Women hold up half the sky…but what about the other genders?

On August 19, 2009, South African runner Caster Semenya won the woman’s 800 meters at the World Championships.  She also set the world record.  This in and of itself is not particularly interesting.  The challenge to her victory is.  Caster Semenya’s physical appearance is not stereotypically female.  This has led some to claim that she is really not a female and a formal protest has been filed.  Many South Africans are outraged at this challenge, identifying it as resulting from “white” racism.  After the race it was revealed that the International Association of Athletics Federations (IAAF) had asked her to take a gender test three weeks before the world championships.  These tests include genetic, gynecological, endocrine, and psychological measures.  Typically when we talk about sex we are referring to something that is biological and when we discuss gender we are referring to how individuls perceive themselves and our perceived by society. Thus gender is more socially constructed than biological. This explains why all four of these tests are conducted to determine eligibility for female sports.  Caster Semenya was allowed to compete in the world championships because the IAAF had no evidence that could be used to stop her from competing.  At the time of this writing these results have not been released. 

There are a number of issues that can explain this challenge.  Some of it may be European racism, but the primary problem here is the notion that the human species contains only two genders with distinct body types and behaviors.  This is a mythological view, but one not held by all cultures.  Plato for example thought that there were three genders, male, female, and hermaphrodite.  Hermaphrodites are defined as individuals that have various mixtures of gonadal structures and secondary sexual characteristics.  The word hermaphrodite comes from the Greek gods Hermes (son of Zeus, messenger) and Aphrodite (Goddess of sexual love and beauty.) Even early Biblical scholars thought Adam was originally a hermaphrodite and that he later became male after the fall2.  Clearly this notion of humanity did not survive later Biblical interpretations in which there are only supposed to be two sexes/genders and their sexual behavior is rigidly proscribed.  The destruction of Sodom and Gomorrah in Genesis results in part from the sexual wickedness of the inhabitants.  The word sodomy (referring to anal intercourse) derives from reference to Sodom.

Fruitfulness (in more ways than one)

One of the ways that scientific hypotheses are judged is their fruitfulness.  That is how many predictions or avenues of investigation are engendered if the hypothesis or theory is true.  Evolutionary biology is particularly fruitful in explaining sex, gender, and mating/sexual behavior and this is one of the weakest aspects of Biblical literalist special creationist ideology.  For example, evolutionary theory requires that reproduction be at the core of the biology of all organisms.  Natural selection is best defined as “differential reproductive success.”  Humans, as complicated as they may be, are not immune to this dictum. 

For example, Biblical literalist interpretations of the human birth process claim that women are cursed to deliver their young in pain because of Eve’s sin (Genesis 3:16.) The passage actually says that God would greatly increase her pain in childbearing, possibly indicating that even without this event there might have been some pain.  Evolutionary biology provides another explanation.  Human females experience more pain in childbirth compared to other closely related primates (Gorilla, Chimpanzee, and Orangutan) because of the size of their offspring’s head compared to the diameter of their pelvis bone3. The increase in human head size and the relative narrowing of the human pelvis result from the mosaic history of human evolution.  On one hand, increasing head size of offspring (and delayed brain development) and on the other upright posture and running are concomitant with the changed pelvis diameter.  These adaptations did not come without cost however, human childbirth is risky and until the age of modern medicine, childbirth was still one the largest sources of death to young women.  This had impacts on the social structure of our species.  Social networks of women may have been crucial in reducing the risk of death during childbirth4.

How Many Genders/Sexes?

During the writing of the Origin of Species, Darwin was particularly impressed by the sexual characteristics/behavior of barnacles.  The more he learned about them, the more he was convinced that his species theory was correct.  He was astounded to find many species in which the males had become completely dependent upon the females (and were only found attached to the female body.)  In these species the entire male anatomy was reduced to simply being a “giant penis5.”  If this occurrence was limited to barnacles than this observation is not particularly important; however there are many species in which parasitic males are the norm, such as the adult of the Schistosoma spp.  In this species the adult male worm spends his entire life in the gynocophoric grove of the female.  His sole function is the production of sperm.

