U.S. flag

An official website of the United States government

NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.

Institute of Medicine (US) Committee on Understanding the Biology of Sex and Gender Differences; Wizemann TM, Pardue ML, editors. Exploring the Biological Contributions to Human Health: Does Sex Matter? Washington (DC): National Academies Press (US); 2001.

Cover of Exploring the Biological Contributions to Human Health

Exploring the Biological Contributions to Human Health: Does Sex Matter?

Show details

Executive Summary

The explosion in the growth of new biological information over the past decade has made it increasingly apparent that many normal physiological functions—and, in many cases, pathological functions—are influenced either directly or indirectly by sex-based differences in biology (also referred to throughout this report as sex differences).

In recent years, considerable attention has been given to the differences and similarities between females and males (1) at the societal level by researchers evaluating how individual behaviors, lifestyles, and surroundings affect one's biological development and health and (2) at the level of the whole organism by clinicians and applied researchers investigating the component organs and systems of humans. However, scientists have paid much less attention to the direct and intentional study of these differences at the basic cellular and molecular levels. Where data are available, they have often been a by-product of other areas of research. Historically, the research community assumed that beyond the reproductive system such differences do not exist or are not relevant.

Scientific evidence of the importance of sex1 differences throughout the life span abounds. Investigators are now positioned to take this work to the next level, at which the mechanisms and origins of such differences can be explored. This will allow scientists and clinicians to understand the implications of these differences for human health. The critical questions to be answered are

  • How can information on sex differences be translated into preventive, diagnostic, and therapeutic practice?
  • How can the new knowledge about and understanding of biological sex differences and similarities most effectively be used to positively affect patient outcomes and improve health and health care?


In November 1999, the Institute of Medicine (IOM) formed the Committee on Understanding the Biology of Sex and Gender Differences in response to combined requests from a consortium of public and private sponsors. The committee members brought expertise from a broad array of disciplines in basic and applied biomedical research. In general, the sponsors asked the committee to evaluate and consider the current understanding of sex differences and determinants at the biological level. Specifically, they asked that the following issues be addressed:

  • the knowledge base on and research priorities for animal and cellular models that could be used to determine when sex and gender differences exist and when they are relevant to biological functioning at the cellular, developmental, organ, organismal, and behavioral levels;
  • current and potential barriers to the conduct of valid and productive research on sex and gender differences and their determinants, including ethical, financial, sociological, and scientific factors; and
  • strategies that can be used to overcome such barriers and promote the acceptance of this research by the scientific community and the general public.

The committee was not charged with the task of preparing a definitive text on all known differences and similarities between the sexes but, rather, was charged with considering factors and traits that characterize and differentiate males and females across the life span and that underlie sex differences in health (including genetic, biochemical, physiological, physical, and behavioral elements). Thus, the focus of this report is on sex-based differences, versus similarities, as they are more likely to successfully demonstrate the need for further research and lead to greater understanding of the significance of sex in human biology and health. Moreover, despite the influence of pregnancy, parity, and parenthood on the manifestation of some diseases and health outcomes, this report does not directly address these issues, as they are deserving of separate and more in-depth attention.

On the basis of its review, the committee arrived at a series of findings and conclusions and developed recommendations that are designed to facilitate scientific endeavors in this area, take advantage of new opportunities in basic and applied research, and fill identified research gaps.


Three common, recurring messages emerged as the committee addressed its primary task (reviewing and evaluating the current state of knowledge about sex differences in health and illness and scientific evidence related to sex differences in health and illness) and as it met with scientific experts across diverse disciplines.

  • Sex matters. Sex, that is, being male or female, is an important basic human variable that should be considered when designing and analyzing studies in all areas and at all levels of biomedical and health-related research. Differences in health and illness are influenced by individual genetic and physiological constitutions, as well as by an individual's interaction with environmental and experiential factors. The incidence and severity of diseases vary between the sexes and may be related to differences in exposures, the routes of entry and the processing of a foreign agent, and cellular responses. Although in many cases these sex differences can be traced to the direct or indirect effects of hormones associated with reproduction, differences cannot be solely attributed to hormones.
  • The study of sex differences is evolving into a mature science. There is now sufficient knowledge of the biological basis of sex differences to validate the scientific study of sex differences and to allow the generation of hypotheses. The next step is to move from the descriptive to the experimental and to establish the conditions that must be in place to facilitate and encourage the scientific study of the mechanisms and origins of sex differences. Naturally occurring variations in sexual differentiation and development can provide unique opportunities to obtain a better understanding of basic differences and similarities between and within the sexes.
  • Barriers to the advancement of knowledge about sex differences in health and illness exist and must be eliminated. Scientists conducting research on sex differences are confronted with an array of barriers to progress, including ethical, financial, sociological, and scientific factors.

