Other courts have held ordinary "product-rule" estimates associated with the DQA test as generally accepted (State v Gentry, 125 Wash.2d 570, 888 P.2d 1105 [1995]). As indicated in Chapter 5, using the ceiling approach for genetic systems that have a small number of alleles per locus and moderate or large allele frequencies is especially difficult to justify.

Explaining the Meaning of a Match

Once two samples are found to have similar profiles, the question arises as to what, if anything, the trier of fact may be told about the significance of this finding. Before forensic experts can conclude that DNA testing has the power to help identify the source of an evidence sample, it must be shown that the DNA characteristics vary among people. Therefore, it would not be scientifically justifiable to speak of a match as proof of identify in the absence of underlying data that permit some reasonable estimate of how rare the matching characteristics actually are.

However, determining whether quantitative estimates should be presented to a jury is a different issue. Once science has established that a methodology has some individualizing power, the legal system must determine whether and how best to import that technology into the trial process (Kaye 1995, p 104-105). If the results are sufficiently probative to be admissible, the conceivable alternatives for presentation range from statements of the posterior probability that the defendant is the source of the evidence DNA (see Chapter 5), to qualitative characterizations of this probability, to computations of the likelihood ratio for the hypothesis that the defendant is the source, to qualitative statements of this measure of the strength of the evidence, to the currently dominant estimates of profile frequencies or random-match probabilities, to unadorned reports of a match.

Few courts, if any, have examined the full range of alternatives, and courts have reached conflicting conclusions as to the acceptability of those modes of presentation that they have examined. Here, we outline the alternatives, identify the considerations that affect their suitability, and discuss the social science research that supplies some information on the possible effects of the various types of presentations on the jury.

The Necessity for Quantitative Estimates

Many courts have held that unless the finding of a match is accompanied by some generally accepted or scientifically sound profile frequency or probability estimate, no testimony about DNA testing is admissible. 80 A few courts, thinking that existing estimates lack acceptance or validity, have excluded quantitative expressions of the frequency of the matching profile while allowing testimony about the match itself.81 The insistence on quantitative estimation has been fueled by the observation in the 1992 NRC report (p 74) that "[t]o say that two patterns match, without providing any scientifically valid estimate (or, at least, an upper bound) of the frequency with which such matches might occur by chance, is meaningless." See, e.g., State v Carter, 246 Neb. 953, 524 N.W.2d 763, 783 (1994) (quoting 1992 report); Kaye (1995a). (suggesting that the better practice is not to refer to probability estimates when introducing DNA results). But cf. Springfield v State, 860 P.2d 435 (Wyo. 1993) (a probability estimate was admissible).

Certainly, a judge's or juror's untutored impression of how unusual a DNA profile is could be very wrong. This possibility militates in favor of going beyond a simple statement of a match, to give the trier of fact some expert guidance about its probative value. As noted above, however, there are a variety of procedures—qualitative as well as quantitative—that might accomplish this objective.

Qualitative Testimony on Uniqueness or Infrequency

In Chapter 5, we asked whether DNA typing has advanced to the point where statements that a particular person is the source of an evidence sample of DNA can be scientifically justified. The 1992 report cautioned that "an expert should—given ... the relatively small number of loci used and the available population data—avoid assertions in court that a particular genotype is unique in the population" (p 92). Because more population data and loci already are available, and still more will be available soon, we are approaching the time when many scientists will wish to offer opinions about the source of incriminating DNA.

In the context of a profile derived from a handful of single-locus VNTR probes, several courts have held that assertions of uniqueness are inadmissible,82 and others have found such testimony less troublesome. 83 We can say only that after one reaches some threshold, the point at which DNA testing is extensive enough to warrant an opinion as to the identity of the source becomes a matter of judgment. Does a profile frequency of the reciprocal of twice the Earth's population suffice? Ten times? One hundred times? There is no "bright-line" standard in law or science that can pick out exactly how small the probability of the existence of a given profile in more than one member of a population must be before assertions of uniqueness are justified (see Chapter 1 for a discussion of how this problem was addressed for fingerprints; see Chapter 5 for discussion of statistical approaches to the problem for DNA typing). There might already be cases in which it is defensible for an expert to assert that, assuming that there has been no sample mishandling or laboratory error, the profile's probable uniqueness means that the two DNA samples come from the same person.84

Opinion testimony about uniqueness would simplify the presentation of evidence by dispensing with specific estimates of population frequencies or probabilities. If the basis of an opinion were attacked on statistical grounds, however, or if frequency or probability estimates were admitted, this advantage would be lost. Nevertheless, because the difference between a vanishingly small probability and an opinion of uniqueness is so slight, courts that decide on a criterion for uniqueness and determine that the criterion has been met may choose to allow the latter along with, or instead of, the former, when the scientific findings support such testimony.

Uniqueness is the limit as the frequency of a profile becomes smaller and smaller. But some experts might testify in qualitative terms even absent a claim of uniqueness; they might prefer to characterize profiles as "rare," "extremely rare," and the like. E.g., People v Venegas, 31 Cal. App. 4th 234, 36 Cal. Rptr. 2d 856, 865 n. 13 (1995). At least one state supreme court has endorsed that more modest approach as a substitute to the presentation of more debatable numerical estimates.85 Although different jurors might interpret the same words differently, the formulas provided in Chapters 4 and 5 produce frequency estimates for profiles of three or more loci that almost always can be conservatively described as "rare."

Quantitative Assessments: Frequencies and Match Probabilities

Except for strong claims of uniqueness, purely qualitative presentations suffer from ambiguity. Professional forecasters, physicians, science writers, students, and soldiers show high variability in translating verbal probability expressions to numerical expressions (Mosteller and Youtz 1990; Wallsten and Budesco 1990). Judges and jurors are likely to show a similar variability in interpreting the meaning of such verbal expressions.86 To help a court or jury to understand the importance of a match, most experts provide quantitative, rather than qualitative, estimates of the frequency of an incriminating profile in one or more races or an upper bound on the frequency. Typically, the figures are presented as an estimated profile frequency or as the "probability of a random-match" or "random-match probability." In some cases, probabilities that the profiles of close relatives would match are given as well. Chapters 4 and 5 describe methods for calculating those quantities. It is accurate to characterize the estimate obtained with those methods as match probabilities if it is established or assumed that the laboratory correctly characterized the human DNA in the samples and that the samples came from reported sources. Thus, the "match probability" might be called the "true match probability,"87 and some experts use the phrase in this sense. In Chapters 4 and 5, all match probabilities are calculated on the assumption that no error has been made.

If a court concludes that the computations satisfy the general-acceptance or scientific-soundness standards, the opponent of the evidence may further argue that the quantitative testimony should be excluded because its prejudicial effect outweighs its helpfulness to the jury. E.g., People v Simpson, No. BA097211 (Super. Ct., Los Angeles Cty., Mar. 20, 1995) (Notice of Objections to Testimony Concerning DNA Evidence); and Taylor v State, 889 P.2d 319 (Okla. Ct. Crim. App. 1995). Three major sources of prejudice have been articulated: that the jury will be awed by small numbers and ignore other aspects of the case, that the jury will misconstrue the probability of a randommatch as the probability that the defendant is not the source of the incriminating DNA, and that the statement of a probability ignores the possibility of a match being declared due to sample mishandling or other blunders.

When the numbers have been presented as estimating the frequency of a profile or the probability of a random-match and have not been mischaracterized as the probability that the defendant is not the source of the incriminating DNA, the argument that numbers will overwhelm the jury rarely has prevailed.88 Only one jurisdiction has routinely excluded quantitatively framed testimony of probabilities or population frequencies in criminal cases for fear of unduly influencing lay jurors,89 and the supreme court of that state carved out an exception to the exclusionary rule for ceiling calculations of DNA profile frequencies (State v Bloom, 516 N.W.2d 159 [Minn. 1994]). Nevertheless, some courts and legal scholars (e.g., Tribe 1971) have theorized that jurors will overvalue the quantitative evidence and undervalue other evidence. For example, the Massachusetts Supreme Judicial Court hypothesized in Commonwealth v Curnin, 565 N.E.2d 440, 441 (Mass. 1991), that "evidence of this nature [a random-match probability of 1 in 59 million], having an aura of infallibility, must have a strong impact on a jury."

Empirical research does not support the common assertion that statistical evidence is overvalued. To the contrary, several studies with mock jurors suggest that decision makers generally make smaller adjustments in their judgments in response to probability evidence than the statistical evidence warrants.90 Nonetheless, the extremely low random-match probabilities associated with much DNA evidence might cause jurors to perceive the evidence as different in quality, as well as quantity. Virtually no studies of juror reactions have assessed the impact of probabilities as extreme as those in Commonwealth v Curnin.91

Courts that are especially concerned that small estimates of the match probability might produce an unwanted sense of certainty and lead a jury to disregard other evidence might wish to adopt procedures to reduce this risk. The party offering evidence has the primary responsibility of informing the jury about the evidence, but the legal system depends also on cross-examination, opposing witnesses, and judicial instructions to guide the jury. The efficacy of the first two approaches rests on the opposing party's capacity to enlist the assistance of informed counsel and well-qualified, expert witnesses. Issues related to the retention and appointment of experts were discussed earlier in this chapter. The third approach—instructing the jury—enables the court directly to address subjects likely to cause confusion or overweighting. Jurors commonly receive judicial instructions on factors to be considered in evaluating the credibility of witnesses. E.g., CALJIC No. 2.20 (3d ed. 1970), cited with approval in People v Hall, 28 Cal.3d 143, 167 Cal. Rptr. 844, 616 P.2d 826 (1980). Similarly, courts might wish to instruct a jury on, for example, factors that affect the adequacy of DNA analysis and the need to consider all the evidence in the case.

The second possible source of prejudice is the jury's potential misinterpretation of the probability of a random-match as the probability that the defendant is not the source. Many court opinions and transcripts of expert testimony present the random-match probability as though it were the conditional probability that the defendant is not the source, given the evidence of a match.92 The random match probability is the conditional probability of the match, given that the defendant is not the source. Transposing the conditionals, as noted in Chapter 5, is sometimes called the "prosecutor's fallacy" and is often condemned in judicial dicta. E.g., State v Bible, 858 P.2d 1152 (Ariz. 1993); and State v Bloom, 516 N.W.2d 159 (Minn. 1994).

