Small Wars Journal

Integration of the Sexes in the Military: Biological Considerations

Fri, 06/15/2018 - 5:40am

Integration of the Sexes in the Military: Biological Considerations

Matthew A. Cronin


A policy decision in 2015 requires the integration of women into all military occupational specialties (MOS), including combat MOS if they meet the appropriate standards.  Human history shows that women participated in war in supporting roles but did not fight as combatants. The risks associated with changing this fundamental characteristic of human society should be assessed considering science. Extensive research in evolutionary biology, sociobiology, anthropology, and genetics, indicate that different physical attributes and societal roles of men and women developed in nature under natural selection pressure for fitness of individuals and populations. Changing the historical pattern by allowing women to fight in war entails risks to the basic structure and fitness of human populations. This indicates that the risks associated with sexual integration of the military have not been adequately assessed.


A policy decision in 2015 requires the integration of women into all military occupational specialties (MOS), including combat MOS if they meet the appropriate standards. I realized there was a need for education of military officers on the topic of sex differences when it became apparent that the foundation of this policy was based on physical standards, not all relevant science. It is obvious, from women’s athletics for example, that some women can meet rigorous physical standards. But whether women can meet standards is the wrong question. The appropriate question is why have men, and not women, historically been the primary combatants in war? There is an immense amount of science that describes sex differences that is relevant to this question that I will summarize in this article.

Review of the literature on the topic of women in the military1 indicates that this policy derived from efforts by feminist lobbyists to give women equal rights. However, denying or minimizing the importance of differences between the sexes became part of the narrative as described in the literature on women in the military1 and feminist literature:

“Male power, male dominance, masculinity, male sexuality, male aggression are not biologically determined.  They are conditioned…What is conditioned can be deconditioned. Man can change.”2 (My italics).

Imagine this statement referring to male cattle (bulls) or male chimpanzees, and its absurdity is apparent (at least to a biologist).  Men and women are different, and the differences are biologically (i.e., genetically) determined.  This should be explicitly recognized in discussing integration of the sexes into combat MOS as has been done by Captain L. F. Serrano (USMC): 

“Acknowledging that women are different (not just physically) than men is a hard truth that plays an enormous role in this discussion.”3

Much has been written about the issue of sexual integration of the military including its effect on readiness and morale, and problems with sexual harassment of women by men1. Integration of the sexes has resulted in a “great paradox”:

“It is one of the great paradoxes… On the one hand, we’re going to throw them together saying they’re all the same, and then there are a million little exceptions and rules to keep [women] apart and treat them special.”4

The point is that men and women are biologically different, and like the emperor with no clothes, this needs to be acknowledged as fact.

A manifestation of the differences of the sexes is the historical pattern that men fight wars, as described by the eminent historian John Keegan:

“Half of human nature – the female half – is in any case highly ambivalent about warmaking…  Warfare is, nevertheless, the one human activity from which women, with the most insignificant exceptions, have always and everywhere stood apart....  Women have followed the drum, nursed the wounded … dug the trenches for men to defend and laboured in the workshops to send them their weapons.  Women, however, do not fight.  They rarely fight among themselves and they never, in any military sense, fight men.  If warfare is as old as history and as universal as mankind, we must now enter the supremely important limitation that it is an entirely masculine activity.” 5 

Note that Keegan doesn’t say that women cannot fight; he says they do not fight in war, and that men and women have had different roles in war6.  I recognize that women have served as combatants in some cases in the past, and the experience of the U.S. military in Iraq and Afghanistan shows that women can, and now do, fight in war.3, 7 However, full integration of women into the military with official sanctioning by society is a major change from historical precedent.  Readers interested in the history of women in war should read other well-researched assessments of the issue.1 

Male-female differences in anatomy, physiology, psychology, and behavior have been studied extensively. The psychologist Jordan Peterson provides keen insights on this topic and notes:

“…the existence of overwhelming, multi-disciplinary scientific literature indicating that sex differences are powerfully influenced by biological factors…”8

The scientific literature to which Peterson refers includes an immense literature in the general field of evolutionary biology, and its component sciences of population genetics, phylogenetics, archaeology, anthropology, sociobiology, paleontology, behavior genetics, developmental biology, molecular biology and others.  However, biology of the sexes has had limited application to the study of the military and war.  I cannot cover the topic thoroughly in this article. A thorough treatment would take volumes, as the reader will see from the extensive references I provide, and the extensive references within them.  The objective of this article is to present insights from biology and an introduction to the scientific literature for military leaders that will stimulate discussion on the issue of integration of the sexes in the U.S. military.


