The Science of Homosexuality


  • Homosexuality is natural, and has been observed in Penguins, Chimpanzees, and other animals; in the wild and in captivity. (Smith, 2009)
  • Genetics plays a role in sexuality, just as it affects all aspects of a person.  Bailey, M. J., & Pillard (1990) showed that identical twins are more likely to both be gay when compared to any other brother (even paternal twins).
  • Homosexuality is determined by factors outside of a person’s control, such as how many older brothers they have (Bogaert, 1998).
  • There is a neurological factor of homosexuality.  When Looking at the third interstitial nucleus of the anterior hypothalamus (INAH 3), homosexual men are more similar to heterosexual women, and homosexual women are hore similar to heterosexual men (Levay, 1991).  



Homosexuality as Influenced by Biology and Society

George Braun



What is it about someone that makes us attracted to them, or even love them?  Can someone choose who they love, or is attraction something that is determined from birth?  That is essentially the big question in the debate over the origins of homosexuality: is sexuality a choice, something that can be altered by environment, or determined by one’s biology since birth?  Although homosexuality has been recognized and written of for millennia, not until recently have scientists taken up the gauntlet to try to explain it, with their focus being on biological and sociological factors that may have an effect on a person’s sexual identity.  Through this paper I will show the research done on the subject, and I hope that the consensus reached by the research will support the notion that people are biologically predisposition towards their sexuality, and that homosexuality is not a choice.  With apologies to the gay, lesbian, bisexual, queer, and transgender community (LGBTQ), for the purpose of writing a clear and more easily read paper, I will use the terms ‘gay’ and ‘homosexual’ to refer to any person who would not identify as straight  In addition, my focus in this paper will be on male homosexuality, as it is more clearly understood and more heavily researched than female homosexuality.


Perhaps one of the simplest biological explanations for differences among individuals is genetics.  As factors such as height, eye and hair color, and even left or right-handedness can be determined by genetics, it seems logical that genetics could also play a role in a person’s sexuality.  One of the earliest studies done to find a connection between genetics and homosexuality was done by Bailey and Pillard.  They compared rates of homosexuality among identical twin brothers, fraternal twin brothers, adopted brothers, and non twin brothers given that one of the brothers in the pair was already known to be gay.  Out of all of the pairs of brothers who responded, 52% of the identical twins, 22% of the fraternal twins, 11% of the adopted brothers, and 9% of the non-twin brothers were both homosexual (Bailey & Pillard, 1990).

Critics of this study point out that the method of sampling for this study is biased since subjects for the study were recruited through homosexual publications instead of a sample taken randomly from the population. They would go on to say that the data would be evidence against sexuality as being determined by genetics (Ankerberg).  One critic, Dr. John F. Ankerberg, points out that “as identical twins have identical genetic makeup, it is much easier to interpret the findings as supporting the nurture rather than nature theory.  If a homosexual orientation is genetic, than 100% of all identical twin brothers should have been homosexual, but only half were.  Therefore, it is easy to conclude that environmental factors, not genes, cause homosexuality” (Ankerberg).  

However Ankerberg is not completely correct, as modern genetics has shown that a simple “if this gene is present than this physical characteristic is present” formula often does not apply, and that most of the time environmental factors have a lot of influence on gene expression.  One such example would be in the case of locusts which begin their adult lives as flightless, hopping insects and then later develop wings and change their behavior if subjected to cramped conditions (Morgan, 2009).  An individual locust does not change its DNA in order to become flighted, and if kept out of the cramped conditions a locust may never gain flight.  Additionally, Ankerberg ignores the reverse of his argument: if environmental factors were the sole cause of sexuality, than surely there would be a higher percentage of identical twin brothers of similar sexuality.  If an individual’s environment was the sole determinate of an individual’s sexuality, and assuming that each twin grew up in at least a very similar environment, in the same household and with similar role models and at the same level in the sibling hierarchy,  then certainly the rate of twins with the same sexuality should certainly be higher than 52%.  In fact one could argue that if environment was the sole determinate, all different pairs of brothers should have more similar percentages, assuming that each pair of brothers grew up in the same household.