After considering the data and examples presented throughout this report, the committee expects that the public, scientific, and policy communities alike will agree that the understanding of sex differences in health and illness merits serious scientific inquiry in all aspects of biomedical and health-related research. Some of the answers have been stumbled upon fortuitously. Until the question of sex, however, is routinely asked and the results—positive or negative—are routinely reported, many opportunities to obtain a better understanding of the pathogenesis of disease and to advance human health will surely be missed.


Every Cell Has a Sex

The biological differences between the sexes have long been recognized at the biochemical and cellular levels. Rapid advances in molecular biology have revealed the genetic and molecular bases of a number of sex-based differences in health and human disease, some of which are attributed to sexual genotype—XX in the female and XY in the male. Genes on the sex chromosomes can be expressed differently between males and females because of the presence of either single or double copies of the gene and because of several other phenomena: of different meiotic effects, X-chromosome inactivation, and genetic imprinting. The inheritance of either a male or a female genotype is further influenced by the source (maternal or paternal) of the X chromosome. The relative roles of the sex chromosome genes and their expressions explain X-chromosome-linked diseases and are likely to illuminate the reasons for the heterogeneous expression of some diseases within and between the sexes.

These findings argue that there are multiple, ubiquitous differences in the basic cellular biochemistries of males and females that can affect an individual's health. Many of these differences do not necessarily arise as a result of differences in the hormonal regime to which males and females are exposed but are a direct result of the genetic differences between the two sexes. Thus, the committee makes the following recommendation:

RECOMMENDATION 1: Promote research on sex at the cellular level.

The committee recommends that research be conducted to

  • determine the functions and effects of X-chromosome- and Y-chromosome-linked genes in somatic cells as well as germ-line cells;
  • determine how genetic sex differences influence other levels of biological organization (cell, organ, organ system, organism), including susceptibility to disease; and
  • develop systems that can identify and distinguish between the effects of genes and the effects of hormones.

Sex Begins in the Womb

Sex differences of importance to health and human disease occur throughout the life span, although their specific expression varies at different stages of life. Some differences originate in events occurring in the intrauterine environment, where developmental processes differentially organize tissues for later activation in the male or female. In the prenatal period, sex determination and differentiation occur in a series of sequential processes governed by genetic and environmental factors. During the prepubertal period, behavioral and hormonal changes manifest the secondary sexual characteristics that reinforce the sexual identity of the individual through adolescence and into adulthood. Hormonal events occurring in puberty lay a framework for biological differences that persist through life and contribute to the variable onset and progression of disease in males and females. It is important to study sex differences at all stages of the life cycle, relying on animal models of disease and including sex as a variable in basic and clinical research designs.

RECOMMENDATION 2: Study sex differences from womb to tomb.

The committee recommends that researchers and those who fund research focus on the following areas:

  • inclusion of sex as a variable in basic research designs,
  • expansion of studies to reveal the mechanisms of intrauterine effects, and
  • encouragement of studies at different stages of the life span to determine how sex differences influence health, illness, and longevity.

Sex is an important marker of individual variability. Some of this sex-related variability results from events that occur in the intrauterine environment but that do not materialize until later in life. Current research varies in its level of attention to these matters.

The committee acknowledges that inclusion of people, animals, or cells and tissues of both sexes in all studies is not always feasible or appropriate. Rather, the committee is urging researchers to regard sex, that is, being male or female, as an important basic human variable that should be considered when designing, analyzing, and reporting findings from studies in all areas and at all levels of biomedical research. Statistical methods can be used to evaluate the effect of sex without necessarily doubling the sample size of every study. In addition, it is particularly important that researchers revisit and revise approaches to studying whole-animal physiology in light of what has been learned in the past decade about major sex differences.

RECOMMENDATION 3: Mine cross-species information.

  • Researchers should choose models that mirror human sex differences and that are appropriate for the human conditions being addressed. Given the interspecies variation, the mechanisms of sex differences in nonhuman primates may be the best mimics for some mechanisms of sex differences in humans. Continued development of appropriate animal models, including those involving nonhuman primates, should be encouraged and supported under existing regulations and guidelines (see the Guide for the Care and Use of Laboratory Animals [National Research Council, 1996]).
  • Researchers should be alert to unexpected phenotypic sex differences resulting from the production of genetically modified animals.