It is important to recognize that there are biological reasons for the historical pattern of men fighting other men in war, to the exclusion of women. There have been excellent empirical assessments of physical performance of men and women in combat-related tasks7 indicating that in general men are more qualified for combat than women.  Also, previous reviews1 of this issue found that pregnancy, menses, and sexual attraction between men and women can affect the dynamics and cohesiveness of a military unit, potentially resulting in reduced effectiveness and increased casualties.

These observations of functional differences indicate that men are more suited to war than women. Science also suggests that the historical pattern of men fighting other men in war is rooted in our evolutionary history.9 The development of the fields of evolutionary medicine and evolutionary psychology10 indicate the potential utility of such a perspective in the study of war.  An example is the assessment of war and natural selection by Colonel P. B. Olsen (USA Ret.) in the “Nature of War Theory".11 Despite our complex civilization, humans retain the genetic legacy of our primitive ancestors that were subject to natural selection (commonly known as survival of the fittest) in hunting and warfare. This topic has been studied by academics over the last 159 years since Darwin’s Origin of Species, but our modern understanding of evolutionary biology has not been integrated into modern military theory and practice.  Colonel Olsen describes the topic succinctly:

“Today’s advances in evolutionary biology are unifying competing theories of natural selection and serve as a timely call for a similar unification of competing theories of war.”11

Assessing natural selection is not easy, especially for humans with whom experimentation is difficult or unethical.  However, inferences can be made from archaeology, anthropology studies of existing hunter-gatherer peoples, and comparative studies of other animals. For example, there is an interesting literature on “war” in animals, particularly chimpanzees, in which inter-group conflicts are violent, fatal, and primarily conducted by males.12

The most fundamental difference among people is sex.  This is not surprising considering that sexual reproduction originated more than 1.2 billion years ago.13 Differences in the anatomy of men and women are obvious, including muscle and fat distribution, bone structure, sex organs, and secondary sexual characteristics (e.g., female breasts, male beards and body hair, voice pitch). There are also differences in physiology and behavior for which the reader should consult the medical, forensic science, and consumer marketing literature.14

Humans have 22 pairs of chromosomes shared by the sexes (called autosomes), and one pair of sex chromosomes (X and Y chromosomes). Men and women have X chromosomes (women have two and men have one) and men have one Y chromosome.15Each parent contributes one sex chromosome to an offspring, the mother an X and the father an X or a Y (i.e., the father’s contribution determines the sex of the offspring). A small segment of DNA on the Y chromosome contains a gene named Testes Determining Factor that initiates the development of testes in men, with the ensuing production of hormones (e.g., testosterone) that result in the development of a male.16

Another basic difference of men and women is the production of eggs and sperm.  An average man will produce approximately 525 billion sperm cells over a lifetime, and an adult man can release between 40 million and 1.2 billion billion sperm cells in one ejaculation. Women, in contrast, are born with an average of 2 million egg follicles, the structures within ovaries from which eggs develop.  A majority of those follicles do not become viable eggs, and only about 450 will ever become mature eggs released for possible fertilization.17

Comparison with Animals

Sexual reproduction has been studied extensively in animals, which should not be directly compared with humans without qualification but provides insights to male-female differences.  There are many aspects of sexual reproduction that differ between male and female animals including:

  • Sexual selection, named by Darwin18, is a form of natural selection for genetically-based traits that enhance an individual’s ability to acquire mates.  Animals with traits that are attractive to the opposite sex, or that improve their ability to compete for mates, are more likely to breed and pass on these traits to offspring.  Sexual selection has been studied in humans and can explain some of the differences of men and women.9
  • Sexual dimorphism, in which the sexes have different anatomy, can result from sexual selection and includes secondary sexual characteristics (in addition to the actual sex organs) which develop at puberty or in the breeding season. (e.g., antlers in male deer, nuptial plumage in birds).
  • Mate Choice is related to sexual selection and sexual dimorphism. Animals exercise discretion in choosing mates, and females exhibit more discretion in mate choice than males in many animals, including humans. This is thought tobe related to the larger female investment of time and energy in gestation and lactation.19
  • Sexual segregation is a pattern in which males and females occupy different areas with different ecology in times of the year other than the mating season and can enhance population fitness.20  

Breeding structures in animals include monogamy (mating between one male and one female) and polygamy (individuals having more than one mate in a breeding season).  There are two types of polygamy: polygyny, when a male has more than one female mate; and polyandry, when a female has more than one male mate.  Polygyny is more common than polyandry in mammals, including humans. In most mammals, including humans, a female can become pregnant by only one male in a breeding season21, followed by long gestation and lactation periods. This makes polyandry disadvantageous for unsuccessful males’ reproductive success.  With polygyny, one male can impregnate many females, enhancing his reproductive success. This can result in sexual selection for different anatomy and behavior of males and females.

