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Proc Natl Acad Sci U S A. 2000 October 24; 97(22): 12385–12388. Published online 2000 October 10. | PMCID: PMC17351 |
Copyright © 2000, The National Academy of Sciences Social Sciences Prostitution and the sex discrepancy in reported number of
sexual partners Devon D. Brewer,*† John J. Potterat,‡ Sharon B. Garrett,* Stephen Q. Muth,‡ John M. Roberts, Jr.,§ Danuta Kasprzyk,¶ Daniel E. Montano,¶ and William W. Darrow‖ *Alcohol and Drug Abuse Institute, University of Washington, 3937 15th Avenue NE, Seattle, WA 98105; ‡El Paso County Department of Health and Environment, 301 South Union Boulevard, Colorado Springs, CO 80910; §Department of Sociology, University of New Mexico, Albuquerque, NM 87131; ¶Centers for Public Health Research and Evaluation, Battelle Memorial Institute, 4000 NE 41st Street, P.O. Box 5395, Seattle, WA 98105-5395; ‖Department of Public Health, Florida International University, 3000 NE 145th Street, ACI-394F, North Miami, FL 33181 Received June 21, 2000; Accepted August 16, 2000. One of the most reliable and perplexing findings from surveys of
sexual behavior is that men report substantially more sexual partners
than women do. We use data from national sex surveys and studies of
prostitutes and their clients in the United States to examine sampling
bias as an explanation for this disparity. We find that prostitute
women are underrepresented in the national surveys. Once their
undersampling and very high numbers of sexual partners are factored in,
the discrepancy disappears. Prostitution's role in the discrepancy is
not readily apparent because men are reluctant to acknowledge that
their reported partners include prostitutes. Across the world, probability
sample household surveys of adult sexual behavior show that men report
substantially more sexual partners than women do (1–13). This finding
is puzzling, because in a closed population of heterosexuals, men and
women actually have the same number of sexual partners in the
aggregate. Explanations for this discrepancy pertain to either
sex-linked reporting bias or sampling bias. Sex-linked reporting bias
means that, for whatever reason, men overreport and/or women
underreport the number of their partners. Sampling bias refers to the
undersampling of women who have had many partners and/or women with
whom sampled men have had sex but are outside the sampling frame. In
our analysis, we evaluated sampling bias related to prostitution as an
explanation for the disparity. Measuring the Discrepancy Data. We used data from the 1988–1991 General Social Surveys (GSS) ( 14) and
the 1992 National Health and Social Life Survey (NHSLS) ( 8). These
cross-sectional surveys involved multistage area probability samples of
adults living in United States households (the NHSLS included adults
under age 60 only). Although the population of the United States is not
closed, we treated it as if it were, given the consistency of the
measured sex discrepancy in countries from each major region of the
world and at different levels of development. We used data based on responses to questions in paper self-administered
questionnaires (SAQs), except as noted. In both surveys, two questions
ask about the number of sexual partners in the last 12 months and 5
years (asked in the GSS since 1991), respectively. Response options
include 1, 2, 3, 4, 5–10, 11–20, 21–100, and more than 100. In the
1991 GSS, some respondents answered these questions in an open-ended
fashion. For each recall period, we recoded open-ended responses to the
closed-ended response categories. Following T. W. Smith's
procedures (personal communication), we recoded response categories
involving ranges to the midpoints and recoded “more than 100”
partners to “101.” For the NHSLS 12-month recall period, we
followed Laumann et al. ( 8) by using the greater of the SAQ
response and the face-to-face response to a similar question. For both
surveys, we defined heterosexuals as those who reported only
opposite-sex partners for a given recall period. Our analyses of the
GSS are based on data from 1988 to 1991 combined. Overall sample sizes
are 5,907 for the GSS and 3,159 for the NHSLS. Analysis and Results. Because of slight differences in the numbers of men and women in the