But despite criticisms of the study, Bailey and Pillard made a very promising observation.  If the research were to be replicated with less bias and result in similar results, it would make a very strong case for the role of genetics in sexuality.  Even if there is no “gay gene” that definitively makes someone gay or straight, there would at least be an undeniable correlation (or statistical relationship) between genetics and sexuality, which may imply the role of genetics in the determination of sexuality.

The Brain

Another of the earlier studies on sexuality revealed a second set of correlations that may indicate a relationship between genetics and sexuality, but in this case the evidence is more tangible than statistics.  The study, originally done in 1991 by Simon LeVay, looked at the anterior hypothalamus of the brain, a region known for regulating male-typical behavior.  Of the four interstitial nuclei (or bundles of nerve cells) of the anterior hypothalamus, three showed no difference between any of the subjects, weather gay or straight, male or female.  However the third interstitial nucleus of the anterior hypothalamus, or INAH 3, was shown to be half the size in homosexual men and heterosexual women as it was in heterosexual men (Levay, 1991).

This would seem, at first, to be undeniable evidence for a biological cause of homosexuality.  If there is a physical difference in brain structure than obviously behavior would be affected. But LeVay, as well as his critics, made sure to point out that correlation does not imply causation, and that these differences in brain structure could be the result, and not the cause.  As LeVay said regarding this finding, “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. …Since I looked at adult brains, we don’t know if the differences I found were there at birth, or if they appeared later” (Knight, 2006).  This is a very important point, as it has been known for a while that when parts of the brain are used more it can affect the size of those regions.  One good example is a study done on London taxi and bus drivers, which compared the size of drivers’ hippocampus (the region of the brain responsible for memory) with the complexity of the routes the drivers had to memorize.  The results of the study showed strong evidence that taxi drivers (who need to memorize the entire city to get people where they need to go) had a larger hippocampus on average than bus drivers (who only needed to memorize a few set routes) (Maguire, Woolett, & Spiers, 2006).

Robert Knight, author of “Born or Bred?”, was quick to point out that all of the homosexual men used in the study were victims of AIDS.  As a result of the disease and treatment the gay men in the study had lower levels of testosterone than a healthy heterosexual man.  Knight stated that these lowered testosterone levels may have affected the size of the hypothalamus.  However, Knight seems to ignore the fact that 6 of the heterosexual men in the study died of aids as well, which should act as a control for his AIDS hypothesis.  Knight also points out that, of the 19 homosexual men and 16 heterosexual men studied, three homosexual men had nuclei similar in size to heterosexual men, and three heterosexual men had nuclei similar in size to homosexual men and heterosexual women.  While Knight makes a good point here, especially considering the small size of the sample, one may point out that there are outliers in nearly every sample study, and that the presence of outliers does not completely negate the correlations found in the study.

Even if LeVay’s study was so biased as to be deemed unusable, it does imply a correlation that should be looked into more.  Looking at other animals, in this case male rats and monkeys, “other researchers have found that injury to this portion of the brain [the hypothalamus] causes males to lose interest in females while continuing to express sexual vigor by such activities as masturbation” (Angier, 1991).  It would not be a far stretch to assume that if a human’s hypothalamus were similarly damaged or differently developed, it may result in a similar condition in human males, possibly causing a man to look towards a homosexual relationship in the absence of attraction to females.  Other more recent studies have continued LeVay’s ideas using healthy, living subjects and modern brain imaging techniques.  One such study has itself shown that, not only do homosexual men have a hypothalamus more similar to heterosexual women, but homosexual women also have a hypothalamus more similar to heterosexual men (Swaab, 2008).

It has been suggested that testosterone levels in utero causes this difference in brain structure.  To test this, a testosterone inhibitor was used on rats, which subsequently caused them to be ‘bisexual’.  The researchers found that this testosterone inhibiter yielded a higher number of vasopressin neurons than usual in the hypothalamus, suggesting that the difference in hypothalamus size, though it can be affected by sexual activity, can also be affected by hormone levels (Swaab, 2008).