Nevertheless, few courts or commentators have recommended the exclusion of evidence merely because of the risk that jurors will transpose a conditional probability (McCormick 1992, § 212). The available research indicates that jurors may be more likely to be swayed by the "defendant's fallacy" than by the "prosecutor's fallacy." When advocates present both fallacies to mock jurors, the defendant's fallacy dominates. That fallacy, as noted in Chapter 5, consists of dismissing or undervaluing the matches with extremely high likelihood ratios because other matches are to be expected in unrealistically large populations of potential suspects. Furthermore, if the initial presentation of the probability figure, cross-examination, and opposing testimony all fail to clarify the point, the judge can counter both fallacies by appropriate instructions to the jurors that minimize the possibility of cognitive errors.93

Finally, defendants and some legal commentators have contended that the risk of a reported match due to laboratory or handling errors dwarfs the probability that a randomly selected profile will match the evidence DNA and renders any profile frequency or random-match probability estimate unfairly prejudicial (People v Barney, 8 Cal. App. 4th 798, 10 Cal. Rptr. 2d 731 [1992]; People v Simpson, No. BA09721 1 [Los Angeles Cty. Super. Ct., Oct. 4, 1994] [Defendant's Motion to Exclude DNA Evidence]; and Koehler, Chia, and Lindsey 1995). The argument that jurors will make better use of a single figure for the probability that an innocent suspect would be reported to match never has been tested adequately.94 The argument for a single figure is weak in light of this lack of research into how jurors react to different ways of presenting statistical information, and its weakness is compounded by the grave difficulty of estimating a false-positive error rate in any given case. But efforts should be made to fill the glaring gap in empirical studies of such matters. Because of the potential power and probative value of DNA evidence, it is important to learn more about juror and judicial response to this evidence in the face of strong and weak nonstatistical evidence.95

Quantitative Assessments: Likelihood Ratios and Posterior Odds

Small values of the probability of a random-match undermine the hypothesis (which we may abbreviate as SC) that the defendant is not the source of incriminating DNA but just happens to have the same profile. Some statisticians prefer to use a likelihood ratio to explain the probative value of a match. As explained in Chapter 5, the likelihood ratio (LR) is related to competing hypotheses about the process that generated the data. With DNA measurements, the hypotheses of most interest are that the DNA samples have a common source (S) and that they do not (SC). LR indicates how many times more probable it would be to observe the data if S, as opposed to SC, were true. As long as LR is greater than 1, the DNA data support hypothesis S. The more LR exceeds 1, the greater the probative value of the data in supporting hypothesis S (see Lempert 1977).

Chapter 5 noted several LRs that might be used to describe the probative value of DNA data. With discrete allele systems and the match-binning analysis of VNTRs, we saw that the LR is 1/P, where P is the probability of a coincidental match.96 For a profile such that P is, say, 1/1,000,000, the LR would be 1,000,000, and an expert might testify that the match is 1,000,000 times as probable under S than under SC. More complicated VNTR profile LRs do not use match windows and bins, but rather consider the extent of the matching at each allele and rely on a continuous representation of the frequency distribution of fragment lengths. With those models, a match that involves almost no separation in all the bands produces an LR that is greater than a match that involves separations at the edges of the match windows for all the bands. Indeed, because these LRs dispense with the somewhat arbitrary dichotomy between matches and nonmatches, they have been termed "similarity likelihood ratios" (Kaye 1995b) and advocated on the ground that they make better use of the DNA data—e.g., Berry 1991a; Evett, Scranage, and Pinchin 1993; Kaye 1995b; Roeder 1994. As with match probabilities, qualitative as well as overtly quantitative presentations can be devised (see Evett 1991, p 201, proposing "a verbal convention, which maps from ranges of the likelihood ratio to selected phrases," such as "strong evidence" or "weak evidence").

Although LRs are rarely introduced in criminal cases,97 we believe that they are appropriate for explaining the significance of data and that existing statistical knowledge is sufficient to permit their computation. None of the LRs that have been devised for VNTRs can be dismissed as clearly unreasonable or based on principles not generally accepted in the statistical community. Therefore, legal doctrine suggests that LRs should be admissible unless they are so unintelligible that they provide no assistance to a jury or so misleading that they are unduly prejudicial. As with frequencies and match probabilities, prejudice might exist because the proposed LRs do not account for laboratory error, and a jury might misconstrue even a modified version that did account for it as a statement of the odds in favor of S. As for the possible misinterpretation of LRs as the odds in favor of identity, that too is a question of jury ability and performance to which existing research supplies no clear answer.

The likelihood ratio is still one step removed from what a judge or jury truly seeks—an estimate of the probability that a suspect was the source of a crime sample, given the observed profile of the DNA extracted from samples. Recognizing that, a number of statisticians have argued that the LR should not be presented to the jury in its own right98 but should be used to estimate the probability that a suspect is the source of a crime sample. E.g., Berry 1991a (but see Berry 1991 b, p 203-204). Thus, a few experts have testified on this posterior probability in court.99

As noted in Chapter 5, the posterior odds (considering the DNA data) that the defendant is the source are the LR times the prior odds (those formed on the basis of other information). That procedure for updating probabilities has a rich history in statistics and law. Known as Bayes's rule, it has been the subject of protracted discussion among legal scholars and statisticians (see generally Allen et al. 1995; Symposium 1991; and Kaye 1988a). One of the more substantial issues raised in the legal scholarship revolves around specifying the prior odds to be updated. For courtroom practice, three methods of presentation have been proposed or used: "expert-prior-odds," "jury-prior-odds," and "variable-prior-odds" (Kaye 1993).

In the expert-prior-odds implementation, a scientist implicitly or explicitly selects a prior probability, applies Bayes's rule, and informs the jury that the scientific evidence establishes a single probability for the event in question. The prosecution relied on a Bayesian analysis of this type in State v Klindt, 389 N.W.2d 670 (Iowa 1986), a gruesome chainsaw-murder case decided before the emergence of DNA testing. The supreme court of Iowa affirmed the admission of a statistician's testimony as to a posterior probability in excess of 99% that a torso found in the Mississippi River was what remained of the defendant's missing wife. (It is doubtful, however, that the Iowa courts appreciated the basis of the calculation.) For years, courts in civil paternity cases that involved testing of antigens have routinely admitted testimony of posterior probabilities. E.g. Kaye 1989; Aickin and Kaye 1983; and McCormick 1992, § 212. However, the practice has met with much less favor in criminal cases where the experts failed to disclose that they had used an ad hoc prior probability of one-half.100 The expert-prior-odds approach has been criticized as requiring a jury to defer to an expert's choice of the prior odds, even though the scientist's special knowledge and skill extend merely to the production of the likelihood ratio for the scientific evidence (Kaye 1993).

Jury-prior-odds implementation requires a jury to formulate prior odds, to adjust them as prescribed by Bayes's rule, and to return a verdict of guilty if the posterior odds exceed some threshold that expresses the point at which the reasonable-doubt standard is satisfied. But that procedure raises serious questions about a jury's ability to translate beliefs into numbers (see Tribe 1971; and Kaye, 1991) and about the desirability of quantifying the vague concept of reasonable doubt (See Nesson 1979, 1985; Shaviro 1989; and Tribe 1971).

Finally, with the variable-prior-odds method, an expert neither uses his or her own prior odds nor demands that jurors formulate their prior odds for substitution into Bayes's rule. Rather, the expert presents the jury with a table or graph showing how the posterior probability changes as a function of the prior probability.101 Although the variable-prior-odds implementation of Bayes's rule has garnered the most support among legal scholars and is used in some civil cases, very few courts have considered its merits in criminal cases.102 How much it would contribute to jury comprehension remains an open question, especially considering the fact that for most DNA evidence, computed values of the likelihood ratio (conditioned on the assumption that the reported match is a true match) would swamp any plausible prior probability and result in a graph or table that would show a posterior probability approaching I except for very tiny prior probabilities.

Importance of Behavioral Research

To make appropriate use of DNA technology in the courtroom, the trier of fact must give the DNA evidence appropriate weight. However, unless the results and meaning of the DNA evidence are clearly communicated, the trier of fact may fail to grasp much of the technical merit of DNA profiling. No research has as yet tested the reactions of triers of fact to the detailed presentations of evidence on DNA profiling that are encountered in the courtroom. We do know that people can make frequent and systematic errors in tasks that require them to assess probabilities or to draw inferences using probabilistic information (see, for example, Bar-Hillel 1980; Edwards and von Winterfeldt 1986; Kahneman et al. 1982; Hogarth and Reder 1987; Nisbett and Ross 1980; Nisbett et al. 1983; Palmerini 1993; Poulton 1989). Yet, despite this plethora of research into information processing in other contexts, we know very little about how laypersons respond to DNA evidence and how to minimize the risk that they will give the DNA evidence inappropriate weight. For example, research generally shows that subjects tend to revise their probability estimates in light of new information less than Bayes's theorem would predict (reviewed by Beyth-Marom and Fischhoff 1983), and some research with mock jurors given written descriptions of blood-group evidence and various types of accompanying expert testimony also suggests that jurors will undervalue match probabilities (see Faigman and Baglioni 1988). However, the studies involve far higher match probabilities than the extreme probabilities associated with DNA evidence, which may evoke a different reaction (see Kaye and Koehler 1991).

Contextual features, such as the method of presenting a question, that are unrelated to a problem's formal structure may substantially influence probability judgments (Reeves and Lockhart 1993). The small amount of research on reactions to probabilistic evidence suggests that methods of presentation may strongly affect reactions to DNA evidence. Unexamined are the effects of testimony about extreme probabilities or laboratory error when DNA evidence is presented by expert witnesses who are subjected to cross-examination. To evaluate the reactions of laypersons to DNA evidence, research is needed in which the respondents are exposed to the methods of presenting DNA evidence typically used in trial settings.

Although scholars have suggested promising ways to present probabilistic assessments in the courtroom (Finkelstein and Fairley 1970; suggesting that jurors be presented with a range of plausible prior probabilities and information about what the likelihood ratio for the trace evidence implies in light of these prior probabilities), almost no empirical evidence yet exists on the effects of such modes of presentation on decision makers. Similarly, although some basic probability concepts can be taught to undergraduates in a half-hour with reasonable success (Fong et al. 1986), research is needed on the appropriate way to instruct jurors adequately on the more sophisticated probabilistic concepts at issue when DNA evidence is presented at trial. If courts are to make informed decisions about the expert presentations that will be allowed or preferred, further research is needed into alternative methods of trial presentation.


This chapter has described some of the legal principles and procedures governing the admission and use of DNA evidence in the courtroom and how this evidence has been received over the last decade. In assimilating scientific developments, the legal system necessarily lags behind the scientific world. Before making use of evidence derived from scientific advances, courts must scrutinize the proposed testimony to determine its suitability for use at trial, and controversy within the scientific community often is regarded as grounds for the exclusion of the scientific evidence. Although some controversies that have come to closure in the scientific literature continue to limit the presentation of DNA evidence in some jurisdictions, courts are making more use of the ongoing research into the population genetics of DNA profiles. We hope that our review of the research will contribute to this process.

In this chapter, we have also discussed how our conclusions and recommendations for reducing the risk of laboratory error, for applying human population genetics to DNA profiles, and for handling uncertainties in estimates of profile frequencies and match probabilities might affect the application of the rules for the discovery and admission of evidence in court. Many suggestions can be offered to make our recommendations most effective: for example, that every jurisdiction should make it possible for all defendants to have broad discovery and independent experts; that accreditation, proficiency-testing, and the opportunity for independent testing (whenever feasible) should be prerequisites to the admission of laboratory findings; that in resolving disputes over the adequacy or interpretation of DNA tests, the power of the court to appoint its own experts should be exercised more frequently; and that experts should not be barred from presenting any scientifically acceptable estimate of a random-match probability. We have chosen, however, to make no formal recommendations on such matters of legal policy; the single recommendation in the chapter concerns scientific evidence—namely, the need for behavioral research that will assist legal decision makers in developing standards for communicating about DNA in the courtroom:

Recommendation 6.1: Behavioral research should be carried out to identify any conditions that might cause a trier of fact to misinterpret evidence on DNA profiling and to assess how well various ways of presenting expert testimony on DNA can reduce such misunderstandings.