population at large and differences in the proportions of men and women
who are heterosexual, estimates must be obtained at the United States
population level rather than simply by relying on the surveys' sample
means for men's and women's numbers of partners. (Detailed
calculations for all analyses are presented in the Appendix,
which is published as supplementary material on the PNAS web site,
www.pnas.org.) To measure the sex discrepancy, we first weighted
responses by the number of adults in respondents' households to
compensate for the fact that persons in large households were less
likely to be interviewed (only one person in a sampled household was
interviewed). Then, for each survey and recall period, we computed the
total numbers of partnerships reported by heterosexual men and women in
the United States based on ( i) the proportions of men and
women in the survey sample who were heterosexual, ( ii)
census figures from 1990 (GSS) and 1992 (NHSLS)
( http://www.census.gov/population/estimates/nation/intfile2–1.txt)
for adult men and women (men and women age 18–59 years for the NHSLS),
and ( iii) the mean numbers of partners reported by
heterosexual men and women in the survey sample. The ratios of the total number of sex partners heterosexual men report
to the number heterosexual women report (see Table
1) show substantial sex disparities, as
indicated by their departure from the expected value of 1. Consistent
with prior research, the discrepancy ratios are larger for the 5-year
recall period than for the 12-month recall period. We obtained very
similar results to those in Table 1 when we used only the data from the
open-ended GSS questions about numbers of partners and when we included
data on bisexuals, treating them as if they were heterosexuals.
| Table 1Unadjusted and prostitution-adjusted ratios of the number
of sexual partners reported by heterosexual men to the number reported
by heterosexual women |
Adjusting the Discrepancy for Prostitution To assess how much of the discrepancy prostitution can explain, we
estimated the prevalence of prostitute women and their number of
partners and then compared these estimates with observations in the GSS
and NHSLS. Prevalence of Prostitutes. Potterat et al. ( 15) estimated the annual prevalence of
full-time equivalent prostitutes in the United States to be 23 per
100,000 population based on a capture–recapture study of prostitutes
found in Colorado Springs, CO, police and sexually transmitted diseases
clinic records between 1970 and 1988. The estimation procedure took
into account prostitutes' mobility by weighting women proportional to
the fraction of a year that they worked in Colorado Springs. The
Colorado Springs Standard Metropolitan Statistical Area during this
period had demographic characteristics and sexually transmitted disease
rates similar to those of the United States as a whole. The 23 per
100,000 estimate reflects the 1970–1988 period overall as well as
annual prevalences observed between 1986 and 1988. Studies of
prostitute prevalence in other North American communities that are less
typical of the United States demographically indicate higher estimates
(see supplementary material). Prostitutes' Number of Partners. We estimated prostitutes' number of partners from our prospective
study of the sexual, drug-using, and social networks of persons
(including prostitutes) presumed to be at high risk for HIV infection
in Colorado Springs ( 16). We recruited respondents from the county
sexually transmitted diseases clinic, HIV testing site, drug treatment
program, and outreach activities, and then enrolled some of
respondents' sexual, drug-using, and social contacts through a
link-tracing design. Although the prostitutes in this study do not
constitute a probability sample of Colorado Springs prostitutes, our
experience indicates that they are demographically and behaviorally
representative of those working between 1988 and 1991. In the first
interview, 98 adult prostitute women reported a mean of 347 male sexual
partners in the last 6 months (median = 103; interquartile range =
11–228; range = 1–5,401). The four prostitutes with the most
partners (2,700–5,401) in this period reported very heavy
cocaine/crack use. Their high level of reported activity is
consistent with field observations of crack-addicted prostitutes ( 17).