But despite the profound relationship between size of brain structures in adult men and women of differing sexualities, there still lies the question which, if answered, may end the ‘nature versus nurture’ debate once and for all.  That question is: are these differences in brain structure present from a very early age, or even from birth?  Now that the technology is available to view a person’s brain simply by scanning them, such as with an MRI or CAT scan, a study needs to be done in which an infant’s hypothalamus is scanned and then checked up on twenty years later.  By that time, each subject should be able to identify as heterosexual or not, and a comparison of hypothalamus size in infancy to their sexuality as an adult would show a clear cause and effect relationship between INAH 3 size and sexuality.  This study, once completed, peer reviewed, and repeated, could be the definitive experiment in this issue.

Birth Order

Of all the studies done looking into possible causes of homosexuality, the study repeated most often, and with the most consistent results, is the study of ‘Fraternal Birth Order’.  Fraternal Birth Order looks at the probability that a man will be homosexual in relation to the number of older biological brothers they have.  Working on Fraternal Birth Order for more than a decade, Anthony Bogaert (1998) has shown that men with no older brothers have a 2-3% chance of being homosexual, while men with three or four older brothers have a 5% chance of being homosexual, at least 20 other studies have shown similar results (Dotinga, 2006).   A critic would likely point out that a man with no older brothers and a man with at least four older brothers would have grown up in drastically different environments, and that environmental factors could solely account for the probability of homosexuality, however in his most recent study Bogaert completely controls these environmental factors.  Bogaert used a massive survey of 944 men of varying family environments, including men who grew up with older step brothers, with and without older biological brothers, and with or without their biological mothers.  Despite the variation in environments, Bogaert replicated his results by showing that one’s probability of being homosexual is solely related to the number of older biological brothers one has, weather they live together or not.  Bogaert suggests that these findings are evidence for a prenatal origin of the Fraternal Birth Order effect, as opposed to the environment an adolescent grows up in (Bogaert, 2006).

Fraternal Birth Order effect is certainly more than just a statistical anomaly, as it has been shown to have very visible effects on a child while it is still in the womb.  Studies have noted that each child a woman carries tends to weigh more than the last; however this increase in weight is not nearly as drastic in boys with older biological brothers as it is in boys with only older biological sisters.  Similarly, the placentas of boys with older biological brothers tend to be heavier than the placentas of boys with only older biological sisters (Puts, Jordan, & Breelove, 2006).  Obviously, these tendencies are not due to the social upbringing of an unborn child, but in other ways they may be products of their environment.

Though Bogaert does not himself suggest a cause for the Fraternal Birth Order effect, there has since emerged a hypothesis that may tie together several of the ideas.  It is called the Maternal Immunization Hypothesis, and it suggests that during birth blood from the mother and son can mix, introducing foreign ‘male proteins’ into the mother’s blood, triggering an immune response which causes the mother to produce ‘male antibodies’.  In Theory, the next time this mother carries a male child, her immune system will have ‘male antibodies’ that may affect the development of the ‘masculine brain’.  Following Bogaert’s research, each subsequent introduction of ‘male protein’ to the mother’s immune system will just stimulate a stronger response against ‘male proteins’ (Puts, Jordan, & Breelove, 2006).  Though studies have yet to find aforementioned proteins and antibodies, the effects such antibodies would have on a developing brain would be obvious.  As mentioned before, a testosterone inhibitor introduced to male rats caused them to become ‘bisexual’, and as also mentioned before, monkeys and rats with injured hypothalamus lost interest in female partners, but did not completely lose their libido.  It may be suggested that these factors, possibly in combination with a man’s social upbringing, may result in their attraction to other men instead of women.