Taking this logic further we can ask: why do males exist at all?  After all if we define female as the capacity to reproduce, then all original life was female.  This question has dominated research in evolutionary biology.  The take home lesson of this research program is that: hermaphroditic species are extremely common in nature, separate sexes evolve under specific conditions, and that individuals can change their sex under specific conditions6.  Given that humans evolved from animal lineages with these features, why would anyone be surprised that human gender/sex determination is both complex and fluid?

Instead of viewing human sex/gender as a dichotomy (male or female) it is more profitable to think of this as continuum from individuals who look and behave in a stereotypically female to those that behave in stereotypically male ways.  To understand this we must understand that sex/gender is influenced by events that occur at the level of genes, organs (specifically gonads and brains), and organ systems (body.)  The expression of genes is influenced by chance and environmental effects and these can have important impacts on development.  In our species, sex determination occurs via the 23rd set of chromosomes, designated X and Y.  The Y chromosome has very few genes, but it carries testes determining factor (TDF).  Generally, individuals who are genetically XY develop testes and a penis.  However, all human embryos start out with identical structures (or are female.)  Therefore it is possible for individuals who are genetically XY to complete development in ways that produce external genitalia that are female.  Fausto-Sterling 2000 summarizes the various common ways that embryos can undergo nondimorphic sexual development.

Cause Frequency/100 Live Births
Non-XX or Non-XY (except Turner’s or Klinefelter’s) 0.06390
Turner Syndrome 0.03690
Klinefelter Syndrome 0.09220
Androgen Insensitivity Syndrome 0.00760
Partial Androgen Insensitivity Syndrome 0.00076
Classic Congenital Adrenal Hyperplasia (CAH) 0.00779
Late onset CAH 1.50000
Vaginal agenesis 0.01690
True hermaphrodites 0.00120
Idiopathic 0.00009
Total 1.72800


These data suggest that around 2% of live births are intersex individuals. These frequencies can differ by population due to genetic drift and it has been shown that the frequency of hermaphrodites in the indigenous S. African population is higher than the value reported in this table7.

When it comes to the sexual behavior aspect of gender, particularly sexual orientation, knowing the frequency of intersex individuals is not particularly informative.  This is because what we think of as gender is influenced by a complicated interaction of genetic, cellular, physiological, and organ interactions.  For example, there are a number of regions in the human brain that influence sexual orientation.  For example three areas around the hypothalamus are sexually dimorphic (SDN-POA, BSTc, and VIP-SCN. ) In non-transgendered heterosexual males, BSTc is 150% the size of (2.5 mm3 versus 1.75 mm3) and has about 33,000 neurons compared to 19,000 neurons in non-transgendered heterosexual females.  VIP-SCN size for homosexual men is bigger (1.7 times and 2.1 times the neurons) than that of heterosexual men or of heterosexual women8.  From this we conclude that gay male brains are different from those of straight men, but not equivalent to those of female brains in this region.   At another region surrounding the hypothalamus, INAH3; heterosexual male size = 0.1 mm3; heterosexual women and gay males = 0.05 mm3.  Consider the combination of brains that result from these 4 regions = 24 power = 16; and if we consider brain and body type combinations (male/female organs and brain types) = 25= 32.  Given that intersex individuals exist with ovo-testes than we could increase the number of combinations to 3 x 16 = 48. In this scenario, those we think of as “women” would be holding up less than half the sky, since 47 other genders exist (even if many of the others are less frequent than “male and female”.)

Using positron emission tomography of sexual attraction a study of 25 heterosexuals of each sex and 20 homosexual also of each sex found that cerebral and amygdale asymmetries of the homosexual group reflected those of the opposite sex in heterosexuals9.   This again suggests that gays may inherit brain structures that more closely resemble the opposite sex, but how this might lead to attraction towards the same sex or whether it is inevitable ‘from the womb’ or still subject to social selection remains to be established.   Thus it is premature to conclude we are born with a rigid sexual orientation.  For example, identical twins which share the same genes display opposite sexual orientation around 50% of the time when one of the twins is gay.