Breeding in nature occurs at times of the year that maximize fitness of offspring.  For example, deer breed in the fall and fetuses gestate until born as fawns in the spring when environmental conditions are favorable for their survival.  In such species females come into estrus, males have increases in testosterone, fertility, and aggression, and there is sexual attraction between the sexes only during the breeding season. In contrast, modern humans are in breeding condition with sexual behavior year-round.22  

Additional insights can be gained from livestock and wildlife management, in which the sex ratio of a population is important.  Because one male can breed many females, management may be tailored to have more females than males in the population.  This is why many hunting seasons are for males only (e.g., buck deer) to allow maximum harvest while maintaining maximum reproduction and numbers of animals in a population. Females (e.g., doe deer) are generally harvested when the goal is to stabilize or decrease the population. With cattle, the more cows there are in a herd, the more calves can be produced.  Cattle operations often work with ratios of one bull per 20 or more cows using artificial insemination or natural breeding. 

The point is that females are of more reproductive value to animal populations than are males. The same is true for human populations.  Humans, before modern society with its abundant food and medicine, had high infant mortality and short life spans. There was strong selection for traits (biological and cultural) that contributed to the production and survival of children to reproductive age.  In our ancestral hunter-gatherer societies, men were the primary hunters and warriors and women gathered food and bore, nursed, and raised children. Such sexual division of labor was efficient and conferred fitness to the group. These conditions might or might not apply to modern society, but they are part of our biological and cultural heritage.  Readers interested in this topic will find a large literature in the fields of evolutionary psychology, sociobiology, and anthropology.9, 10  


There are two primary points the reader should take from this paper. First, science indicates that the historical pattern of men fighting men is rooted in basic differences of the sexes that developed as evolutionary adaptations that enhanced fitness in nature. Second, there is a vast scientific literature on the differences between the sexes and why such differences exist. This scientific knowledge can contribute to an understanding of the historical pattern of men fighting men in war and can inform decisions regarding policy on the roles of the sexes in the U.S. military.

Clearly, this topic is complex and controversial, but changing a historical pattern by allowing women to fight in war entails risks to the basic structure and fitness of human populations, as well as military effectiveness. The ability of some women to meet the standards of an MOS is irrelevant to the primary issue of the basic biological causes of the historical pattern of men fighting wars. I will defer specific recommendations regarding the policy of integration of the sexes to military professionals.3, 7 However, I recommend that the military officer corps and policy makers become familiar with biological factors relevant to differences of the sexes. My opinion is that the risks associated with full integration of the sexes in the military have not been adequately assessed and should be reconsidered in light of biology.23

End Notes

1 These books are thoroughly-researched assessments of women in the military:

2 Browne, K. 2007. Co-Ed Combat. Sentinel, New York.

3 Gutmann, S. 2000. The Kinder, Gentler, Military. Scribner, New York.

4 Maginnis, R.L. 2013. Deadly Consequences: How Cowards are Pushing Women into Combat. Regnery, Washington, D.C.

5 Mitchell, B. 1989. Weak Link: the Feminization of the American Military. Regnery Gateway, Washington, D.C.

6 Mitchell, B. 1998. Women in the Military: Flirting with disaster. Regnery, Washington, D.C.

7 Van Creveld, M. 2001. Men, Women and War, Cassell & Co., London.

8 Catherine Itzin (Editor) 1992. Pornography: Women, Violence, and Civil Liberties A Radical New View. Oxford University Press, New York. The quote is on page 13. Interestingly, this quote is cited on page 330 of: Grossman, D. 2009. On Killing. Back Bay Books, Little, Brown, and Company, New York.

9 Serrano, L.F. 2014. Why Women Do Not Belong in the U.S. Infantry, Marine infantry isn’t broken, it doesn’t need to be “fixed”.  Marine Corps Gazette Volume 98, Issue 9, pages 36-40, September 2014.

10 Gutmann, S. 2000. The Kinder Gentler Military. Scribner, New York. The quote is from former JAG lawyer Henry Hamilton on page 209.

11 Keegan, J. 1993. A History of Warfare. Vintage Books, Random House Inc., New York.  The quote is on pages 75-76. 

12 Notable exceptions of women fighting in war include Russia in World War 2, Israel, and others, and are described in the books cited in note number1. The novelty of the recent sexual integration of the U.S. military is that it is officially sanctioned as a permanent policy.