Sex Affects Behavior and Perception

Basic genetic and physiological differences, in combination with environmental factors, result in behavioral and cognitive differences between males and females. Sex differences in the brain, sex-typed behavior and gender identity, and sex differences in cognitive ability should be studied at all points in the life span. Hormones play a role in behavioral and cognitive sexual dimorphism but are not solely responsible. In addition, sex differences in the perception of pain have important clinical implications. Research is needed on the natural variations between and within the sexes in behavior, cognition, and perception, with expanded investigation of sex differences in brain organization and function. To better understand the influences and roles of factors that may lead to sex differences, the committee makes the following recommendations:

RECOMMENDATION 4: Investigate natural variations.

  • Examine genetic variability, disorders of sex differentiation, reproductive status, and environmental influences to better understand human health.

Naturally occurring variations provide useful models that can be used to study the influences and origins of a range of factors that influence sex differences.

RECOMMENDATION 5: Expand research on sex differences in brain organization and function.

New technologies make it possible to study sex-differential environmental and behavioral influences on brain organization and function and to recognize modulators of brain organization and function. Innovative ways to expand the availability of and reduce the cost of new technologies need to be explored.

Sex Affects Health

Males and females have different patterns of illness and different life spans, raising questions about the relative roles of biology and environment in these disparities. Dissimilar exposures, susceptibilities, and responses to initiating agents and differences in energy storage and metabolism result in variable responses to pharmacological agents and the initiation and manifestation of diseases such as obesity, autoimmune disorders, and coronary heart disease, to name a few. Understanding the bases of these sex-based differences is important to developing new approaches to prevention, diagnosis, and treatment.

RECOMMENDATION 6: Monitor sex differences and similarities for all human diseases that affect both sexes.

Investigators should

  • consider sex as a biological variable in all biomedical and health-related research; and
  • design studies that will
    • control for exposure, susceptibility, metabolism, physiology (cycles), and immune response variables;
    • consider how ethical concerns (e.g., risk of fetal injury) constrain study designs and affect outcomes; and
    • detect sex differences across the life span.

The Future of Research on Biological Sex Differences: Challenges and Opportunities

Being male or female is an important fundamental variable that should be considered when designing and analyzing basic and clinical research. Historically, the terms sex and gender have been loosely, and sometimes inappropriately, used in the reporting of research results, a situation that should be remedied through further clarification. Conducting studies that account for sex differences might require innovative designs, methods, and model systems, all of which might require additional resources. Studies relying on biological materials would benefit from a determination and disclosure of the sex of origin of the material, and clinical researchers should attempt to identify the endocrine status of research subjects. Longitudinal studies should be designed to allow analysis of data by sex. Once studies are conducted, data regarding sex differences, or the lack thereof, should be readily available in the scientific literature. Interdisciplinary efforts are needed to conduct research on sex differences.

RECOMMENDATION 7: Clarify use of the terms sex and gender.

Researchers should specify in publications their use of the terms sex and gender. To clarify usage and bring some consistency to the literature, the committee recommends the following:

  • In the study of human subjects, the term sex should be used as a classification, generally as male or female, according to the reproductive organs and functions that derive from the chromosomal complement.
  • In the study of human subjects, the term gender should be used to refer to a person's self-representation as male or female, or how that person is responded to by social institutions on the basis of the individual's gender presentation.
  • In most studies of nonhuman animals the term sex should be used.

RECOMMENDATION 8: Support and conduct additional research on sex differences.

Because differences between the sexes are pervasive across all subdisciplines of biology, all research sponsors should encourage research initiatives on sex differences. Research sponsors and peer-review committees should recognize that research on sex differences may require additional resources.

RECOMMENDATION 9: Make sex-specific data more readily available.

Journal editors should encourage researchers to include in their reports descriptions of the sex ratios of the research population and to specify the extent to which analyses of the data by sex were included in the study. If there is no effect (absence of a sex difference), that should be stated in the results. When designing or updating databases of scientific journal articles and other information, informatics developers should devise ways of reliably accessing sex-specific data.

RECOMMENDATION 10: Determine and disclose the sex of origin of biological research materials.