 Genetic contributions to gay sexual orientation

Hamer and Pattatucci 1993 found that gay men tended to cluster in families10.  This study recruited homosexual men (and their families) from HIV clinics in Washington, D.C. area.  The brother of a gay man had a 13% chance of being gay, while the brother of a heterosexual man had only about a 2% chance. This result indicates that genes are involved in determining gay sexual orientation.  Subjects rated their sexuality on the Kinsey scale (0 for exclusive heterosexuality to 6 for exclusive homosexuality.)  These ratings were revealing.  For example, only 70% of the individuals who self-described as heterosexual claimed that they never had experienced homosexual activity to orgasm.  Which means that 30% of the self-described heterosexual men admitted to having some homosexual experiences in their past.  Similarly > 40% of the men who described themselves as homosexual admitted to having heterosexual activity to orgasm.  This again underscores the complexity of human gender.

The Hamer study localized the genetic contribution in this population to five genetic loci on the tip of the X chromosome that where statistically significantly higher in gay males.   This raises an interesting evolutionary question.  How could these genes circulate at such high frequency if homosexual males have a lower reproductive output than heterosexual males?  The answer is that no gene could increase in frequency by natural selection if its net impact is to lower the reproductive output of those that carry it.  Two hypotheses have been advanced to explain the existence of genes that increase the likelihood that its carrier will be gay.   Imagine that the role of these specific genetic markers is to signal that the carrier should be attracted to men.

If the five markers are inherited together on the X chromosome, they can be symbolized as one locus = s.  This results in five phenotypes:

•XX =    “normal” woman

•XsX =    normal woman (possibly more highly attracted to men.)

•XsXs= “super” woman (most highly attracted to men.)

•XY =     heterosexual man

•XsY=     homosexual man

If we assume that only heterosexual men and heterosexual women are having children, then the predisposition to be a gay male can be produced by heterosexual males and their mating with any woman that carries the s allele.  If the women carrying the s alleles are more likely to mate than women without, that would explain the persistence of the “gay gene.”  Obviously this explanation only applies to gay maleness but not any genetic contribution to lesbianism.  There is no reason why a mechanism of this kind couldn’t be in operation.  Such genes are labeled as “sexually antagonistic.”  A second hypothesis that has been applied to how gay behavior may evolve is through the mechanism of kin-selection and multiple gender families.  Roughgarden describes this in a number of non-human species11.  In short, this operates because an individual can contribute to the reproductive success of a relative, even if they do not reproduce themselves.  If multiple gender families are more successful than dual gender families, than genes contributing to this behavior would spread, even if they resulted in homosexual behavior.

Fruitfulness Explained and its consequences

Genesis 1:20 describes how God blessed the animals and exhorted them to be fruitful and multiply after their kind.  Beyond this poetic language it does not describe specific mechanisms and read literally what it claims is wrong.  Reproduction implies evolution.  For this reason, anything in an organism’s reproductive biology is a great interest to evolutionary biologists.   This is just as true in humans as in any other species.  Applying evolutionary theories to sex and gender tells us a great number of things, including how someone could have a male-looking body, female genitalia, and female gender behavior (or any other combination.)  This reality applied to the world of athletic competition means that the present gender categories need revision.  This reality applied to the world of human society as a whole means that it can no longer rationally support anti-gay bigotry.  Case in point, North Carolina has a “crimes against nature” statute.  As written it is applied mainly to oral and anal sexual acts, generally only applied to homosexuals.  This act is discriminatory and its origin is based in the mythology of dichotomous gender and sex.  It and all remaining discrimination against gay and lesbians should be struck down immediately.  It is time for rational understanding of the human condition to determine our laws.


1. Is she really a he? Women’s 800 meter runner shrugs off gender storm to take gold. http://www.dailymail.co.uk/news/worldnews/article-1207653/Womens-800m-gold-medal-favourite-Caster-Semenya-takes-gender-test-hours-World-Championship-race.html

2. Fausto-Sterling, A, Sexing the Body: Gender Politics and the Construction of Identity, (New York, NY: Basic Books), 2000.