13 Balke, L.A. 2017. Gender-Integration in the United States Marine Corps.  A report submitted in partial fulfillment of the requirements for the degree of Master of Military Studies. United States Marine Corps Command and Staff College, Marine Corps University, Quantico, Virginia. 

14 Kamarck, K.N. 2015. Women in Combat: Issues for Congress. December 3, 2015. Congressional Research Service 7-5700 R42075.

15 Marine Corps Operational Test and Evaluation Activity. Ground Combat Element Integrated Task Force Experimental Assessment Plan. Staff Study, 2015.

16 United States Marine Corps Ground Combat Element Integrated Task Force Research ONR Award #N00014-14-1-0021 Final Report August 14, 2015. Neuromuscular Research Laboratory Department of Sports Medicine and Nutrition, University of Pittsburgh.

17 Peterson, J.B. 2018. 12 Rules for Life, An Antidote to Chaos. Random House Canada, Toronto. The quote is on page 311.

18 These topics are described in detail in the literature on sociobiology and anthropology. See:

19 Ehrlich, P.R. 2000. Human Natures. Island Press, Washington, D.C.

20 Geist, V. 1978. Life Strategies, Human Evolution, and Environmental Design: Towards a Biological Theory of Health. Springer-Verlag, New York. 

21 Shaw, R.P. and Y. Wong. 1989. Genetic Seeds of Warfare. Unwin Hyman Inc., Winchester, MA.

22 Stringer, C. 2012.  Lone Survivors: How we came to be the only Humans on Earth.  Times Books, Henry Holt and Company, New York.

23 Vayda, A.P. 1974. Warfare in ecological perspective. Annual Review of Ecology and Systematics 5:183-193.

24 Wilson, E.O. 1975. Sociobiology. Harvard University Press, Cambridge, MA.

25 Wilson, E.O. 2012 . The Social Conquest of Earth. Liveright, New York.

26 Buss, D. 2015. Evolutionary Psychology: The New Science of the Mind, Fifth Edition. Psychology Press, Philadelphia, PA.

27 Nesse, R.M. 2001. How is Darwinian medicine useful? Western Journal of Medicine 174(5):358–360.

28 Templeton, A.R. 2010. Has human evolution stopped? Rambam Maimonides Medical Journal, RMMJ 2010;1(1):e0006. doi:10.5041/RMMJ. 10006.

29 Olsen, P.B. 2011. Natural Selection and Nature of War, Small Wars Journal, 14 November 201l.

30 Olsen, P.B. 2011. U.S. Army War College Strategy Research Project (USAWC, 22March2}Ll).

31 Inter-group fighting of chimpanzees has been compared to human wars.  Most killings by chimps were gang attacks and males were the most frequent attackers and victims. Jane Goodall in a 2014 article stated:  "We used to think it was only humans who waged war, but we find that chimpanzees, like humans, have this rather unpleasant ability to create an in-group and an out-group."

32 Goodall, J. 2010. Through a Window: My Thirty Years with the Chimpanzees of Gombe. Houghton Mifflin Harcourt, Boston and New York.

33 Morris, I. 2014. War! What Is It Good For?: The Role of Conflict and the Progress of Civilisation from Primates to Robots. MacMillan, New York.

34 Balter, M. 2014. Why do chimps kill each other? Science Magazine online news, 17 September 2014.

35 Barras, C. 2014. Only known chimp war reveals how societies splinter. New Scientist online, 7 May 2014.

36 Wilson, M. L. et al. 2012. Ecological and social factors affect the occurrence and outcomes of intergroup encounters in chimpanzees. Animal Behaviour 83(1):277-291.

37 Wilson, E.O. 2012. The Social Conquest of Earth. Liveright, New York. Wilson describes the chimp wars on page 73. 

38 Zimmer, C. et al. 2009. On the Origin of Sexual Reproduction. Science 324 (5932):1254-1256.

39 Most traits in plants and animals are influenced by both genetics and environment. Claiming only genetic or only environmental (e.g., cultural influences) causes for complex traits without empirical evidence should be posed as hypotheses in need of testing. The rapidly advancing fields of quantitative genetics, epigenetics, genomics, and phenomics rigorously assess genetic and environmental components of variation of plants and animals. However, some traits, including sex, are clearly determined by genetics. Literature on differences in men and women is extensive.

