Biological Causes of Same-sex Attraction

SummarySome scientists have searched for a direct genetic cause of same-sex attraction and found little evidence. But science has not shown that homosexuality is an inborn or biologically-determined characteristic.  Most scientists today give such genetic theories little credibility.

Some scientists have searched for a direct genetic cause of same-sex attraction—a gene or chromosome that actually determines sexual orientation. (Friedman and Downey, p. 149) Some studies hint at a biological component, but no study proves that sexual orientation is an innate (unchangable), biologically-fixed property of human beings. Scientific evidence simply does not support the idea that people are “born that way.” (Mayer)

While there is evidence that biological factors such as genes and hormones are associated with sexual behaviors and attractions, there are no compelling causal biological explanations for human sexual orientation. While minor differences in the brain structures and brain activity between homosexual and heterosexual individuals have been identified by researchers, such neurobiological findings do not demonstrate whether these differences are innate or are the result of environmental and psychological factors. (Mayer)

In addition, longitudinal studies of adolescents suggest that sexual orientation may be quite fluid over the life course for some people, with one study estimating that as many as 80% of male adolescents who report same-sex attractions no longer do so as adults (Mayer). 

Nevertheless, some news reports have misrepresented or sensationalized the facts, leading some people to the mistaken notion that homosexuality is a genetic trait. Most scientists today give genetic theories little credibility. (Lewontin) The Gay and Lesbian Medical Association recommends that people not use the argument that homosexuality is biological because there is simply not enough evidence. Drs. Neil & Briar Whitehead state, "No genetically determined human behavior has yet been found.” (Neil & Briar Whitehead, 1999) Most researchers and scientists believe there is no single cause, and that the factors may be different for different people.

Summaries of the more significant research in these biological areas is described below:

Twin studies

Eight major studies of more than 10,000 sets of identical twins during the last two decades all arrive at the same conclusion: gays were not born that way. "At best genetics is a minor factor," says Dr. Neil Whitehead, PhD in biochemistry and statistics.

Identical twins have the same genes or DNA and they are nurtured in equal prenatal conditions. Therefore, if homosexuality is caused by genetics or prenatal conditions and one twin is gay, the co-twin should also be gay. If both twins are not gay, then homosexuality cannot be genetically dictated. "The predominant things that create homosexuality in one identical twin and not in the other have to be post-birth factors."

"Summarizing the studies of twins, we can say that there is no reliable scientific evidence that sexual orientation is determined by a person’s genes. But there is evidence that genes play a role in influencing sexual orientation." (Mayer, p. 31)

Read summaries of the results of these twin studies.

Brain studies

In 1991, Dr. Simon LeVay, a neurobiologist at the Salk Institute in La Jolla, California, reported his findings from studying the brain structures of forty-one cadavers. (LeVay, pp. 1034–37) He concluded that an area of the hypothalamus (the INAH3) was smaller in homosexual men than in heterosexual men. (It was also found to be smaller in women than in heterosexual men.) However, these findings do not show any direct link between the hypothalamus and sexual orientation and, furthermore, are dubious at best because of the following reasons:

  • It has not been determined that the INAH3 is involved in the development of sexual orientation. (Byne and Parsons, pp. 228–39)
  • The sample size of this study was small (only forty-one). Furthermore, Dr. LeVay did not know the sexual histories of the cadavers he studied. Nineteen men apparently were homosexual and he assumed that the other sixteen men and six women were heterosexual. (Byne and Parsons, pp. 234-35)
  • Since most of the subjects had died of AIDS, the HIV virus may have affected the brains in various ways, especially the hypothalamus, which is a major player in the immune system. LeVay himself admitted this was a serious flaw in the study. (There is also the possibility of research bias. Dr. LeVay told Newsweek that his lover’s death from AIDS prompted him to find an inborn cause for homosexuality, a quest so important that he would give up his scientific career if he did not find it.)
  • There were many inconsistencies in the findings. Three of the allegedly heterosexual men had a smaller INAH3 than the mean size for the "homosexual" men and three of the "homosexual" men had a larger INAH3 than the mean size for "heterosexual" men. (Cameron, p. 45)
  • Many neuroscientists charge that LeVay deviated from protocol when he measured volume rather than the number of neurons in the INAH3. This is critical, since the area LeVay measured is very small (about the size of a snowflake).
  • Dr. LeVay himself cautions that the results of his study "do not allow one to decide if the size of INAH3 in an individual is the cause or consequence of that individual’s sexual orientation." (Science, p. 1036)