We trust that our efforts to explain the state of the forensic science and some of the social science findings that are pertinent to resolving these issues will contribute to better-informed judgments by courts and legislatures.

Appendix 6A

The following tables summarize the law in the United States on the admissibility of estimates of profile frequencies or random-match probabilities of DNA types. Table 6.1 lists the leading cases or statutes in each jurisdiction with a parenthetical explanation of the result in each case. Table 6.2 presents this information in a more abbreviated format. In many of the more recent cases, both an interim-ceiling and product-rule estimates were presented. The tables do not show whether an opinion holds or suggests that the product-rule estimate would have been inadmissible had the ceiling estimate not been included. Many other subtleties and issues that arise in these cases are not captured in this brief summary.

TABLE 6.1. Leading Cases and Statutes on Admissibility of Inclusionary DNA Evidence by Jurisdiction, as of June 1995.


Leading Cases and Statutes on Admissibility of Inclusionary DNA Evidence by Jurisdiction, as of June 1995.

TABLE 6.2. Admissibility of Inclusionary DNA Evidence by Jurisdiction, as of June 1995.


Admissibility of Inclusionary DNA Evidence by Jurisdiction, as of June 1995.



Unless otherwise indicated, our observations apply to all the technologies for DNA analysis described in this report.


State v Bogan, 905 P.2d 515, 522-23 (Ariz. Ct. App. 1995), rev. granted.


State v Bogan, 905 P.2d 515, 522-23 (Ariz. Ct. App. 1995), (dissenting opinion challenging the majority's conclusion that the "'tenuous distinction between molecular genetics and other scientific disciplines' should [not] cause DNA opinion evidence to be treated differently from other opinion testimony that is customarily allowed to support other kinds of scientific evidence") rev. granted; Neufeld and Colman (1990) (advocating more rigorous standards for forensic science generally); Saks and Koehler (1991) (calling for more rigorous validation of many forensic tests).


State v Schwartz, 447 N.W.2d 422, 427 (Minn. 1989) ("access to the data, methodology, and actual results is crucial so a defendant has at least an opportunity for independent expert review").


See, e.g., United States v Yee, 129 F.R.D. 629 (N.D. Ohio 1990) (even before 1993 amendment to Federal Rule of Criminal Procedure 16(a)(1)(E), a federal magistrate judge granted discovery of matching criteria, environmental insult studies, population data, and proficiency tests as "predicate materials" essential to the defense in a DNA-testing case); State v Schwartz, 447 N.W.2d 422, 427-28 (Minn. 1989) (although a laboratory disclosed its protocol, laboratory notes, autoradiographs, and frequency tables, its refusal to supply "more specific information on its methodology and population data base" was a reason to exclude the findings); People v Davis, 196 A.D.2d 597, 601 N.Y.S.2d 174 (Sup. Ct. 1993) (Lifecodes was required on constitutional grounds to turn over statistical data underlying a DNA probability estimate); cf. State v Feldman, 604 A.2d 242, 244 (N.J. Super. 1992) (defense was entitled to discovery related to a databank search of the Automated Fingerprint Information Service). A few statutes governing the admissibility of DNA tests include provisions for pretrial discovery of the state's report. E.g., 10 Md. Code Ann. Cts. & Jud. Proc. § 10-915(3)(b).


See, e.g., State v Dykes, 847 P.2d 1214, 1217-1218 (Kan. 1993) (request of a defendant claiming American Indian ancestry to obtain discovery of a data bank denied, but court permitted discovery of state laboratory's notes, autoradiographs and testing protocol); Spencer v Commonwealth, 384 S.E.2d 785, 791 (1989) (written laboratory reports discoverable, but rules expressly excluded discovery of expert's underlying "work notes [or] memorandum"), cert. denied, 110 U.S. 1171 (1990); cf. United States v Iglesias, 881 F.2d 1519, 1523 (9th Cir. 1989) (discovery of log notes, protocols, or other internal documents of chemists analyzing heroin was denied).


See, e.g., United States v Shue, 766 F.2d 1122, 1135 (7th Cir. 1985) (oral report of FBI photographic expert not discoverable pursuant to Federal Rule 16), cert. denied, 484 U.S. 956 (1987). Rule 16 was amended in 1993 to require that the government disclose to a defendant a summary of the expert testimony that the prosecution intends to offer on direct examination and the bases therefore. It is still unclear whether this provision will cause experts to provide more detailed written documentation than they previously furnished. Many states do not have a counterpart to this subdivision. Other jurisdictions make all discovery related to scientific tests discretionary, and still others explicitly provide for the discovery of oral reports of examinations or tests. See Giannelli and Imwinkelried (1993), vol. 1, § 3.2.


See generally McCormick 1992. § 203, p 875 n 40; Berger 1994, p 63; Kaye and Freedman 1994. p 337. In State v Carter, 246 Neb. 953, 524 N.W.2d 763 (1994), the Nebraska Supreme Court, in reversing a conviction involving a PCR DQA test on the grounds that the 1992 NRC report indicated lack of general acceptance of calculations that assumed Hardy-Weinberg proportions, noted the absence of testimony from a population geneticist. See also Swanson v State, 308 Ark. 28, 823 S.W. 812 (1992) (an argument that a serologist lacked a PhD and was not qualified as an expert in population genetics and therefore could not testify about probabilities was not preserved for appeal): Powell v State, 598 S.W.2d 829 (Tex. Ct. Crim. App. 1994) (argument that molecular biologist should not have been allowed to testify "concerning probabilities of matching DNA patterns because ... the witness had not been qualified as an expert in the field of population genetics" not made at trial, and therefore not preserved for appeal). Trial judges ordinarily are accorded great discretion in evaluating the qualifications of a proposed expert witness, and the decisions depend on the background of each witness. E.g., United States v Davis, 40 F. 3d 1069 (10th Cir. 1994) (the court rejected the argument that a witness "was not qualified to testify regarding population genetics" because ''acceptance of an expert's qualifications will be disturbed only for a clear abuse of discretion" and the witness "had thirteen years experience working for the FBI," "a Master's degree in cell biology," and "six months of specialized training in DNA profiling"): State v McFadden, 458 S.E. 2d 61 (S. Car. 1995) (there was no abuse of discretion in allowing a microbiologist employed by the state's forensic laboratory to testify as to the nature of databases and as to product-rule estimates of 1/(710 million) for blacks and 1/(1.7 billion) for whites); State v Lewis, 654 So. 2d 761 (La. Ct. App. 1995) (the court of appeals held that the trial court abused its discretion by denying expert status to a technician who presented herself as an expert in molecular biology and DNA analysis; the technician had been the assistant director of a laboratory for a year and had no doctoral degree, but belonged to invitational professional organizations, had received numerous academic awards, had testified as an expert in other cases, and had written 14 articles in collaboration with the laboratory director—in promotional rather than scientific publications).


E.g., State v Colbert, 257 Kan. 896, 869 P.2d 1089 (1995) (in view of general acceptance of VNTR databases, estimate of match probability admissible despite expert's concessions that he was not a population geneticist and was not qualified to explain how the databases applied to the town of Coffeyville).


In the federal courts, Federal Rule of Evidence 706 authorizes a court to appoint an expert. Many jurisdictions have similar rules. As with the appointment of defense experts, the federal courts have relied on the Criminal Justice Act § 06(A)(E).


Cecil and Willging (1994, p 542). In some cases, defendants have sought court-appointed experts to review the work of the state's experts. E.g., Taylor v Commonwealth, 1995 WL 808189 (Va. Ct. App. 1995) (unpublished opinion refers to corroboration "by an independent DNA expert appointed by the trial court on defendant's motion"). Other opinions refer to independent experts without indicating the manner of their appointment. E.g., Williams v State, 265 Ga. 351, 455 S.E. 2d 836 (1995) (observing that "an independent geneticist concurred with the DNA findings").


Statutes applicable to criminal cases include 11 Del. Code § 3515; Ind. Code § 37-4-13; 15 La. Stat. Ann. § 441.1; 10 Md. Code Ann. § 915; Minn. Stat. § 634.25; Tenn. Code Ann. § 24-7-117 (1993 Supp.); Va. Stat. § 19.2-270.5; NRC 1992, p 141-142. The Tennessee statute, for example, provides that "in any civil or criminal trial, hearing or proceeding, the results of DNA analysis ... are admissible in evidence without antecedent expert testimony that DNA analysis provides a trustworthy and reliable method of identifying characteristics in an individual's genetic material upon a showing that the offered testimony meets the standards of admissibility set forth in the Tennessee Rules of Evidence" (Tenn. Code Ann. § 24-7-117[b][1]). "DNA analysis" is defined broadly to mean "the process through which deoxyribonucleic acid (DNA) in a human biological specimen is analyzed and compared with DNA from another biological specimen for identification purposes" Id. at § 24-7-117(a). Some statutes explicitly identify a type of DNA analysis, e.g., 10 Md. Code Ann. § 915(b) ("an analysis that utilizes the restriction fragment length polymorphism analysis of DNA"). For discussions, see Moenssens, Starrs, Henderson and Inbau (1995, § 15.20) (surveying criminal and civil statutes); Kaye and Kanwischer (1988) (cataloging civil statutes); Liebeschuetz (1991); Jakubaitis (1991); O'Brien (1994).


For an unusual exception, see Kelly v State, 792 S.W. 2d 579 (Tex. App. 1990) (admitting a VNTR profile match where the state produced five experts who were seriously challenged by only one defense expert, who said that "radioactive technology was too new to be generally accepted in the scientific community"), aff'd, 824 S.W. 2d 568 (Tex. Crim. App. 1992). Although the vast bulk of the cases finding general acceptance have come in the context of VNTR profiling, similar principles and methods of molecular biology underlie the detection of coding DNA polymorphisms, STRs, minisatellite repeat mapping, and the like.


When a court takes judicial notice, it accepts a matter as true without requiring that it be proved. Judicial notice is reserved for matters of common knowledge or those that are capable of "accurate and ready determination by resort to sources whose accuracy cannot be questioned" Fed. R. Evid. 201(b). The 1992 NRC report suggested that the following "underpinnings" would be subject to judicial notice (p 149): "The study of DNA polymorphisms can, in principle, provide a reliable method for comparing samples; each person's DNA is unique (with the exception of identical twins), although the actual discriminatory power of any particular DNA test will depend on the sites of DNA variation examined; [and] the current laboratory procedure for detecting DNA variation (specifically, single-locus probes analyzed on Southern blots without evidence of band shifting) is fundamentally sound, although the validity of any particular implementation of the basic procedure will depend on proper characterization of the reproducibility of the system (e.g., measurement variation) and the inclusion of all necessary scientific controls."


