Human Sexuality Part 2: Is Homosexuality Genetic?


Genesis of Homosexuality

How does a person become gay? Is it genetic? Is it a learned behavior? Or perhaps it is a mixture of the two? In order to solve this puzzle, let us look at five major studies performed by non-Christian, secular scientists on the biological origin of sexual orientation. Therefore, bias encountered in the data is decidedly devoid of any, and all Christian bias.

Study 1:
Simon LeVay, (1991), “A Difference in Hypothalamic Structure Between Heterosexual and Homosexual men”, Science, vol. 253, pages 1034-7

In this study, neurobiologist Simon LeVay studied 41 cadavers: 6 heterosexual females, 16 heterosexual males and 19 homosexual males.

coloured portion is hypothalamus

Apparently, all 19 homosexual cadavers were the victims of the AIDS virus. Simon stated the the INAH3 portion of the brain’s hypothalamus (responsible for controlling sexual appetite, anger, body temperature and sleep) was on average 2 to 3 times larger in heterosexual males than in homosexual men.


Evaluating the data:

All 19 homosexual cadavres had AIDS. Only 6 of the 16 heterosexual men died of the disease. Please note that human testes tend to shrink in late term AIDS victims. Since gonadal (testes) hormones are known to regulate the size of several hypothalamic nuclei in animals, it may be that the change in size of testes causes hypothalamus shrinking.  Therefore the disease effects of AIDS cannot be excluded as a cause for the differences in the brain tissues of LeVay’s cadavers. In other words, abnormal hormone levels in AIDS patients could be to blame for the differences found in the hypothalamus, NOT sexual orientation. It is also important to note what LeVay himself wrote concerning his famous 1991 study:

“It’s important to stress what I didn’t find. I did not prove that homosexuality is genetic, or find a genetic cause for being gay. I didn’t show that gay men are born that way, the most common mistake people make in interpreting my work. Nor did I locate a gay center in the brain.”

Furthermore researchers who reviewed LeVay’s work have questioned his ability to determine which of his cadavers were the homosexual men and whether or not drugs use accounted for the observed changes.[1] Of more minor importance is that LeVay is an active homosexual who founded the Institute of Gay and Lesbian Education, which is significant seeing that the ambiguity of his results were strong enough to overcome even his own obvious bias.

 LeVay (1993) also pointed out that in rats, a particular hypothalamic nuclei (in this case, the medial pre-optic nucleus) is highly susceptible to levels of certain hormones in the blood.

Medial Pre Optic Area in rats
Medial Pre Optic Area in rats

He states that during a critical period of development lasting from a few days before birth to a few days afterwards, altered hormonal levels affect the size of the pre-optic nucleus. Before or after this period, changing hormonal levels has absolutely no effect on the nucleus size.

This indicates that hormonal factors may have an impact in the size, shape and development of brain tissue. This would include nuclei (in this case, in the hypothalamus) that have effects on sexual behavior. Such evidence does create a potential for natural factors (mother’s drug use, diet, etc…) to have impact on brain structures that influence sexual appetite.

 Take note though, the tissues affected by the above-mentioned hormones controls several things, not simply sexual behavior. Sleep, hunger and emotions are just some of the other factors that are influenced NOT controlled by nervous tissue. And before we concede that these changes would inevitably lead to the homosexual lifestyle we need much more evidence than an isolated sample of tissue size changes in rats. We need behavioral observations that demonstrate homosexual effects by specific hormonal changes in rats and then in humans. This has not been shown. Furthermore, this example is not genetic but physiological. In other words, the tissues were altered after conception, not by conception. Do not forget, however that even if homosexuality were found to have biological triggers, the negative effects of this lifestyle would still benefit from rehabilitation.

 Also of interest is a study of male rams that displayed almost purely homosexual preferences. They were found to have hypothalamus sizes and chemical activities equivalent to their female counterparts and different from their heterosexual male peers[2]. If such studies could be reproduced en masse, it would make a convincing debate for the biological genesis of homosexual behavior. However, there are serious problems with using the ram study because within the ram population, fierce combat ensues to establish a small class of dominant males. The “losers” then form a fringe group in which “situational homosexuality” occurs much like human male prison populations.[3] When dominant rams are killed off by hunters, battle or natural means, the fringe group rams once again jostle violently for heterosexual access as the new dominant rams. This renders their formerly homosexual behaviour temporal and only as a means of dominating other frustrated bachelor rams to form a hierarchy within that subpopulation.[4] It must also be noted that the prolonged behaviour of suppressed rams may itself cause changes in brain tissue[5] [6] and therefore the ram study loses its potency.