Anne Fausto-Sterling, a professor of medical science at Brown University, said, "My freshman biology students know enough to sink this study." (Gorman, p. 61)

Also see (Mayer, p. 39-41)

Chromosome studies

In 1993, Dr. Dean Hamer announced that he had found a correlation between DNA markers on the X chromosome (region Xq28) and sexual orientation in a selected group of homosexual men and their relatives over age eighteen. In other words, "it appears that Xq28 contains a gene that contributes to homosexual orientation in males." (Hamer, pp. 325) In his book, Dr. Hamer stated, "We can make only educated guesses about the importance of Xq28 in the population at large." He concludes that "Xq28 plays some role in about 5 to 30 percent of gay men. The broad range of these estimates is proof that much more work remains to be done." (Hamer and Copeland, pp. 145–46) Scientists have since questioned the validity of these findings and what they purport to show. (Scientific American 1994, pp. 50–55) Dr. Hammer has been charged with research improprieties and is under investigation by the federal government for improperly excluding from his study men whose genetic makeup contradicted his findings. (Scientific American 1995, p. 26) A later study by the University of Western Ontario "found no consistent pattern of DNA similarity on the X chromosome." (Washington Post, pp. 5–6)

Hormone studies

Studies have shown that in some cases the mothers of homosexual males suffered a high degree of stress during their pregnancy. Since stress affects hormonal levels, some researchers suggest that decreased levels of testosterone could lead to a demasculinization of the developing brain. However, multiple studies over the years have not been able to substantiate the theory, and the available evidence is to the contrary. Ehrhardt and Meyer-Bahlburg wrote, "In the majority of intersex patients with known hormone abnormalities, the sexual orientation follows the sex of rearing. Consequently, we have to assume that prenatal hormone conditions by themselves do not rigidly determine sexual orientation." (Ehrhardt and  Meyer-Bahlburg, p. 1316) Dr. John Money also states there is no evidence that prenatal hormonalization alone determines sexual orientation. (Money 1987, p. 398)

Experiments have been conducted wherein testosterone was given to homosexual males, both those who were effeminate and those who were not. "When there were any behavioral changes at all, the subjects became more like themselves than ever. Their sex drives were usually increased and sometimes their effeminate mannerisms as well (when they had any), but there were never any directional changes in their sexual interests. From these experiments . . . it has become abundantly clear that the sex hormones play a considerable role in powering human sexuality, but they do not control the direction of it." (Tripp, p. 12)

Also see (Mayer, p. 34-39)

Biological conclusions

Some scientists have intently tried to discover scientific proof that same-sex attraction is genetic. Some studies hint at a biological component, but have not proven that same-sex attraction is simply an inborn or biologically-determined characteristic.

Nevertheless, biology may play some role in influencing behavior or feelings. Some people seem susceptible to particular actions and may be drawn toward them or become addicted to them more easily than other people. (Oaks, p. 9) One person may be able to dabble with gambling, while another becomes a compulsive gambler. Some may drink only socially, while others have an unusual attraction to alcohol. Studies indicate that genetics may be a factor in susceptibilities to some behavior-related disorders, such as aggression, obesity, or alcoholism. Likewise, there are theories that claim biological predispositions influence the development of homosexual attractions when other life experiences are also present. (Friedman and Downey,  p. 149)

Beyond such predispositions, some scientists search for more direct genetic causes—a gene or chromosome that actually determines sexual orientation. (Friedman and Downey,  p. 149) None of these studies has shown any direct genetic cause of homosexuality.

Drs. Byne and Parsons of the Department of Psychiatry at Columbia University reviewed the biologic theories of human sexual orientation in 1993 and concluded, "[T]here is no evidence at present to substantiate a biologic theory." (Byne and Parsons 1993, p. 228) No study suggests that a simple cause–effect relationship exists. (Hubbard and Wald) And Dr. Earl Wilson wrote, "the disputed evidence for physical causes of male homosexuality is even weaker when it comes to lesbianism." (Wilson, p. 76)

Regardless of the role that genetics play in the development of sexual attractions, people who experience these attractions can make conscious choices about their behaviors. Although researchers have found a certain gene present in 77% of the alcoholic patients (Dallas, 1992, pp. 20–23), we know that alcoholics can control their behavior and lead productive lives. You have control over your destiny. You have moral agency and can determine the course of your life.

Read more about other causes of same-sex attraction