E.g., United States v Perry, Crim. No. 92-474 (D.D.C. Jan. 11, 1994) (order taking "judicial notice of the reliability of the technique of DNA profiling"); State v Montalbo, 73 Haw. 130, 828 P.2d 1274 (1992) (taking judicial notice that "the DNA paradigm is not controversial and is widely accepted in the relevant scientific community"); People v Adams, 195 Mich. App. 267, 489 N.W.2d 192 (1992) ("trial courts may take judicial notice of the reliability of DNA identification testing," but "the prosecutor must establish in each particular case that the generally accepted laboratory procedures were followed"); State v Woodall, 182 W. Va. 15, 385 S.E.2d 253 (1989) (taking judicial notice of general scientific acceptance where there was no expert testimony, but holding that inconclusive results were properly excluded as irrelevant). But cf. State v Hammond, 221 Conn. 264, 604 A.2d 793 (1992) (''Unlike some courts . . . , we regard DNA typing as too novel for its reliability to be judicially noticed at this time.").


E.g., State v Peters, 192 Wis. 2d 674, 534 N.W.2d 867 (Ct. App. 1995) ("Unlike judges in Frye and Daubert jurisdictions, this role is much more oblique and does not involve a direct determination as to the reliability of the scientific principle on which the evidence is based .... Although Wisconsin judges do not evaluate the reliability of scientific evidence, they may restrict the admissibility of such evidence through their limited gatekeeping functions."). For a survey of the reactions of state courts to Daubert, see Meaney (1995).


McCormick (1992, § 203). With many, if not most, types of scientific evidence, admissibility does not seem to turn on the choice of the label. For example, by and large, polygraph evidence is inadmissible in both general-acceptance and sound-methodology jurisdictions. With DNA identification, however, a different pattern might be emerging. Over the last several years, appellate courts in Frye jurisdictions have seemed more prone than appellate courts in other jurisdictions to regard the admission of single-locus VNTR tests as error. See State v Anderson, 118 N.M. 284, 295-96, 881 P. 2d 29, 40-41 (1994) (collecting cases); State v Streich, 658 A. 2d 38 (Vt. 1995) ("We note that the courts that refuse to accept statistics based on the unmodified product method continue to rely on the more narrow Frye standard.").


Blake et al. (1992, p 707) report that "[t]he first use of PCR in a criminal case" occurred in a 1986 Pennsylvania case entitled Commonwealth v Pestinikis. This application of an early form of the DQA test appears to be the first instance of forensic DNA testing of any kind in this country. The first appellate opinion on the admissibility of DNA testing is Andrews v State, 533 So. 2d 841 (Fla. Dist. Ct. App. 1988), and it involved VNTR profiling.


See, e.g., Cobey v State, 559 A.2d 391 (Md. Ct. Spec. App. 1988) (prosecution produced five experts to testify to general acceptance of VNTR probes; defense called no experts); Kaye (1991. p 357 n 17); Thompson and Ford (1989).


Caldwell v State, 393 S.E.2d 436 (Ga. 1990) (finding Lifecodes "straight binning method satisfactory," but because the laboratory's calculation that the frequency of the profile in the population was 1/24,000,000 rested on the assumption of HW proportions was inconsistent with its database, held that the more conservative figure of 1/250,000 derived from that database would have to be used).


Commonwealth v Curnin, 565 N.E.2d 440 (Mass. 1991) (holding that Cellmark DNA evidence in a rape case had been erroneously admitted in the absence of a showing of the general acceptance of the validity of the product rule, which gave a frequency of 1/(59 million)).


State v Schwartz, 447 N.W.2d 422, 428 (Minn. 1989) (responding to VNTR analysis, said to produce a "banding pattern [whose frequency] in the Caucasian population is approximately I in 33 billion," the court concluded that "DNA typing has gained general acceptance in the scientific community" but that "the laboratory in this case did not comport" with "appropriate standards," and further holding the statistical conclusion to be inadmissible because even if the computation is accurate, "we remain convinced that juries in criminal cases may give undue weight and deference to presented statistical evidence'').


Other courts have also refused to admit some forms of DNA evidence. See, e.g., United States v Two Bulls, 918 F.2d 56 (8th Cir. 1990), vacated for rehearing en banc but appeal dismissed due to death of defendant, 925 F.2d 1127 (8th Cir. 1991); People v Castro, 545 N.Y.S.2d 985 (Sup. Ct. 1989); cf. Perry v State, 586 So.2d 242 (Ala. 1991) (remanding for hearing on Lifecodes's adherence to proper procedures and acceptability of statistical methods).


See, e.g., United States v Jakobetz, 747 F. Supp. 250 (D. Vt. 1990) (applying relevance standard), aff'd, 955 F.2d 786 (2d Cir. 1992), cert. denied, 113 S.Ct. 104 (1992); United States v Yee, 134 F.R.D. 161 (N.D. Ohio 1991) (applying general-acceptance standard), aff'd sub nom. United States v Bonds, 12 F.3d 540 (6th Cir. 1993); cf. State v Pierce, 597 N.E.2d 107 (Ohio 1992) (applying relevance standard, no defense experts); Satcher v Commonwealth, 421 S.E.2d 821 (Va. 1992) (applying general-acceptance standard and statute, no defense experts).


For reviews of the case law, see, e.g., Kaye (1993, 1994); Thompson (1993). It remains possible that some unreported cases have reached a contrary result.


E.g., Fishback v People, 851 P.2d 884, 893 (Colo. 1993) (trial courts may take judicial notice of the acceptability of the techniques used in RFLP analysis); State v Moore, 885 P.2d 457 (Mont. 1994) ("the theory underlying DNA and RFLP technology is not open to serious attack"); State v Streich, 658 A.2d 38 (Vt. 1995) (this "part of the scientific debate has essentially ended in favor of DNA admissibility").


United States v Jakobetz, 747 F. Supp. 250 (D. Vt. 1990), aff'd, 955 F.2d 786 (2d Cir. 1992), cert. denied, 113 S. Ct. 109 (1992); United States v Yee, 134 F.R.D. 161 (N.D. Ohio 1991), aff'd sub nom, United States v Bonds, 12 F.3d 540 (6th Cir. 1993). Those arguments have no application to PCR-based methods that use discrete markers.


From a statistical standpoint, the window is best understood in terms of the "standard error" of measurement—a quantity that indicates the variability in repeated measurements of DNA fragments of the same size—and can differ from one laboratory to another. As explained in Chapter 5, match windows must be wider than the normal variability to permit a declaration that two fragments match in most of the cases when they are actually the same length. But the window should not be so wide as typically to produce declarations of matches between fragments that are not about the same size (see Chapter 5). The result is a wide range of possible match windows. Cf. Roeder 1994, p 275 ("the 'objective' match criterion ... is, in fact, simply an arbitrary rule"). In these circumstances, it has been suggested that expert testimony that narrower windows would have excluded a defendant is tautological and more prejudicial than probative (Kaye 1993, 1995). A brief argument to the contrary is made in Thompson (1993).


Cf. People v Venegas, 36 Cal. Rptr. 2d 856 (Ct. App. 1995) (observing that because "the [1992] NRC report expressed approval of the fixed bin method as an alternative to the NRC's 'floating bin' method . . . there is no need for the FBI to abandon that method in order to find consensus in the NRC methodology"), rev. granted, 39 Cal. Rptr. 2d 408, 890 P. 2d 1117.


See Blake et al. (1992, p 720) ("As of September 1991, the HLA-DQA test has been introduced as courtroom evidence into 44 cases and has been evaluated in 25 admissibility hearings in 20 different states"). As of November 10, 1995, 34 cases in which PCR-based DNA testing had been conducted could be retrieved from the Westlaw "allcases" database of court opinions. A survey with responses from 49 forensic laboratories, conducted in November 1994, revealed over 280 cases where PCR-based typing results were introduced in courts in 37 states (Perkin Elmer Corp. 1995, at 1).


See, e.g., Harrison v State, 644 N.E.2d 1243, 1251 (Ind. 1995) ("The words 'DNA test results' are not magic words which once uttered cause the doors of admissibility to open."); State v Russell, 125 Wash.2d 570, 882 P.2d 747 (1995) ("The issue in this case is thus not whether the underlying theory of DNA testing is generally accepted, but whether the PCR technique is generally accepted."); State v Grayson, No. K2-94-1298, 1994 WL 670312 (Minn. Dist. Ct. Nov 8, 1994) (although RFLP testing is accepted in Minnesota, court re-examined PCR-based testing according to the Frye standard). Consequently, judicial opinions on the admissibility of PCR-based evidence illuminate the procedures that judges use in determining the validity of a new DNA technology, in addition to elucidating particular legal issues generated by the PCR method of DNA typing. See, e.g., State v Gentry, 125 Wash.2d 570, 888 P.2d 1105 (1995) (1995) (6-week Frye hearing).


Serritt v State, 647 So. 2d 1 (Ala. Crim. App. 1994); People v Amundson, 34 Cal. App. 4th 1151, 41 Cal. Rptr. 2d 127 (1995) ("Since [the 1992 NRC] report was written, the reliability of PCR testing for forensic use has consistently been proven by the testimony of experts, hundreds of authoritative scientific articles and other literature supporting this typing technique, and by the overwhelming acceptance of PCR testing in dozens of judicial decisions."); People v Groves, 854 P.2d 1310 (Colo. Ct. App. 1992); State v Hill, 859 P.2d 1238 (Kan. 1995) (generally accepted); State v Hoff, 904S.W.2d 56 (Mo. Ct. App. 1995) (generally accepted); State v Moore, 885 P.2d 457 (Mont. 1994) (DQA inclusion and exclusions satisfy Daubert standard); State v Williams, 599 A.2d 960 (N.J. Super. Ct. 1991) ("hundreds" of scientific articles); State v Lyons, 863 P.2d 1303 (Or. Ct. App. 1993) (PCR methodology used forensically in eight states and adopted by several state and private forensic laboratories and FBI; extensive peer-reviewed literature); Trimboli v State, 826 S.W.2d 953 (Tex. Crim. App. 1992); Clarke v State, 813 S.W.2d 654 (Tex. App. 1991); State v Gentry, 888 P.2d 1105 (Wash. 1995) (generally accepted); State v Russell, 125 Wash. 2d 24, 882 P. 2d 747 (1994) (court notes extensive validation studies on PCR testing in holding the Frye test satisfied).


State v Carter, 524 N.W.2d 763, 769 (Neb. 1994); State v Grayson, No. K2-94-1298, 1994 WL 670312 (Minn. Dist. Ct. Nov 8, 1994) ("means to exclude possible defendants rather than identify"); State v Penton, No. 9-91-25, 1993 WL 102507 (Ohio Ct. App. Apr. 7, 1993) ("Unlike RFLP/DNA analysis, PCR/DNA can not get you down to one person but excludes a percentage of the population."), app. dismissed, 619 N.E.2d 698, 617 Ohio St. 3d 1464 (1993).