We doubled the 6-month mean to obtain an estimated mean of 694 male
partners in the last 12 months for these women. This doubled figure is consistent with an estimate derived from
prostitutes' reported number of male partners in the last 5 years. For
the latter estimate, we assumed that the rates of entry into and exit
from prostitution were equal for the Colorado Springs cohort of
prostitutes, which implies that these women were, on average, halfway
through their prostitution careers ( 18). Because prostitute women in
Colorado Springs have a mean career length of 5 years ( 15), prostitute
women in this sample most likely worked as prostitutes for only 2.5 of
the last 5 years, on average. Therefore, currently active adult
prostitutes' ( n = 98) mean of 2,171 reported male
partners in the last 5 years corresponds to an estimated mean of 868
male partners per year (2,171 partners per 2.5 years) for working adult
prostitutes. We opted for the more conservative estimate based on
doubling the 6-month mean. Other research in the United States during the past 25 years tends to
indicate higher numbers of partners for prostitute women, but these
results seem to derive from methodological features of these studies
(see supplementary material). These 10 other studies of prostitute
women in 17 different communities (involving a total of 2,319 women)
either did not include representative samples of prostitutes or used
recall periods less appropriate for estimating the number of partners
in a year. Numbers of partners reported by prostitutes are not likely
to be overestimates. In one study, South African prostitutes'
retrospective estimates of the number of their partners in the past
week were actually less than (only 59% of) those recorded in daily
diaries for the same period ( 19). Analysis and Results. With these estimates of the national prevalence of prostitute women and
their number of male sex partners, we calculated the number of
prostitutes and their number of partnerships expected to be reported
for the last 12 months and 5 years in the GSS and NHSLS. In merging
information from different sources, we sometimes excluded observations
from particular studies or adjusted estimates of key parameters (as
detailed below) to ensure that the age ranges of respondents and
partnerships reported in the different studies were comparable. The GSS
and NHSLS involved only adult respondents, and the NHSLS also excluded
adults over age 59. The studies of prostitutes and their clients,
however, often involved some juveniles and individuals over age 59. For the 12-month recall period, we estimated the adult prostitute
prevalence rate to be 22.1 per 100,000, correcting for that fraction
(4% in the Colorado Springs sample) of prostitutes who were under age
18. Because studies cited in the supplementary material show that
virtually all working prostitutes are under age 45, we computed the
number of women aged 18–44 in a survey expected to be prostitutes from
this adult prevalence rate, census figures, and a survey sample's age
distribution. We then computed the expected total number of
partnerships reported by prostitutes by multiplying the expected number
of prostitutes by 694, the estimated mean number of partners for
prostitutes in a year. [For the NHSLS, we reduced this expected number
of partnerships for prostitutes by the percentage of commercial
partnerships (3%) in the last 6 months that were reported by clients
over age 59 in the Colorado Springs study.] For both national surveys,
we defined prostitutes in the last year as those women who indicated,
in response to one question, that they had received payment or paid for
sex in the last 12 months. Our calculations for the 5-year recall period followed those for the
12-month recall period except that we (i) defined
prostitutes as heterosexual women who reported ever having received
payment or paid for sex (GSS) or been paid by a man for sex (NHSLS) and
were younger than age 45 sometime in the last 5 years; (ii)
multiplied the expected number of prostitutes' partnerships by 5; and
(iii) for the NHSLS, corrected for partnerships between
prostitutes and men who were age 55–59 in the last 5 years but
older than 59 at the time of the NHSLS, based on data from the
Colorado Springs study reported by clients of prostitutes. The observed number of partnerships reported by prostitute women in the
national surveys falls far short of the number expected. For example,
in the GSS between 1988 and 1991, there was one prostitute woman and
she reported between 21 and 100 male sexual partners in the last 12
months. This contrasts with 1.67 expected prostitutes reporting an
expected total of 1,158 partnerships with men. To compensate for this shortfall, we added prostitutes' expected
partnerships that were not reported in the national surveys to
heterosexual women's reports and computed a prostitution-adjusted mean
number of partners for heterosexual women. We then used this
prostitution-adjusted mean for recomputing the estimated number of
partnerships for heterosexual women in the United States. We also
reduced the estimated number of partnerships reported by heterosexual
men by the number of prostitutes' partnerships in the United States
multiplied by the proportion of Colorado Springs prostitutes'
partnerships attributable to prostitutes who were juveniles throughout
the recall period (2% for the last year and 1% for the last 5 years).