Study 2:
Laura S. Allen and Roger A. Gorski
, (1992), “Sexual orientation and the size of the anterior commissure in the human brain”, Proceedings of the National Academy of Sciences of the U.S.A., vol. 89, pages 7911-7202

 After examining 200 autopsied brains, Allen and Gorski found that a bundle of neurons called the Anterior Commissure (AC), was larger in women and gay men than in heterosexual men. There is no known sexual regulation function of the Anterior Commissure, yet the differences may have cognitive impacts that could conceivably affect sexual desires and perceptions.


Evaluating the data:

Of first importance is the fact that the AC has no known relationship to sexual behaviour. Second, some of the women’s AC’s were up to 3 times larger than those of other women subjects within the same patient pool. It was only when averaging the findings that a general trend was found showing a larger AC for women than heterosexual men. If a small variation in the number, size and volume of a small packet of neurons can make the radical transformation of a straight man into a gay man, what occurs when there is an even larger variation in this same packet between two women? If this brain matter really had such a powerful and highly specific effect, should we not then see an extremely profound spectrum of sexual (and possibly other) behavior between women who posses AC’s 3 times the size of their female peers? Yet this has not been observed. We therefore know that sexual behavior variations cannot be attributed to this small portion of neurons that range so wildly in size and number without any sexual effects in women.

Also, 27 of the 30 homosexual men had AC’s whose size was within the range of those of the 30 straight men. Thereby nullifying the research’s explanatory power[7]. Furthermore, an under-reported 1988 study found precisely the opposite: men had larger AC’s than women[8].

Also of importance is that AIDS may have played a part in tissue size and weight as the disease is known to affect brain matter.

Study 3:
Bailey, J.M., Pillard, R.C., (1991) Archives of General Psychiatry,  48:1089-1096.

Researchers Bailey and Pillard interviewed 56 homosexual men who had identical twins, 54 homosexuals who had fraternal twins and 57 homosexuals who had adoptive brothers. Of this sample, 52% of identical twins shared their homosexuality with their twin brothers, compared to only 22% of fraternal twins and 11% of adoptive brothers. baby-twinsThe conclusion was that genetics had an impact on the likelihood of someone’s sexual orientation.

Evaluating the data:

The high predilection for homosexuality was at least in part due to the fact that the twins involved (both fraternal and identical) grew up together. Family and life experience most likely influenced the homosexual development. In the end, there is no way to distinguish what percentage their environment or what percentage their genetics played in developing their homosexual behaviour. Note the fact that 11% of adoptive brothers were gay. This is higher than the national average, providing evidence that the familial environment was perhaps a strong influence in homes which ended up producing gay individuals.

However, the strength of this research is that identical twins (whom share the EXACT same DNA) are two and a half times more likely to be gay then fraternal twins. Yet, if genetics were the sole cause of homosexuality, why would 48% of men who are genetically identical to gay men not be gay? The answer is that clearly, homosexuality is not 100% genetically fixed. But perhaps it is significantly influenced by genetics. Environmental circumstances can exact very strong pressure, but they do not seal, or inescapably destine a person the way genetics do (and even genetics are open to epigenetic phenomena). Which would explain how two identical men, who are essentially clones of each other can end up reacting differently (free choice) while truly living the exact same experience (both in their family life and in their genetics). In the end, this study proves beyond a shadow of a doubt that homosexuality is at best, 52% genetic. This is a generous estimate seeing as most (if not all) pro-gay researchers believe environment is a more important contributor to sexual orientation over and above genetics.


More importantly, a 14,000 person identical twin study (as opposed to 56 in this study) in Australia found the number of identical twins who shared their homosexuality was 39%.[10] Therefore, the actual genetic impact on sexual orientation is still an unknown and inexact figure. On a final note, familial causes of homosexuality are obviously not responsible for 100% of what renders and individual gay. If it were, then closer to 100% of brothers (identical, fraternal or simply brothers) would be gay. The answer must therefore lie outside both nature and family nurture (see next section on nurture).


Study 4:
D.H. Hamer et al, (1993) Science, 261:321-7

In studying the genetics of gay men who had gay brothers, Hamer found that such people had an excess of gay relatives on the mother’s, but not the father’s, side of the family. Reasoning that this might indicate that sexual orientation might be linked to the sex determining X chromosome on the 23rd pair of chromosomes (inherited only from the mother) Hamer conducted a linkage analysis to determine if any DNA markers on the X sex chromosome would be inherited along with the supposed gene for sexual orientation.