In the reported cases, PCR typing usually was done with a DQ Alpha kit. But see People v Morales, N.Y.L.J.. Oct. 26, 1994, at 34, col. 6 (Rockland County Ct.). (Amplitype PM or Polymarker test, as well as DQA test, admitted); cf. State v Russell, 125 Wash. 2d 24, 882 P. 2d 747, 768 (1994) (as other modes of testing are developed, "any concerns about implementation in a given case are matters to be addressed to the trial court pursuant to E[vidence] R[ule] 702").


These loci can be analyzed with further PCR-based tests, with VNTR systems, or with traditional protein markers. See, e.g., People v Simpson, No. BA097211 (Super. Ct., Los Angeles Cty., 1995) (VNTR, DQA, Polymarker, ABO, and PGM markers); State v Gentry, 125 Wash.2d 570 (Wash. 199 5) (using ABO, GM, haptoglobin, and PCR-based results, an expert testified that the combined frequency in the Caucasian population was 0.18%, whereas the frequency for the PCR type was 8%).


E.g., People v Amundson, 41 Cal. Rprt. 2d 127 (Ct. App. 1995) ("the report's observation that PCR analysis has not yet received 'full acceptance' for forensic use is not a valid criticism . . . a new scientific technique need only have gained 'general acceptance'"), rev. granted; State v Russell, 882 P.2d 747, 762 (Wash. 1994). The 1992 NRC report expressed reservations about PCR-based testing for forensic use (p 70), the dangers of contamination (p 65-67), differential amplification (pp. 64-65), and misuse of testing kits by "nonexpert laboratories" (p 69).


See State v Gentry, 125 P.2d 570 (Wash. 1995) (dissent cites 1992 report as establishing that PCR testing was "not yet generally accepted as a methodology capable of consistently producing reliable results on forensic samples").


But see State v Russell, 882 P.2d 747, 767 (Wash. 1994) (also discussing dangers of differential amplification and misincorporation).


E.g., State v Lyons, 863 P.2d 1303, 1309 (Or. Ct. App. 1993) ("The potential for contamination presents an 'open field' for cross-examination at trial, but does not indicate that the PCR method is inappropriate for forensic use."); State v Russell, 882 P.2d 747 (Wash. 1994) (discussing 1992 NRC report, but finding PCR-based evidence admissible after noting that over 30 forensic laboratories were performing DQA testing as of March 1991, that the FBI began using the Cetus kit in 1992. that the British Home Office had adopted DQA as its screening test, and that problems of laboratory error are "either detectable or preventable" when proper techniques and laboratory procedures are used). In theory, a court could find a particular PCR-based test performed in such a substandard way as to justify exclusion of the evidence. Cf. State v Moore, 885 P.2d 457, 474-75 (Mont. 1994) (DQA test results admissible despite concern about contamination expressed in 1992 NRC report because "the experts handling the piece of brain tissue were aware of the possibility of contamination, and took appropriate steps to avoid and detect contamination").


State v Streich, 658 A.2d 38 (Vt. 1995) ("The [RFLP] process is not error-free, but adherence to accepted procedures and controls minimizes this error. . . . We cannot find any recent decision under any standard of admissibility which refuses to admit the DNA match result based on the invalidity or risk of error of the underlying technology.").


Compare United States v Two Bulls, 918 F.2d 56 (8th Cir. 1990), vacated for rehearing en banc but appeal dismissed due to death of defendant, 925 F.2d 1127 (1991); Perry v State, 586 So. 2d 242 (Ala. 1991); People v Barney, 8 Cal. App. 4th 798, 10 Cal. Rptr. 2d 731 (1992); Commonwealth v Curnin, 409 Mass. 218, 565 N.E.2d 440 (1991) (dictum); Commonwealth v Rodgers, 605 A.2d 1228 (Pa. Super. 1992); Barnes v State, 839 S.W.2d 118 (Tex. App. 1992) (requiring clear and convincing evidence of compliance) with State v Bible, 858 P.2d 1152 (Ariz. 1993), cert. denied, 114 S.Ct. 1578 (1994); People v Stremmel, 258 N.E.2d 93 (111. App. 1994); Davidson v State, 580 N.E. 2d 238 (Ind. 1991); State v Vandebogart, 616 A.2d 483 (N.H. 1992); State v Cauthron, 120 Wash. 2d 879, 846 P.2d 502 (1993).


Cf. McCormick ( 1992, § 203, at 875 n.41) (proposing external proficiency testing as a prerequisite to admissibility); Jonakait (1991). Courts also could refer to regular participation in accreditation programs, proficiency-testing and independent audits when instructing the jury, allowing jurors to draw a negative inference from the absence of these quality control mechanisms. In addition to providing the jury with valuable guidance, wide use of this instruction would encourage laboratories to participate in such activities. An instruction might read: "In evaluating the quality of the DNA evidence, you might wish to consider the laboratory's participation or nonparticipation in the following quality control activities: (1) accreditation; (2) proficiency-testing, particularly proficiency-testing with blind samples; and (3) independent audits."


In the federal courts, Rule 16(a)(l)(C) of the Federal Rules of Criminal Procedure—which authorizes, on defendant's request, inspection of tangible objects in the government's possession—has been interpreted to mandate a defendant's right to test or retest a sample in the government's control. See, e.g., United States v Butler, 988 F.2d 537 (5th Cir. 1993) (cocaine), cert. denied, 114 S.Ct. 413 (1993). Some states similarly construe their jurisdiction's criminal-discovery rules as mandating retesting (Annotation 1984). Other states have statutes or rules that specifically provide for the retesting of physical evidence. See, e.g., Iowa Code Ann. § 813.2, R. 13(2)(b)(1) (1979); La. Code Crim. Part 71 (West 1981 ); State v Schwartz, 447 N.W.2d 422, 427 (Minn. 1989) (relying on a Minnesota rule of criminal procedure giving defense counsel the right to "inspect and reproduce any results of any ... scientific tests, experiments or comparisons made in connection with the particular case" to conclude that, whenever practical, "a defendant should be provided with the actual DNA sample(s) in order to reproduce the tests"). In addition, some authority supports a constitutional right to retest, stemming from the requirement of due process. See, e.g.. Moore v State, 748 P.2d 732, 735 (Ok. Crim. App. 1987) (Oklahoma constitution requires the state to afford the accused an opportunity to re-examine and retest unless the sample was consumed by government testing; illegal substance); State v Thomas, 421 S.E.2d 227, 234 (W.Va. 1992) (if the prosecution conducts a test, such as an electrophoretic blood test, that consumes the sample being tested, the state must "preserve as much documentation of the test as is reasonably possible to allow for a full and fair examination of the results by a defendant and his experts"). Other courts, however. have found that even when retesting was refused, sufficient protection for the accused was afforded by the right to cross-examine the prosecution's expert. See, e.g., Frias v State, 547 N.E.2d 809, 813 (Ind. 1989) (cocaine), cert. denied, 495 U.S. 921 (1990); People v Bell, 253 N.W.2d 726. 729 (1977); Montoya (1995).


Compare State v Faraone, 425 A.2d 523, 526 (R.I. 1981) ("The court may [after the defendant moves for testing] in its discretion, provide for appropriate safeguards, including where necessary, the performance of such tests at the state laboratory under the supervision of the state's analyst.") with Prince v Superior Court, 8 Cal. App. 4th 1176, 1179, 10 Cal. Rptr. 2d 855, 857 (1992) (defendant entitled to independent testing).


See, e.g., State v Faraone, 425 A.2d 523, 526 (R.I. 1981) (requiring defendant to file a motion "setting forth the circumstances of the proposed analysis, the identity of the expert who will conduct such analysis, his qualifications, and scientific background").


See, e.g., Hicks v State, 352 S.E.2d 762, 769 (Ga.) (it is not an error to deny a request for independent analysis of blood samples if the trial court had offered the defendant an opportunity to have the state crime laboratory perform additional tests and had left open the possibility of a forensic expert for the defendant if necessity was shown; capital case), cert. denied, 482 U.S. 931 (1987).


Cf. Harrison v State, 644 N.E.2d 1243, 1253 (Ind. 1995) (in finding no abuse of discretion in failing to appoint a defense expert, the court noted that "there was every reason to believe that" experts who had performed testing for the prosecution were neutral).


Statutes also play a role; some set limits, which can be quite low, on the compensation for experts and other defense services. E.g., Tex. Crim. Proc. Code Ann. art. 26.05 § 1(d) (Vernon 1989 & Supp. 1994) (maximum, $1,000).


See, e.g., Dubose v State, 662 So.2d 1189 (Ala. 1995) (it is a due-process violation not to provide an expert "to refute the testimony of the Lifecodes witnesses . . . to independently test the samples, to question whether the DNA results, in fact, showed a match, or to explain that scientific opinion may be divided"); Cade v State, 658 So. 2d 550 (Fla. Ct. App. 1995) (trial court abused its discretion under state statute by denying defense request for appointment of DNA expert even though there was no showing of specific need, but only the general observation that "I can't tell the Court what I'm looking for because it's so complicated"), rev. denied, 663 So. 2d 631 (Fla. 1995); Husske v Commonwealth, 448 S.E.2d 331, 335 (Va. Ct. App. 1994) (it depends on how important the scientific issue is in the case and how much help a defense expert could have given; it is an error not to provide an expert to challenge the numbers and assertions of a population geneticist).


Harrison v State, 644 N.E.2d 1243 (Ind. 1995) (there is no error in failing to provide an expert in a capital case; the court stresses that Ake was concerned with expertise of a very subjective nature, whereas DNA testing involved precise physical measurements performed by an independent laboratory, and the defendant had not made any showing of what the laboratory might have done inaccurately); State v Harris, 866 S.W.2d 583 (Tenn. Crim. App. 1992) (insufficient showing of particularized need). The more detailed a showing the court requests as to precisely what issues in the case necessitate expert assistance, the less likely it is that a defense counsel unsophisticated about DNA testing will be able to satisfy the court.


Many subsidiary questions can arise: Would such a comment chill the defendant's right to effective assistance of counsel because defendants would not avail themselves of the opportunity to retest if this provided ammunition for the prosecution? Would such a comment impermissibly shift to the defendant a burden to produce evidence? To what extent does it matter whether the defense has called witnesses in its behalf instead of merely cross-examining the prosecution's expert? Would such a comment be unfair if the defendant were indigent and the jurisdiction did not provide defendants with experts in DNA typing? proceed ex parte (see 18 U.S.C. § 3006A (e)), in many instances this provision would preclude the prosecution from knowing a defendant's plans. With DNA evidence, however, the prosecution will know that the defense wishes to retest because the samples have to be turned over. Nevertheless, it has been suggested that the restriction on discovery should be interpreted as barring the prosecution from calling the defense expert (Contra State v McDaniel, 485 N.W.2d 630 [Iowa 1992]).