However, there are those whom still suggest, despite multiple studies showing correlation between biological factors and sexuality, that sexuality is purely a choice, is unnatural, and is caused solely by one’s upbringing.  One study often pointed to shows that homosexuality is not evenly distributed across the globe.  A specific example is the lack of homosexuals among Orthodox Jews, whom presumably have a fairly strict upbringing where social norms are stringently enforced (Prager, 1993).  However, where some argue that this strict upbringing prevented men from being harmed by a society that would cause them to be homosexual, it could conversely be argued that this strict upbringing merely caused those men who realized that they were homosexual to hide or repress their urges.  This is especially meaningful when considering that in the Torah sodomy is a sin on par with the abomination of sacrificing children (Prager, 1993).

One would also think that if homosexuality is affected by society and an individual’s environment instead of by their biology, than it could be ‘fixed’.  In fact many institutions claim to be able to do this, and there are people (often associated with religious institutions) whom call themselves “ex-gay”.  Weather these ex-gays truly are ‘reformed’ or simply motivated to hide their orientation is up for debate, however.  Openly gay individuals would argue the latter, and studies recently published by the American Psychological Association have shown that attempts to change one’s sexuality can be both harmful and ineffective.  Regarding the studies, the APA stated that “efforts to switch a person’s sexual orientation through aversive treatments might cause harm, including loss of sexual feeling, suicidality, depression and anxiety” and that, “at most, certain studies suggested that some individuals learned how to ignore or not act on their homosexual attractions…” and furthermore “this result was much less likely to be true for people who started out only attracted to people of the same sex [as opposed to bisexual or pansexual individuals]” (CNN, 2009).

Recently, in addition to research on human sexuality, more research has been done showing apparently homosexual acts in animals besides humans.   A very public example included two male penguins in the Central Park Zoo which seemed to have an irregularly close companionship; making mating displays to each other, attempting to incubate an egg together, and both ignoring the female penguins in the exhibit (Smith, 2009).  Other examples animals of animals displaying homosexual behavior have now been cited in several different species, in captivity as well as in the wild.  One species of particular interest is the Bonobo; a species of chimpanzee thought to be most closely related to humans.  Studies have shown “that whether observed in the wild or in captivity, nearly all are bisexual, and nearly half their sexual interactions are with the same sex. Female bonobos have been observed to engage in homosexual activity almost hourly” (Smith, 2009).

With so much evidence, it is irrational to think that biology does not have a role in determining an individual’s sexuality.  Though it may be impossible to say that there is a ‘gay gene’, or even that biology has the whole say in the matter of sexuality, it is ignorant to say that homosexuality is chosen by any individual.  But that is not to say that environment has no effect at all.  Just as individuals with a family history of certain medical conditions can worsen or lessen the effects through their eating and exercise habits, perhaps sexuality is similarly predisposed biologically to be more sensitive to environmental factors.  This should not be taken to mean that homosexuality should be compared to disease.  How tan or freckled a person can get is another example of a way biology can predisposition an individual in one way or the other.  It is not socially acceptable to criticize someone for having a heart attack, or contracting cancer, when they have a family history (biological predisposition) of heart attacks or cancer, so why do some people feel that it is okay, or even correct, to criticize homosexuals for being who they are?  Even if environment plays a role in determination of sexuality, does one get to choose the family, location, or social setting they were born into?

While doing research for this paper I found it upsetting that so much of the work done to discredit the science done to find the cause of homosexuality was done by theologians and others associated with Christianity.  Despite multiple rephrasings of “do unto others as they would have do unto you” and “love thy neighbor as you love thyself” throughout the Bible, some still cling to the infrequent condemnations of Gays.  Why are those passages upheld while condemnations of shellfish and suggestions to take multiple wives and sell daughters into slavery are ignored?  Jesus lead by example, associating himself with adulterers and lepers alike; people who were ostracized for their choices, as well as for factors beyond their control.  In the end, my explanation is that people are simply choosing to ignore the facts out of bias and homophobia.  Hopefully with continued research and media exposure logic and reason will one day win out over fear.  Until then I will continue to play the devil’s advocate and ask: “So, when did you choose to be straight?”



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