Evaluating the data:

In 33 of 40 pairs of gay brothers he found such a marker near the tip of the long arm of the X chromosome, in a location called Xq28, an area that contains several hundred genes. Hamer recently replicated this finding in a new set of families (S. Hu et al., Nature Genetics, 11:248, 1995).


gay-geneEvaluating the data:

The first objection is that 7 of the 40 specimens did not show this marker. Therefore  homosexuality does not require these genes and is not purely genetic (even if this marker were strongly associated with sexual orientation, we have not yet even understood its function, nevertheless proven how it could cause as radical an effect as homosexuality).

More notable is that a study by George Ebers contradicts the X linkage theory. A group led by George C. Ebers, a professor of neurology at the University of Western Ontario in Canada, has failed to replicate Hamer’s findings in a study that is currently being submitted for publication. “We’ve been collecting families that have more than one gay person for five years, and we’ve gone through something like 400 pedigrees,” explains Ebers. “In those [families] there is really no support for the idea that male homosexuality is X-linked. The DNA tests that were done didn’t even support Dean’s idea a bit. There wasn’t even a trend toward increased sharing of haplotypes down there at Xq28.”

Recently, Hamer’s research has been cast into doubt not only by arguments over his interpretation of the data, but also by allegations of scientific misconduct. According to a front-page article by John Crewdson in the Chicago Tribune (June 25, 1995), an anonymous former member of Hamer’s lab has alleged that Hamer engaged in selective presentation of data in his 1993 Science paper. Crewdson reported that an investigation has been launched by the Office of Research Integrity (ORI) of the United States Department of Health and Human Services.

Study 5:
Meyer-Bahlburg, et al (1995). Prenatal estrogens and the development of homosexual orientation. Developmental Psychology, 31, 12-21.

Women who were exposed to a synthetic form of estrogen known as DES while in the womb, were found to fantasize about same sex pairing more often than women who were not DES-exposed.


Evaluating the data:

Questionnaires about thought-life are less than scientific. Furthermore, the women who were found to be gay were mostly bisexual. And many were bisexual in thought not deed. Therefore they did not constitute a positive, strictly gay tendency (in actual lifestyle) as a result of DES exposure. The researchers themselves agreed that the study was not strongly conclusive.

Exposure to atrazine (a common weedkiller) increased hermaphroditic offspring in a group of male Xenopus frogs[11].

It is important to note that the condition of hermaphroditism does not inescapably make its human victims bisexual or even gay. Hermaphrodites are usually predominantly male or female looking (in facial and body features) and vary in sexual orientation from purely heterosexual to strictly homosexual. However, these studies do bring up interesting observations that could point to a partly biological influence in sexual behaviors. If exposure to substances such as atrazine were strongly linked to homosexuality, than geographic areas where such chemicals were used would be shown to produce large numbers of homosexual men and women. This has not been shown.

Concluding thoughts on the Argument for Natural Etiology of Homosexual Behaviour

Well, the above evidence is amongst the most widely reported by persons and groups that are in favor of legitimizing the gay lifestyle. It is amongst the strongest proof (if not the strongest) of a genetic or biological cause of homosexuality. Yet at its very best, the evidence is inconclusive. None of the above quoted studies constitute a straightforward case in favor of biology being the sole cause of homosexuality. There may be, at best, a hormonal influence on sexual behavior. But built-in gay programming is still a hypothesis on which the jury is out and the evidence not in. Furthermore, the sheer genetic propensity for an action in no way renders it desirable, healthy or free from the rigors of rehabilitation. We would not excuse someone who is “naturally” angry if they hit their spouse or killed someone.

Neil Whitehead, PhD adds some perspective on biology and behaviour:

“For perspective, it is valuable to compare genetic contributions to homosexuality with the question – is a girl genetically compelled to become pregnant at 15? Her genes might give her physical characteristics that make her attractive to boys [and her attracted to boys]- but whether she gets pregnant will depend greatly on whether her community is Amish or urban, conservative or liberal, whether they use contraceptives, and whether the parents are away for the evening….. …..In this, homosexuality proves to be no different from such unrelated behaviors as violence, being extroverted, or getting divorced. All may be influenced by genes, but not overwhelmingly determined by them.”