See State v Gentry, 888 P.2d 1105, 1121-1122 (Wash. 1995) (the trial court ruled that questions to experts about whether they had retested forensic samples were permissible but that questions about whether they could have done so were impermissible and gave curative instruction on the prosecution's burden of proof; appellate court stated: "While it is questionable whether asking scientific experts whether they did, or could have, conducted duplicate testing is error at all, in this case any possible error in confusing the jury as to the burden of proof was cured by the trial court's simultaneous curative instructions."). See also State v Jobe, 486 N.W.2d 407, 418 (Minn. 1992) (the court found that the prosecutor's question to the defense's DNA expert, who was critical of FBI's testing procedures, as to "whether he could do that type of procedure in his laboratory if samples were provided" did not impermissibly shift the burden of proof, because it did not suggest that the "appellant was obligated to pursue independent testing").


When the defense calls its expert to testify, it waives some privileges: United States v Nobles, 422 U.S. 225, 239 (1975) (work product); United States v Alvarez, 519 F.2d 1036, 1046-47 (3d Cir. 1975) (attorney-client privilege).


Compare Giannelli (1991, p 819) ("retesting comes with a price tag. The prosecution could introduce evidence that samples had been turned over to the defense with the opportunity for retesting and then comment to the jury on the defense's failure to introduce the test results.") with Scheck (1994, p 1969 n 33) ("there should be no requirement that the results [of defense tests] be disclosed").


In Ake v Oklahoma, 470 U.S. 68 (1985), the Supreme Court rested its decision requiring the state to provide expert psychiatric assistance to the defendant on the due-process clause. It has been suggested that a rule that infringes on the right of a defendant to obtain expert assistance by making that right costly "subverts this due process right" (Maringer 1993, p 656 n 11). Due process also is implicated to the extent that a comment impermissibly shifts the prosecution's burden of proof to the defendant.


See, e.g., Prince v Superior Court, 8 Cal. App. 4th 1176, 1179, 10 Cal. Rptr. 2d 855, 857 (1992) (an order that would require defendant to turn over the results of DNA testing even if it did not introduce this evidence at trial denied the defendant the effective assistance of counsel); State v Melvins, 382 A.2d 925 (N.J. Super. 1978) (an order requiring production of copies of a defense expert's investigative reports violated the Sixth Amendment if the defense was not planning to call the expert as witness). But see State v McDaniel, 485 N.W.2d 630, 633 (Iowa 1992) (prosecution use of an expert provided an indigent defendant violates neither due process nor effective assistance of counsel).


The majority rule appears to be that the attorney-client privilege covers communication between experts and attorneys and that consequently the privilege prevents the prosecution from calling a defense-retained expert as a government witness (see Giannelli and Imwinkelried 1993, § 5-10). There are, however, cases that find either that the expert is not an agent of the attorney or that the privilege applies only to experts, such as psychiatrists, who rely on the defendant's communication in reaching their opinions. Id. See also Mosteller (1986).


See Imwinkelried (1990) (arguing that work product, rather than privilege, should apply so that prosecution can obtain a defense witness's opinion on a showing of need).


Rule 16 of the Federal Rules of Criminal Procedure requires the defense to advise the prosecution of the results of scientific tests only if it intends to introduce the results in its case in chief. Especially when coupled with a requirement that defense applications for "investigative, expert or other services"


See California v Trombetta, 467 U.S. 479 (1984) (failure to preserve a breath sample did not amount to a lack of due process). With the advent of PCR testing, the prosecution's choice of a method that will consume a sample, rather than replicate it, might become an issue. See, e.g., People v Griffin, 761 P.2d 103, 107. 46 Cal.3d 1011, 1021-1022, 251 Cal. Rptr. 643, 647-48 (1988) (en banc) (surveys cases in which courts have suggested that prosecution has burden of showing that its destruction of the sample was reasonable). When the defense tests a sample to which the prosecution has not had access, it may not keep the results secret if the testing consumes the sample. See, e.g., State v Cosey, 652 So.2d 993, 994 (1995); People v Cooper, 809 P.2d 865, 889, 53 Cal.3d 771, 815, 281 Cal. Rptr. 90, 114 (1991) (en banc).


Arizona v Youngblood, 488 U.S. 51 (1988) (the prosecution's failure to preserve evidence and to perform genetic-marker tests did not amount to a denial of due process). But cf. Colo. Rev. Stat. § 16-3-309 (1986) (setting forth factors that a court should consider in deciding whether to admit results of a test that consumed a sample, making independent testing impossible).


Obviously, the defense can make such an arrangement only if it is provided adequate notice of the prosecution's plans for testing and if it has retained an expert. See, e.g., Commonwealth v Gliniewicz, 500 N.E.2d 1324, 1327 (1986) (in holding that the prosecution's actions violated a pretrial agreement, the court noted that "defendants received no notice of the impending tests, and thus were not able to have their own expert present to observe and potentially to refute the subjective aspects of the [blood] testing"). The value of having defense experts present, however, has been questioned (see Wooley 1995).


See, e.g., People v Garries, 645 P.2d 1306 (Colo. 1982) (test results were suppressed where blood samples were consumed, and the defendant had no opportunity to be present and no photographs were taken); State v Schwartz, 447 N.W.2d 422, 427 (Minn. 1989) (because "forensic samples are often so small that the entire sample is used in testing, . . . access to the data, methods, and actual results is crucial"). See also People v Griffin, 761 P.2d 103, 107, 46 Cal.3d 1011, 1021-1022. 251 Cal.Rptr. 643, 647-48 (1988) (en banc) (surveys cases in which courts have suggested that prosecution has burden of showing that its destruction of the sample was reasonable).


State v Streich, 658 A.2d 38 (Vt. 1995) ("We note that the courts that refuse to accept statistics based on the unmodified product method continue to rely on the more narrow Frye standard," but the court reached the same result under the Daubert standard).


For example, one early criticism of the allele-frequency estimates focused on the FBI's reliance on a database consisting of FBI agents. E.g., United States v Jakobetz, 747 F. Supp. 250 (D. Vt. 1990), aff'd, 955 F.2d 786 (2d Cir. 1992), cert. denied, 113 S. Ct. 109 (1992); United States v Bonds, 12 F.3d 540 (6th Cir. 1993).


State v Bible, 858 P.2d 1152, 1186 (Ariz. 1993); State v Buckner, 125 P.2d 915 (Wa. 1995) (although "the sample must be truly random," the ceiling calculation could account for a departure from randomness).


Neither the courts nor the experts are always careful to specify the population that is of interest. Population structure is less of an issue when one seeks to estimate the frequency within a racial group than in a small, genetically isolated subpopulation. See, e.g., Kaye (1993) (suggesting that the published criticism of the usual multiplication procedure occurs only in the context of making subpopulation estimates).


See, e.g., Lindsey v People, 892 P.2d 281 (Colo. 1995) (suggesting that general acceptance was more easily found before 1992 NRC report); State v Carter, 246 Neb. 953, 524 N.W.2d 763, 782 (1994) ("The report . . . is persuasive regarding the lack of general acceptance.... Before [its] issuance . . . statistical estimates calculated by forensic laboratories were routinely ruled admissible in most cases; however, since the issuance, an overwhelming majority of courts have excluded evidence of a match after finding that there is no general acceptance as to the statistical probability calculations due to the division in the scientific community on the issue of population substructure."). For a more complete review of the cases after the publication of the 1992 report, see Kaye (1995).


People v Wilds, 37 Cal. Rptr. 2d 351 (Ct. App. 1995), rev. granted. See also People v Amundson, 41 Cal. Rptr. 2d 127 (Ct. App. 1995) ("the scientific landscape has once again changed"), rev. granted; People v Marlow, 41 Cal. Rptr. 2d 5 (Ct. App. 1995) ("Since Lewontin's and Hartl's article . . . numerous studies have been published in scientific journals compiling VNTR frequency data from around the world. These studies have empirically demonstrated the very conservative nature of the frequency calculation methods employed by forensic laboratories. . . . The weight of authority in the published peer-reviewed literature overwhelmingly supports the proposition that VNTR frequency differences due to ethnicity or substructuring have little impact on DNA population frequency estimates . . . ." ), rev. granted; People v Soto, 35 Cal. Rptr. 2d 846 (Ct. App. 1994), ("We now have data showing that population substructuring is not 'forensically significant' in estimating the random likelihood of a particular DNA profile."), rev. granted; Taylor v State, 889 P.2d 319 (Okla. Ct. Crim. App. 1995) (reiterating the conclusion of People v Soto that "several scientific developments


The supreme court of Nebraska, for example, while finding a PCR-based match sufficiently reliable to satisfy its Frye standard, reversed a conviction on the grounds that no general acceptance exists with regard to the calculation of the probability to which the state's experts testified: State v Carter, 524 N.W.2d 763 (Neb. 1994) (relies on 1992 NRC report as indicating lack of general acceptance and states that limiting statistical estimates to two racial groups when the racial or ethnic background of the perpetrator is unknown is prejudicial).


People v Venegas, 36 Cal. Rptr. 2d 856 (Ct. App. 1995) (an FBI expert reported a three-locus match for "DNA characteristics as shared by one person out of 65,000 in the general population, and one out of 30,000 in the southwestern United States Hispanic population"; "the defense genetics expert ... concluded that . . . depending on his choice of methodology, . . . one out of 35 or one out of 378 persons shared appellant's DNA profile."), rev. granted. Discrepancies sometimes occur in the figures quoted for ceiling frequencies by prosecution experts alone. E.g., People v Marlow, 41 Cal. Rptr. 2d 5 (Ct. App. 1995) (such estimates ranged from 1/105,000 to 1/27,000), rev. granted; cf. Taylor v State, 889 P.2d 319 (Okla. Ct. Crim. App. 1995) (the state's expert produced figures of 1/(97 billion), 1/(334 billion), and, "using Lifecodes's current, more conservative approach" of "straight binning," 1/(10 billion)). Discrepancies between unadjusted figures and ceiling frequencies are even more dramatic. E.g., id. (1/7,400,000-1/33,000,000 without adjustment); People v Wilds, 37 Cal. Rptr. 2d 351 (Ct. App. 1995) ("Cellmark's original probability estimate of a random-match included a calculation of I in 186 billion. Using a more conservative approach, the estimate was reduced to I in 66 million. At a pre-trial hearing, Dr. Kidd applied statistically unreasonably conservative' assumptions to Cellmark's data and calculated a probability of I in 1.86 million. Based on an expanded data base which was available at the time of trial, Dr. Kidd revised his probability estimate to 1 in 4.5 million."), rev. granted; Lindsey v People, 892 P.2d 281 Colo. 1995) (estimates of "the probability that Lindsey's DNA profile would match the profile of a randomly selected African American individual . . . ranged from one in 340 billion down to one in 21 million using more conservative frequency calculations"); Brim v State, 654 So. 2d 184 (Fla. Dist. Ct. App. 1995) ("the FBI procedure [used by the Florida state laboratory] generated a probability that only one out of 1.4 billion whites and one out of 2.5 million blacks would share the DNA code ... The modified ceiling principle indicated that only one in just over 9,000 individuals would share the perpetrator's genetic DNA code."); Commonwealth v Lanigan, 419 Mass. 15, 641 N.E.2d 1342 (1994) (interim ceiling estimate changed VNTR probability "from only one in more than 2,000,000 ... to a range of one in 311,000 to one in 108,000"). Still larger figures can be obtained with the ''counting method." E.g., People v Marlow, 41 Cal. Rptr. 2d 5 (Ct. App. 1995) (the laboratory's expert witness reported figures ranging from high of 1 in 33 million to a low of I in 7.4 billion for the frequency of the defendant's four-locus VNTR profile in various populations; the defense expert reported that it "might be as common as one in 211"), rev. granted. Because the opinions are not always clear about which "conservative" method is being used, it is not always easy to discern how much of the variation in the estimates can be attributed to ambiguities in the interim ceiling method or to choices among other competing procedures.