The origin and sex chromosome constitutions of cells or tissue cultures used for cell biological, molecular biological, or biochemical experiments should be stated when they are known. Attempts should be made to discern the sex of origin when it is unknown. Journal editors should encourage inclusion of such information in Materials and Methods sections as standard practice.

RECOMMENDATION 11: Longitudinal studies should be conducted and should be constructed so that their results can be analyzed by sex.

The health status of males and females can vary considerably, both within and between the sexes, across the life span—from intrauterine development to old age. Most longitudinal studies have been designed with specific disease end points, thereby precluding consideration of many other relevant developmental issues and other diseases, disorders, and conditions.

RECOMMENDATION 12: Identify the endocrine status of research subjects (an important variable that should be considered, when possible, in analyses).

Data on cycles (menstrual, circadian, etc.) are often lacking. Most studies do not define which part of the cycle participants were in at the time of study, note participants only by age and not whether they are pre-or postmenopausal, or are based on only one cycle.

RECOMMENDATION 13: Encourage and support interdisciplinary research on sex differences.

Interdisciplinary research is generally accepted as valuable and important. Opportunities for interdisciplinary collaboration to enhance the understanding of sex differences, however, have not been fully realized. The committee recommends the continued development of interdisciplinary research programs and strategies for more effective communication and cooperation to achieve the following goals:

  • synergy between and among basic scientists, epidemiologists, social scientists, and clinical researchers;
  • enhanced collaboration across medical specialties; and
  • better translational—or bench-to-bedside—research and interlevel integration of data (cellular, to animal, to human).

RECOMMENDATION 14: Reduce the potential for discrimination based on identified sex differences.

The committee noted that, historically, studies on race, ethnicity, age, nationality, religion, and sex have sometimes led to discriminatory practices. The committee believes, therefore, that these historical practices should be taken into consideration so that they will not be repeated. The past should not limit the future of research but should serve as a guide to its use. Ethical research on the biology of sex differences is essential to the advancement of human health and should not be constrained.


Despite the progress made in focusing on women's health research and including women in clinical trials, such research will have limited value unless the underlying implications—that is, the actual differences between males and females that make such research so critical—are systematically studied and elucidated. Such research can enhance the basis for interpreting the results of separate studies with males and females, helping to clarify findings of no essential sex differences and suggesting mechanisms to be pursued when sex differences are found.

BOX 1Summary of Barriers to Progress in Research on Sex Differences


  • There is inconsistent and often confusing use of the terms sex and gender in the scientific literature and popular press.

Research Tools and Resources

  • The conduct of research on sex differences and longitudinal research may require more complex studies and additional resources.
  • Information on sex differences can be difficult to glean from the published literature.
  • Useful information on the sex of origin of cell and tissue culture material is often lacking in the literature.
  • There is a lack of data from longitudinal studies encompassing different diseases, disorders, and conditions across the life span.
  • There is a lack of consideration of hormonal variability.

Interdisciplinary and Collaborative Research

  • The application of federal regulations is not uniform.
  • Opportunities for interdisciplinary collaboration have been underused.

Non-Health-Related Implications of Research on Sex Differences in Health

  • There is a lack of awareness that the consequences of genetics and physiology may be amenable to change.
  • The finding of sex differences can lead to discriminatory practices.

BOX 2Summary of Recommendations

Recommendations for Research

  • Promote research on sex at the cellular level.
  • Study sex differences from womb to tomb.
  • Mine cross-species information.
  • Investigate natural variations.
  • Expand research on sex differences in brain organization and function.
  • Monitor sex differences and similarities for all human diseases that affect both sexes.

Recommendations for Addressing Barriers to Progress

  • Clarify use of the terms sex and gender.
  • Support and conduct additional research on sex differences.
  • Make sex-specific data more readily available.
  • Determine and disclose the sex of origin of biological research materials.
  • Conduct and construct longitudinal studies so that the results can be analyzed by sex.
  • Identify the endocrine status of research subjects.
  • Encourage and support interdisciplinary research on sex differences.
  • Reduce the potential for discrimination based on identified sex differences.



The committee defines sex as the classification of living things, generally as male or female according to their reproductive organs and functions assigned by the chromosomal complement, and gender as a person's self-representation as male or female, or how that person is responded to by social institutions on the basis of the individual's gender presentation. Gender is shaped by environment and experience.

Copyright 2001 by the National Academy of Sciences. All rights reserved.
Bookshelf ID: NBK222287


Recent Activity

Your browsing activity is empty.

Activity recording is turned off.

Turn recording back on

See more...