 3. Shultz, A.H., Sex differences in the pelvis of primates, Am. J. Physical Anthropology 7: 401-424.

4. Trevathan, W., Evolutionary obstetrics, in Evolutionary Medicine, Trevathan, W, Smith, E.O, and McKenna, J.J, Eds, (New York, NY: Oxford University Press), 1999.

 5. Desmond, A. and Moore, J. , Darwin: Life of a Tormented Evolutionist, (New York, NY: Norton), 1994.

 6. An excellent discussion of this topic can be found in Rose, M.R. and Mueller, L.D, Evolution and Ecology of the Organism, (Upper Saddle River, NJ: Prentice Hall), 2006.

 7. Danso A.P, Nkrumah F.K, The challenges of ambiguous genitalia. Cent. Afr. J. Med. 38:367–371, 1992; Blackless, M, Charuvastra, A, Derryck, A, Fausto-Sterling, A, Lauzanne, K, and Lee, E, How Sexually Dimorphic Are We? Review and Synthesis, Am.  J. Human Biology 12:151–166, 2000.

 8.  Roughgarden, J, Evolution’s Rainbow: Diversity, Gender, and Sexuality in Nature and People, (Berkeley , CA: University of California Press), 2004.

 9. Savic, I and Lindstrom, P, PET and MRI show differences in cerebral asymmetry and functional connectivity between homo-and heterosexual subjects, PNAS 10.1073/pnas.0801566105Â, 2008.

10. Hamer et al., A linkage between DNA markers on the X chromosome and male sexual orientation, Science261: 321-327, 1993.

 11.  Roughgarden, J, Evolution’s Rainbow: Diversity, Gender, and Sexuality in Nature and People, (Berkeley , CA: University of California Press), 2004; see specifically chapter 6: Multiple Gender Families.

Joseph Graves

About Joseph Graves

Dr. Joseph Graves, Jr. received his Ph.D. in Environmental, Evolutionary and Systematic Biology from Wayne State University in 1988. In 1994 he was elected a Fellow of the Council of the American Association for the Advancement of Science (AAAS.) In April 2002, he received the ASU-West award for Scholarly Research and Creative Activity. His research concerns the evolutionary genetics of postponed aging and biological concepts of race in humans, with over sixty papers and book chapters published, and had appeared in six documentary films and numerous television interviews on these general topics. He has been a Principal Investigator on grants from the National Institutes of Health, National Science Foundation and the Arizona Disease Research Commission. His books on the biology of race are entitled: The Emperor's New Clothes: Biological Theories of Race at the Millennium, Rutgers University Press, 2001, 2005 and The Race Myth: Why We Pretend Race Exists in America, Dutton Press, 2004, 2005. A summary of Dr. Graves’s research career can be found on Wikipedia, and he is also featured in the ABC-CLIO volume on Outstanding African American scientists. In November 2007, he was featured in the CNN Anderson Cooper 360 program on Dr. James Watson. He has served as a member of the external advisory board for the National Human Genome Center at Howard University. In January 2006, he became a member of the “New Genetics and the African Slave Trade” working group of the W.E.B. Du Bois Institute of Harvard University, chaired by professors Henry Louis Gates and Evelyn Hammonds. He is currently serving on the Senior Advisory Board for the National Evolutionary Synthesis Center (NESCent) at Duke University. He has been an active participant in the struggle to protect and improve the teaching of science, particularly evolutionary biology in the public schools. In 2007, he became a member of the inaugural editorial board of Evolution: Education and Outreach, published by Springer-Verlag. He has been a leader in addressing the under representation of minorities in science careers, having directed successful programs in California and Arizona. He currently serves as a member of the board of the Guilford Education Alliance. From 2005 – 2009, he has been a leading force in aiding underserved youth in Greensboro via the YMCA chess program.
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