The 1992 report was not explicit on this point. The interim ceiling principle does not purport to measure the frequency of an incriminating profile in the reference population, but rather an upper limit of the random-match probability that is unrelated to the reference population. In late 1994, one of the authors of that report, and an early advocate of the ceiling procedure, expressed his belief that the committee intended to offer ceiling frequencies as a supplement rather than as a necessary substitute for estimates derived from data on the population or subpopulation of interest (Lander and Budowle 1994).


State v Sivri, 646 A.2d 169 (Conn. 1994); State v Carter, 246 Neb. 953, 524 N.W.2d 763 (1994) (the NRC report indicates a lack of general acceptance of HW proportions, which renders testimony of a PCR-based DQA test result said to include about 7% of the population inadmissible under Frye).


United States v Porter, 618 A.2d 629 (DC App. 1992); People v Watson, 629 N.E.2d 634 (111. App. 1994); Commonwealth v Lanigan, 596 N.E.2d 311 (Mass. 1992); State v Vandebogart, 616 A.2d 483 (Vt. 1992). In Franson v Mitchell, 206 111. Dec. 399, 645 N.E. 2d 404 (I11. Ct. App. 1994), the court determined that questions about the effect of population structure made a "probability of paternity" of 99.99% and a "cumulative paternity index" (likelihood ratio) of 29,217,637 inadmissible under Frye. It remanded for a determination of whether the more conservative methods proposed in the 1992 report had achieved general acceptance for parentage determinations. As we observed in Chapter 5, however, the ceiling methods were not proposed or designed for computing a paternity index, and the 1992 report's call for "conservative" procedures was influenced by its interpretation of the ''beyond a reasonable doubt" standard of proof used in criminal, but not civil, cases.


People v Venegas, 36 Cal. Rptr. 2d 856 (Ct. App. 1995), rev. granted; State v Bloom, 516 N.W.2d 159 (Minn. 1994); State v Alt, 504 N.W.2d 38 (Minn. Ct. App. 1993); State v Streich, 658 A.2d 38 (Vt. 1995); State v Cauthron, 846 P.2d 502 (Wash. 1993).


People v Wallace, 14 Cal. App. 4th 651, 17 Cal. Rptr. 2d 721 (1993). See also Hayes v State, 660 So. 2d 257 (Fla. 1995) (dictum); State v Hollis, No. 92-1-04603-9 (Wash. Super Ct. King County, June 1993), appeal pending, No. 3307-1-L.


State v Johnson, 183 Ariz. 623 (Ct. App. 1995) ("most of the remaining debate stems from criticisms that the ceiling method is too conservative, that evidence of population substructure is lacking, and that further study is needed to determine the best means of presenting probability statistics to juries, not [from any doubts about] the ceiling method's validity as a reliable and highly conservative forensic tool"), rev. granted; People v Venegas, 36 Cal. Rptr. 2d 856 (Ct. App. 1995) (noting general agreement that interim ceiling calculations have "forensic reliability"), rev. granted; United States v Porter, 618 A.2d 629 (DC Ct. App. 1992); Commonwealth v Lanigan, 419 Mass. 15, 641 N.E.2d 1342 (1994) ("the great weight of opinion appears to be" that "the answer given by the ceiling principle is . . . either irrationally conservative and thus absurd or a reasonable means of producing admissible probability evidence untainted by potential problems of population substructuring"); State v Alt, 504 N.W.2d 38 (Minn. Ct. App. 1993); State v Vandebogart, 616 A.2d 483 (N.H. 1994) (affirming trial court's findings on remand "that there is 'universal' consensus in the scientific community of geneticists and forensic DNA scientists that the interim ceiling principle properly accounts for the possibility of population substructure by providing a highly conservative estimate" and that, although those estimates "may be so conservative as to be deemed not accurate, it is nonetheless generally accepted ... that any possible errors in such estimates favor the defendant"); State v Streich, 658 A.2d 38 (Vt. 1995) ("There is general acceptance within the scientific community that the ceiling principle over-compensates for any population substructure or allele linkage.'').


For example, in United States v Chischilly, 30 F.3d 1144 (9th Cir. 1993), a jury convicted the defendant of raping and murdering a woman in a remote part of the Navajo Indian reservation in Arizona. Single-locus VNTR tests performed by the FBI indicated a match between sperm found on the victim and a sample of the defendant's blood. A population geneticist testified that a probability of 1/2,563 would be a "conservative estimate" of the probability of a match with a randomly selected American Indian. That probability was not obtained with the 1992 NRC report's ceiling method, but by looking to the largest profile frequency among particular tribes represented in FBI's American Indian database. The US Court of Appeals for the Ninth Circuit upheld the admission of the testimony. It reasoned that "evidence of opposing academic camps in virtual scholarly equipoise amidst the scientific journals" demonstrated more than "minimal support within a [scientific] community" and that this degree of acceptance, in combination with the other considerations listed in Daubert, militated in favor of admission. Other federal and state courts, applying the scientific-soundness standard, have held far smaller genotype-frequency estimates-both ceiling and population-specific-admissible: United States v Jakobetz, 955 F.2d 786, 800 (2d Cir.), cert. denied, 113 S.Ct. 104 (1992) (pre-Daubert opinion holding population-specific frequencies admissible); United States v Bonds, 12 F.3d 540 (6th Cir. 1993); United States v Martinez, 3 F.3d 1191 (8th Cir. 1993), cert. denied, 114 S.Ct. 734 (1994); Commonwealth v Lanigan, 419 Mass. 15, 641 N.E.2d 1342 (1994) (interim ceiling estimate properly admitted under Daubert on retrial following reversal under Frye of original conviction obtained with usual product-rule estimate); State v Duran, 881 P.2d 48 (N.M. 1994); State v Peters, 192 Wis. 2d 674, 534 N.W.2d 867 (Ct. App. 1995) (ceiling frequency properly admitted with other estimates against American Indians under relevancy standard when the trial court found that the interim ceiling method satisfied Daubert); State v Springfield, 860 P.2d 435 (Wyo. 1993) (population-specific frequencies admissible when accompanied by ceiling frequency). But see State v Streich. 457 A.2d 440 (Vt. 1995) ("even under Daubert it is inappropriate to allow evidence based on the unmodified product method. In the lexicon of Daubert, we are concerned that the accuracy of the results cannot be ensured by testing, there is an unknown potential for error, and these calculations are not generally accepted within the scientific community. The endorsement of the ceiling principle by the NRC and more recently by leading advocates in the dispute, including a representative of the FBI, leads us to this conclusion.").


State v Carter, 246 Neb. 953, 524 N.W.2d 763 (1994) (1992 NRC Report indicates lack of general acceptance of HW proportions, which renders testimony of a DQA test result said to include about 7% of the population inadmissible under FrYe); People v Morales (Rockland County Ct.), N.Y.L.J., Oct. 26, 1994. at 34, col. 6.


E.g., People v Wallace, 14 Cal. App. 4th 651, 17 Cal. Rptr. 2d 721 (1993); State v Hollis, No. 2-1-04603-9 (Wash. Super. Ct., King County, June, 1993), appeal pending, No. 3307-1-L; People v Barney, 8 Cal. App. 4th 798, 10 Cal. Rptr. 2d 731 (1992); People v Atoigue, DCA No. CR 91-95A (Guam Dist. Ct. App. Div. 1992); State v Carter, 246 Neb. 953, 524 N.W.2d 763, 782, 783 (1994) ("The calculation of statistical probability is an essential part of the process used in determining the significance of a DNA match . . .We hold that evidence of a DNA match will not be admissible if it has not been accompanied by statistical probability evidence that has been calculated from a generally accepted method."). Contra Commonwealth v Crews, 640 A.2d 395, 402 (Pa. 1994) ("The factual evidence of the physical testing of the DNA samples and the matching alleles, even without statistical conclusions, tended to make appellant's presence more likely than it would have been without the evidence, and was therefore relevant.").


State v Bogan, 183 Ariz. 506, 905 P.2d 515 (Ct. App. 1995), rev. granted; State v Hummert, No. CR 90-05559 (Super. Ct. Maricopa Co. Apr. 16, 1991), rev 'd for not excluding testimony thought to assert that match was unique, 183 Ariz. 493, 905 P.2d 493 (Ct. App. 1994), rev. granted; State v DeSpain, No. 15589 (Super. Ct. Yuma Co., Feb. 12, 1991); State v Pennell, 584 A.2d 513 (Del. 1989); State v Schwartz, 447 N.W.2d 422, 428 (Minn. 1989); State v Alt, 504 N.W.2d 38 (Minn. Ct. App. 1993); Polk v State, 612 So. 2d 381 (Miss. 1993) (the trial court admitted testimony of a match but excluded an accompanying population frequency estimate); State v Moore, 885 P.2d 457, 467, 468 (Mont. 1994) (the defendant was barred from challenging the fact that the trial court "refused to allow testimony concerning the statistics, but allowed the experts to testify that the RFLP and PCR test results were 'consistent' with [the defendant's DNA]," but "whether, and if so, to what extent we will allow DNA evidence without the accompanying statistical evidence in other criminal cases will be decided in a future case."); Rivera v State, 840 P.2d 933 (Wyo. 1992) (suggesting that the better practice is not to refer to probability estimates when introducing DNA results). But cf. Springfield v State, 860 P.2d 435 (Wyo. 1993) (a probability estimate was admissible).


See State v Hummert 183 Ariz. 493, 905 P.2d 493 (Ct. App. 1994), rev. granted; State v Cauthron, 846 P.2d 502, 516, 518 (Wash. 1993) (experts from a testing laboratory presented no "probability statistics" but one expert claimed that "the DNA could not have from anyone else on earth"); State v Buckner, 890 P.2d 460 (Wash. 1995) (testimony that the profile "would occur in only one Caucasian in 19.25 billion" and that because "this figure is almost four times the present population of the Earth, the match was unique" was improper).


