SameSexAttraction.org

Biological Causes of Same-sex Attraction


Some 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 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 have not proven that same-sex attraction is an inborn or biologically-determined characteristic. If you read the reports published by the researchers, you find that they admit their current findings are not conclusive and simply hint at what some of the causes may be. Furthermore, these studies have not been able to be replicated. Sadly, 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

Drs. Michael Bailey and Richard Pillard studied identical and fraternal twins. (Bailey and Pillard, pp. 1089–96) They identified homosexual males who had identical twin brothers and found that 52% of the brothers were also homosexual. Among fraternal twins, they found the ratio to be 22%. They concluded that since identical twins had a higher incidence of mutual homosexuality than fraternal twins, there must be a genetic component in the development of homosexuality.

However, if genetics caused the homosexuality, the correlation between the identical twins (who have exactly the same genes) should have been much higher—even 100%. Since all the twins in this study were raised together, it is impossible to determine whether genetics or the same family environment contributed to the brothers’ homosexuality. If the genes are identical and the brothers are raised in the same family environment, a correlation of only 52% in identical twins shows that other factors are involved.

Many question the validity of the twins study. The researchers commented that since their subjects were not selected by random sampling, they may have collected a biased sample with skewed results. (Bailey and Pillard, p. 1094) Another twin study was conducted a year later, which showed a correlation of only 25%. (British Journal of Psychiatry, pp. 407–409.)

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)

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)

Biological conclusions

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, such attractions are changeable and treatable. In analogy, although the City of Hope National Medical Center researchers found a certain gene present in 77% of the alcoholic patients they studied, we have not abandoned treatment for alcoholism. (Dallas 1992, pp. 20-23) Many former alcoholics have changed their behavior and lead productive lives. You have control over your destiny. You have moral agency and can determine the course of your life. Regardless of any biological thread, thousands of people who experience same-sex attractions have made changes in their lives for the better.

Read more about other causes of same-sex attraction