Suicide hotline workers have attested to the negative effects the gay-gene myth has had on homosexual teens who are on the brink of taking their lives. From the teen’s perspective, if there is no way out of the lifestyle which has made them so miserable, death becomes a welcomed repose[12]. If there is even a chance that homosexuality is behavioral, it would be advantageous to these desperate teens.

Some have cited the occurrence of homosexuality in the animal world as evidence of its “naturalness”. Unfortunately for proponents of this view, we also observe that many things exist in the animal world that we (thankfully) do not imitate. Tomcat mothers will sometimes respond to the shrill voice of their young by killing them. This is due to their inability to voluntarily switch their “kill small prey-instinct” to their “play-instinct”, and end up confusing its young for their dinner. Cannibalism is also epidemic throughout the lower vertebrate and invertebrate world. Instinctual glitches prevent members of the same species from recognizing their peers as non-enemies.[13]

Also, non-human animals have greatly simplified cognition. Therefore, one behavior is often repeated in a myriad of occasions. Dogs bark in the same manner to express different emotions. And many animals use sexual stimuli to express non-sexual emotional urges. An example of this is the Bonobo chimp tribes. Bonobos react to intense emotions such as intimidation and fear by dispelling them through sexual acts. In one study, a foreign object (in this case a cardboard box) was introduced into a cage full of Bonobos. The anxiety caused by the unknown stimuli immediately increased sexual contact by most of the Bonobos present. When a weaker Bonobo attempted to play with the box, a stronger Bonobo would intervene. To prove submission and ease tension, the weak Bonobo mounted the other or engaged in scrotal rubbing. In effect, it is reasonable to conclude that the use of sexual behavior is to substitute for higher order functions such as “smooth talking” or other strictly human tactics these monkeys are incapable of performing.[14] As a result, a belief in genuine animal homosexuality is unwarranted by any evidence presented thus far.

So the question then becomes: what causes homosexuality? If biology cannot account for the full spectrum of homosexual development, non-biological factors are next in line in our investigation. Is there, in fact, proof that a person’s circumstances, environment and up-bringing, not biology, are the single strongest factors in forming a homosexual identity and lifestyle? Stronger, indeed, than any known genetic cause?

Please see Part 3 in our Human Sexuality series to see the discussion on behavioural and environmental influences for homosexual lifestyles.


[1] P. Billings and J. Beckwith, “Born Gay?” Technology Review, July 1993, p. 60. Paul Billings, M.D., is the former chief of the Division of Genetic Medicine at California Pacific Medical Center in Palo Alto, California and is now head of Internal Medicine at the Palo Alto Veteran’s Administration Hospital; Jonathan Beckwith, M.D., is American Cancer Society Research Professor in the Department of Microbiology and Molecular Genetics at Harvard Medical School.

[2] Charles E. Roselli, John A. Resko, Fred Stormshak, Hormonal Influences on Sexual Partner Preference in Rams, Archives of Sexual Behavior, Volume 31, Issue 1, February  2002, Pages 43 – 49

[3] Fisher,A; Matthews,L (Date not cited).

[4] Shackleton,DM (1991): Social maturation and productivity in bighorn sheep: are young males incompetent? Appl. Anim. Behav. Sci. 29, 173-184.

[5] G. Gabbard, “Psychodynamic Psychiatry in the ‘Decade of the Brain,'” American Journal of Psychiatry, 149, no. 8 (1992), pp. 991-98.. This study found that infant monkeys who are repeatedly and traumatically separated from their mothers suffer more or less permanent alterations in both blood chemistry and brain function.

[6] R. Post, “Transduction of Psychosocial Stress into the Neurobiology of Recurrent Affective Disorder,” American Journal of Psychiatry 148, no. 8 (1992), pp. 999-1010.


[8] S. Demeter et al., “Morphometric analysis of the human corpus callosum and anterior commissure,” Human Neurobiology 6 1988), pp. 219-26.

 [9] Bailey, JM; Dunne,MP; Martin,NG (2000): Genetic and Environmental influences on sexual orientation and its correlates in an Australian twin sample. J. Pers. Social Psychology 78, 524-536.

[10] Bailey, JM; Dunne,MP; Martin,NG (2000): Genetic and Environmental influences on sexual orientation and its correlates in an Australian twin sample. J. Pers. Social Psychology 78, 524-536.


[12] Judith Halberstam and Ira Livingston, eds., “Posthuman Bodies,” The Advocate, July 1997, pp. 135-147


[14] Frans B. M. de Waal, “Bonobo Sex and Society,” Scientific American, Mar. 1995, pp. 82-88,

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