State v Zollo, 36 Conn. App. 718 (1995) ("testimony that the chance that the DNA sample came from someone other than the defendant was 'so small that ... it would not be worth considering"' was not inadmissible as an opinion on an ultimate issue in the case "because his opinion could reasonably have aided the jury in understanding the [complex] DNA testimony"); People v Heaton, 266 I11. App. 3d 469, 640 N.E.2d 630 (1994) (an expert who used the product rule to estimate the frequency at 1/52,600 testified over objection to his opinion that the "defendant was the donor of the semen"); State v Pierce, No. 89-CA-30 (Ohio Ct. App. 1990) (affirming admission of testimony that the probability would be one in 40 billion "that the match would be a random occurrence," and "the DNA is from the same individual''), aff'd, 64 Ohio St. 3d 490, 597 N.E.2d 107 (1992); cf. State v Bogan, 905 P.2d 515, 522-23 (Ariz. Ct. App. 1995) (it was proper to allow a molecular biologist to testify, on the basis of a PCR-based analysis known as RAPD, that he "was confident the seed pods found in the truck originated from" a palo verde tree near a corpse); Commonwealth v Crews, 640 A.2d 395 (Pa. 1994) (testimony of an FBI examiner that he did not know of a single instance "where different individuals that are unrelated have been shown to have matching DNA profiles for three or four probes" was admissible under Frye despite objection to lack of a frequency estimate, which had been given at a preliminary hearing as 1/400).


See, e.g., State v Bloom, 516 N.W.2d 159, 160 n.2 (Minn. 1994) (a population geneticist was prepared to testify that "in his opinion the nine-locus match constituted 'overwhelming evidence that, to a reasonable degree of scientific certainty, the DNA from the victim's vaginal swab came from the [defendant], to the exclusion of all others'").


State v Bloom, 516 N.W.2d 159, 166-67 (Minn. 1994) ("Since it may be impossible to reach a consensus on how to estimate, with any degree of precision, the probability of a random-match, and given the great difficulty in educating the jury as to precisely what that figure means and does not mean, it might make sense to simply try to arrive at a fair way of explaining the significance of the match in a verbal, qualitative, nonquantitative, nonstatistical way."). See also Kreiling (1993).


Cf. United States v Fatico, 458 F. Supp. 388, aff'd, 603 F .2d 1053 (2d Cir. 1979), cert. denied, 444 U.S. 1073 (1980) (a survey of district judges revealed that their assessment of the probability of guilt associated with the "beyond a reasonable doubt" standard ranged from 76% to 90%).


Some commentators distinguish between the probability of a reported match (including the risk of sample mishandling or laboratory error that would produce a false positive result) and the probability of a true (but coincidental) match for a person selected at random. E.g., Koehler 1993b.


For cases rejecting this argument, see, e.g., United States v Chischilly, 30 F.3d 1144 (9th Cir. 1994) (citing cases); State v Weeks, 891 P.2d 891 P.2d 477 (Mont. 1995); State v Schweitzer, 533 N.W.2d 156, 160 (S.D. 1995) (reviewing cases).


State v Carlson, 267 N.W.2d 160 (Minn. 1978); McCormick (1992, § 210). The opinions of the Minnesota Supreme Court also posit "a real danger that the jury will use the evidence as a measure of the probability of the defendant's guilt or innocence" (State v Schwartz, 447 N.W.2d 422, 428 (Minn. 1989], quoting State v Boyd, 331 N.W.2d 480, 483 [Minn. 1983]).


For example, Goodman (1992) varied the frequency of the suspect's blood type in a hypothetical homicide case in which the sample of blood from the scene of the crime matched that taken from the defendant and not the victim. Although the mock jurors with frequency information were more likely to convict than those who received no frequency information, and although guilty verdicts decreased as the frequency of a random-match went from 0.001 to 0.1, frequencies of 0.001, 0.01, and 0.05. did not produce differing rates of conviction. Other research on blood-type evidence has produced similar results (Faigman and Baglioni 1988; Thompson and Schumann 1987).


See Kaye and Koehler 1991 (reviewing studies). Koehler and colleagues (1995) provided a brief written summary of a homicide case in which the case evidence was circumstantial and weak apart from the DNA evidence. In two studies, one with college students and a replication with jurors, respondents were assigned to one of three laboratory error-rate conditions (absent, 0.02, 0.001) and were either provided with a 1/(1 billion) probability of a random-match or given no information on the probability of a random-match. Conviction rates were influenced by the information on probability of a random-match but unaffected by the presence or level of the laboratory error-rate information. Yet conviction rates with information on probability of a random-match averaged 44% for the students (44% with and 44% without laboratory error information) and 49% for the jurors (54% with and 44% without laboratory error information), reinforcing the impression that jurors are not overwhelmed by statistical DNA evidence.


Examples are collected in Kaye (1993) and Koehler (1993a). See also, e.g., United States v Martinez, 3 F.2d 1191, 1194 (8th Cir. 1993) ("The second step of the DNA identification process then involves a determination of the probability that someone other than the contributor of the known sample could have contributed the unknown sample."); Greenwood v United States, 659 A.2d 825, 826 (DC Ct. App. 1995) ("The parties stipulated that DNA evidence established that the probability that Greenwood was the source of the semen found on the victim's underpants was not less than 2000 to 1."); People v Heaton, 266 III. App. 3d 469, 640 N.E.2d 630 (1994) (an expert was said to have testified that "the probability of another Caucasian . . . was 1 in 52,600"); Commonwealth v Crews, 640 A.2d 395, 400 (Pa. 1994) ("DNA analysis generally can provide only statistical probability; e.g., there is one chance in four hundred or one chance in four million that the DNA samples come from someone else.''); Taylor v State, 889 P.2d 319 (Okla. Ct. Crim. App. 1995) (Lifecodes's expert was said to have "testified that the likelihood that an African American other than Taylor contributed the DNA ... was one in 97 billion"); Taylor v Commonwealth, No. 1767-93-1, 1995 WL 80189 (Va. Ct. App. Feb. 28, 1995) (an unpublished opinion reporting that "DNA analysis of semen obtained from the victim and defendant's blood established a probability of I in 128 million that a black male other than defendant was the perpetrator").


As regards the transposition fallacy, such an instruction might be framed along these lines: "In evaluating the expert testimony on the DNA evidence, you were presented with a number indicating the probability that another individual drawn at random from the [specify] population would coincidentally have the same DNA profile as the [blood stain, semen stain, etc.]. That number, which assumes that no sample mishandling or laboratory error occurred, indicates how distinctive the DNA profile is. It does not by itself tell you the probability that the defendant is innocent."


The only study comparing reactions to separate and combined estimates found that subjects were insensitive to information on error rates when the random-match probability and the laboratory-error rate were presented separately (Koehler, Chia, and Lindsey 1995). The "evidence" in the mock case, however, was presented in the form of a single sentence unaccompanied by explanation or argument.


In the short run, it is appropriate to alert jurors both to the value of the statistical evidence and to its limitations. In the longer run, research should be conducted to evaluate the impact of DNA testimony on juror decision-making and the effects of alternative approaches, such as likelihood ratios or instructions in applying Bayes's theorem, on jury comprehension. Studies are needed that test reactions to the kind of DNA evidence that is presented in the courtroom, along with witness explanations, attorney arguments, and judicial instructions. Such hypotheses as the suggestion that jurors faced with the estimated laboratory-error rate and the random-match probability might average the two (Lempert 1993) or that jurors will fail to use information on laboratory-error rates cannot be evaluated in a useful way if jurors are not provided with the kind of assistance that they would receive in the relevant legal setting. Additional research on juror reactions should test the ability of jury instructions, videotaped expert explanations, and other educational efforts to facilitate appropriate interpretation of DNA evidence.


As discussed in Chapter 5, with match-binning, the numerator is slightly less than I because there is a very small chance that two measurements of the same band will not satisfy the match criteria (see Kaye 1995b).


Likelihood ratios were used in State v Klindt, 389 N.W.2d 670 (Iowa 1986) (discussed later), and are admitted routinely in parentage litigation, where they are known as the "paternity index" (see Chapter 5). E.g., Kaye 1989; Aickin and Kaye 1983; McCormick 1992, § 212. Some state statutes use them to create a presumption of paternity (Kaye 1990a,b,c). The practice of providing a paternity index has been carried over into criminal cases in which genetic parentage is used to indicate the identity of the perpetrator of an offense. E.g., State v Skipper, 228 Conn. 610, 637 A.2d 1101 (1994); Davis v State, 476 N.E.2d 127 (Ind. Ct. App. 1985); State v Weeks, 891 P.2d 477 (Mont. 1995); State v Spann, 130 N.J. 484, 617 A.2d 247 (1993): State v Jackson, 320 N.C. 452, 358 S.E.2d 679 (1987). Some of the appellate courts in some of these cases disapproved of the biostatistical presentations, but none specifically condemned the use of the likelihood ratio.


E.g., Evett 1991, p 201 ("Just leaving a court with a likelihood ratio does not seem enough."): cf. Fienberg 1992 (criticizing presentation of a relative likelihood function).


See Smith v Deppish, 807 P.2d 144 (Kan. 1991) (the state's "DNA experts informed the jury that ... there was more than a 99 percent probability that Smith was a contributor of the semen"); State v Thomas, 830 S.W.2d 546 (Mo. Ct. App. 1992) (a geneticist testified that "the likelihood that the DNA found in Marion's panties came from the defendant was higher than 99.99%"): Commonwealth v Crews, 640 A.2d 395, 402 (Pa. 1994) (an FBI examiner who at a preliminary hearing had estimated a coincidental-match probability for a VNTR match "at three of four loci" reported at trial that the match made identity "more probable than not").


State v Skipper, 228 Conn. 610, 637 A.2d 1101 (1994) (reasoning that this application of Bayes's theorem violated the presumption of innocence and suggesting in dictum that any use of Bayes's theorem would be impermissible); State v Hartman, 426 N.W. 2d 320 (Wis. 1988). The undisclosed use of a prior probability of one-half was standard in civil cases and first was criticized in Ellman and Kaye (1979). The courts that routinely admitted such testimony probably did not recognize the Bayesian nature of the "probability of paternity" laid before them, but courts unmistakably apprised of the foundations of these probabilities have continued to approve of them. A few courts have imposed restrictions on the practice. E.g., Commonwealth v Beausoleil, 490 N.E.2d 788 (Mass. 1986); Plemel v Walter, 735 P.2d 1209 (Or. 1987). For discussion and criticism, see Kaye (1988b, 1989).


Finkelstein and Fairley (1970). For LR = 1,000,000, the posterior probability approaches I for all but invisible values of the prior probability. For example, the prior probability would have to be about 1/1,000,000 or less to keep the posterior probability to less than one-half.


See State v Skipper, 228 Conn. 610 (1994) (stating in dictum that it would be an error to use the variable-prior-odds approach); State v Spann, 130 N.J. 484, 617 A.2d 247 (1993) (remanding for possible consideration of the use of a Bayesian graph of the probability of paternity). The Skipper opinion is criticized by many of the discussants in Allen et al. (1995).

Copyright 1996 by the National Academy of Sciences. All rights reserved.
Bookshelf ID: NBK232607


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