All calculations for these adjustments are based on
household-size-weighted data. After adjusting for these prostitution-related factors, the ratios for
the sex discrepancy in the reported number of sexual partners hover
slightly above and below 1 (see Table 1), indicating that prostitution
can account for essentially all of the disparity. In the NHSLS it is
possible that, in fact, no genuine discrepancy in reporting exists. For
the 12-month recall period, one bisexual prostitute woman (not included
in our analyses) reported more than 100 partners. If she had an
above-average number of partners for a prostitute and primarily male
partners, her report might eliminate most or all of the discrepancy.
Likewise, the discrepancy for the 5-year recall period could also
possibly be eliminated with similar assumptions about the two bisexual
prostitute women who reported more than 100 partners. (No nonprostitute
woman reported more than 100 partners for either recall period in the
national surveys.) In any event, prostitution would still explain the
disparity. There are several reasons why prostitutes, especially those with
typical numbers of partners, are unlikely to be represented in
household surveys in the United States. First, many prostitutes
temporarily reside in institutions (such as jails/prisons, homeless
shelters, and halfway houses) or live in other lodgings (such as
motels/hotels and rooming houses) not considered to be households,
placing them outside of sampling frames for such surveys. On enrollment
in the Colorado Springs study, 34% of the 98 adult women who worked as
prostitutes in the last 6 months did not live in households. Similarly,
in a study that involved prostitute women representing the main locales
and sectors of prostitution in Seattle in 1991–1993, 46% of the 57
adult women who worked as prostitutes in the last 3 months did not live
in households on entry to the study ( 20). In addition, 15% of a
probability sample of 1,024 street prostitutes in Los Angeles County in
1990–1991 were homeless, ** as were 17%
of 1,963 street prostitutes in New York City contacted by outreach
workers in 1989–1992 ( 21). Second, prostitute women's work schedule
(usually beginning in the late afternoon and often stretching into the
early morning) ( 22) conflicts with the GSS interviewers' schedule
(weekdays after 3:00 p.m., weekends, and holidays) ( 14). Furthermore,
prostitute women in the United States tend to be quite mobile ( 15, 23)
and have high mortality ( 24). Men's Underreporting of Contact with Prostitutes Prostitution's role in explaining the discrepancy is not readily
apparent because heterosexual men underreport contact with prostitutes.
We defined clients of prostitutes as heterosexual men who indicated
they had paid for sex or received payment for sex (GSS/NHSLS) or paid
for sex (NHSLS) in the last 12 months. In both the GSS and the NHSLS,
even if all of the partners reported by acknowledged clients of
prostitutes are assumed to have been prostitutes, the sum of these
partnerships constitutes only 15–23% of the expected number of
prostitutes' partnerships. This result is not likely due to
undersampling of clients, as clients do not seem to be underrepresented
in household surveys. In the GSS and NHSLS, clients and nonclients in
the last year do not differ meaningfully in terms of age, education,
employment status, or income. [We based these and the following
analyses of clients in the NHSLS on the whole NHSLS sample
( n = 3,432), which includes the oversample of blacks
and Hispanics.] Furthermore, studies in which clients have been
sampled directly at prostitution venues indicate that they resemble
adult men in the United States overall on these and other demographic
characteristics ( 25, 26). Responses to repeated questioning about involvement in prostitution
indicate men's reluctance to acknowledge contact with prostitutes. In
two different parts of the Colorado Springs interview, heterosexual men
were asked about contact with prostitutes in the last 5 years, with the
second question referring to prostitutes in Colorado Springs only.