40 Houle, D., D.R. Govindaraju, and S. Omholt. 2010. Phenomics: the next challenge.  Nature Reviews Genetics 11:855-866.

41 Barban, N. et al. 2016. Genome-wide analysis identifies 12 loci influencing human reproductive behavior. Nature Genetics 48(12):1462-1475.

42 Carlson, N.R. 2007 Physiology of behavior.  Pearson Education Inc., New York.

43 Wadman, M. 2016. Pregnancy resculpts women’s brains for at least 2 years. Science Magazine online news, 19 December 2016.

44 Franzen, H. 2001. Enzyme Lack Lowers Women's Alcohol Tolerance. Scientific American, 16 April 2001.

45 Hanson, D. 2011. Women's Response to Alcohol Suggests Need for Gender-Specific Treatment Programs. Scientific American, 16 December 2011.

46 Barletta, M. 2006. Marketing to women.  Dearborn Trade Publishing, Chicago.

47 Chromosomes are structures in cells containing DNA.  There are chromosomal aberrations that result in deviations of the XX female and XY male patterns (e.g., XO females with only one X chromosome, or XYY males with two Y chromosomes).  These chromosome aberrations are relatively rare.

48 Weingarten, C.N. and S.E. Jefferson. 2009. Sex Chromosomes: Genetics, Abnormalities, and Disorders. Nova Science Publishers, Hauppauge, New York.

49 Berta P. et al. 1990. Genetic evidence equating SRY and the testis-determining factor. Nature 348(6300): 448–50. 

50 Walters, P.D., M.C. Wallis, and J.A. Marshall Graves. 2007. Mammalian sex--Origin and evolution of the Y chromosome and SRY. Seminars in Cell and Developmental Biology  18(3):389-400.

51 A recent study suggests sex development includes other factors in addition to SRY/TDF.

52 Zhao, F. et al. 2017. Elimination of the male reproductive tract in the female embryo is promoted by COUP-TFII in mice. Science 357.6352:717-720.

53 Olson, E.R. 2013. Why are 250 million sperm cells released during sex?  LiveScience online, 24 January 2013.

54 Darwin, C. 1871. The Descent of Man and Selection in Relation to Sex. Prometheus Books, New York, 1998. Originally published in London in 1871.

55 Geist, V. 1978. Life Strategies, Human Evolution, and Environmental Design: Towards a Biological Theory of Health. Springer-Verlag, New York. 

56 Frances, A.J. 2013. The power of sexual selection. Psychology Today online, 15 February 2013.

57 Jones A.G. and N.L Ratterman. 2009. Mate choice and sexual selection: What have we learned since Darwin? Proceedings of the National Academy of Sciences 106 Supplement 1:10001–10008.

58 Geist, V. and R.G. Petocz. 1977. Bighorn sheep in winter: do rams maximize reproductive fitness by spatial and habitat segregation from ewes? Canadian Journal of Zoology 55(11):1802-1810.

59 Clutton-Brock, T.H. et al. 1982.  Red Deer: Behavior and Ecology of Two Sexes.  Edinburgh University Press, The University of Chicago Press, Chicago.

60 Multiple paternity, in which there is more than one father of multiple offspring in one litter, occurs in dogs and other species, and rarely in humans.

61 Worland, J. 2015. The Science of How Women Can Have Twins With 2 Different Fathers. Time Health online.

62 Women have year-round estrus cycles, each averaging 28 days, which are influenced by the 27-29 day lunar cycle.

63 Zimecki, M. 2006. The lunar cycle: effects on human and animal behavior and physiology. Postepy Higieny I Medycyny Doswiadczalnej (Advances in Hygiene and Experimental Medicine, Polish) 60:1-7.

64 Regarding the risk of reducing the military’s effectiveness with a new policy of sexual integration, consider the potential relevance of the medical Hippocratic Oath: “First, do no harm”.  The so-called “precautionary principle”, frequently invoked by environmentalists, also applies.  The precautionary principle states that if an action or policy has a suspected risk of causing harm, in the absence of scientific consensus the burden of proof that it is not harmful falls on those taking that action.  These axioms regarding the important topics of medicine and the environment could be applied to sexual integration of the military and indicate that sexual integration is unwise if there is a risk of negatively impacting the military’s effectiveness.

About the Author(s)

Matthew A. Cronin received a Ph.D. in Biology from Yale University in 1989, an M.S. in Biology from Montana State University in 1986, and a B.S. in Forest Biology from State University New York, College of Environmental Science and Forestry in 1976.  Dr. Cronin was a U.S. Coast Guard officer from 1981 to 1984.  He has worked in the government, university, and private sectors in wildlife and livestock ecology, genetics, and evolution.