Eleven of the 110 clients acknowledged prostitute partners only in
response to the second question, and 2 additional men who did not
report contact with prostitutes were known to be clients from
prostitutes' naming them specifically as clients in another part of
the interview. In the NHSLS, 7 of the 13 heterosexual men who admitted
to contact with prostitutes in the last year acknowledged this only in
response to the second question on the topic (the two questions were
presented in separate SAQs). Methodological experiments involving audio computer-assisted
self-interviewing (ACASI), which is thought to promote accurate
reporting, also point to men's underreporting contact with
prostitutes. In a national probability sample of males aged 15–19 in
the United States ( 27), the proportion of those reporting contact with
prostitutes was almost four times higher with ACASI than with paper
SAQs, which were used in the GSS and NHSLS. However, ACASI and SAQs did
not differ appreciably in the proportion of respondents reporting more
than five lifetime sexual partners. In a probability sample of Hong
Kong men returning from Shenzhen in mainland China, the percentage of
those reporting contact with prostitutes in mainland China in the last
6 months was 37% higher with ACASI than with SAQs (32% vs. 24%)
( 28). In the GSS and the NHSLS, clients reported, on average, seven to nine
times more partners for the last year than nonclients did. In the
Colorado Springs study, on average, prostitutes accounted for 48% of
the partners for heterosexual men reporting more than 15 partners in
the last 6 months (n = 15) but only 17% of the
partners for heterosexual men reporting 15 or fewer partners in the
last 6 months (n = 228). These observations suggest
that the primary source of underreporting contact with prostitutes may
be men who do not acknowledge being clients but still report a large
number of partners that likely includes many prostitutes. Indeed,
clients in the Colorado Springs study and the NHSLS who admitted
contact with prostitutes only after being asked twice reported as many
partners on average as did clients who admitted contact with
prostitutes when first asked. In our calculations, prostitution-related partnerships make up
30–32% of the adjusted number of women's partnerships for the last
12 months, but 54–61% of the adjusted number for the last 5 years.
Prostitution allows men to accrue new partners at a higher
rate than nonprostitute women, which causes the unadjusted ratios to
increase with longer recall periods. In prior research, the discrepancy could not be eliminated by removing
those respondents who reported involvement in prostitution or by
reducing men's number of partners by an estimate of admitted clients'
number of prostitute partners ( 2, 13, 29, 30). These previous results
are consistent with ours and can be explained by heterosexual men
underreporting their contact with prostitutes. Einon ( 18) addressed and dismissed the prostitution explanation for the
discrepancy in the British household survey ( 5). However, her analysis
of the lifetime number of reported partners is undermined by the use of
point and annual, rather than lifetime, prevalences of prostitutes, and
thus does not adjust for the cumulative number of partners that all
prostitutes from multiple cohorts had over respondents' lifetimes.
Furthermore, available empirical information indicates that
prostitution also can account for the discrepancies in the British ( 5)
and Ivorian ( 29) surveys for recall periods between 1 and 5 years (see
supplementary material). Other types of sampling bias cannot account for the sex
discrepancy in reported number of partners. In 1990, the number of men
from the United States who traveled overseas is balanced almost
perfectly by the number of men from overseas countries who traveled to
the United States ( 31). Foreign men visiting the United States also
tend to be younger than men from the United States traveling overseas.
These facts imply that the number of sexual partnerships (commercial
and otherwise) men from the United States have in other countries is
likely canceled out by the number of partnerships foreign men have in
the United States. Moreover, the excess partners reported by adult men
cannot be accounted for by partnerships with adolescent females. In a
probability sample of 882 18-year-olds in Detroit, the sex discrepancy
ratio in reported number of lifetime sexual partners is 1.57 ( 32). This
ratio would need to be substantially less than 1 for adolescent females
to account for a significant share of adult men's excess partners. In sum, prostitutes are underrepresented in national household sex
surveys, and their undersampling can account for the sex discrepancy in
reported numbers of sexual partners. These results suggest that
respondents' reports of the number of their sex partners, although
possibly limited in other ways, may not be significantly affected by
sex-linked reporting bias. We thank Daniel David, Pamela Miles, and Nancy Sutherland for
library assistance, Ning Gu and Cheryl Williams for help in accessing
data, and Barbara Metzger for copyediting suggestions. Sevgi Aral,
Stuart Brody, Norman Brown, Linton Freeman, Hilary Kinnell, Barbara
Leigh, Martina Morris, A. Kimball Romney, Michael Wiederman, and four
anonymous reviewers provided helpful comments. This research was
supported in part by National Institute on Drug Abuse Grants DA09928
and DA10640. | | GSS | General Social Survey | | | NHSLS | National
Health and Social Life Survey | | | SAQ | self-administered questionnaire |
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