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Biological Causes of Same-Sex Attraction

Summary: 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 R. C. and Downey, J. “Neurobiology and Sexual Orientation: Current Relationships,” Journal of Neuropsychiatry 5, 1993, 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. (R. C. Lewontin, et. al., Not in Our Genes, Pantheon Books, New York, 1984 and Hubbard, R. and Wald, E., Exploding the Gene Myth, Beacon Press, Boston, 1993.) 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.” (Whitehead, Neil L. & Briar. My Genes Made Me Do It!, Huntington House Publishers, 2014, 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.”

Same-sex attraction (homosexuality) is caused by non-shared factors—things that happen to one twin but not the other, or a personal response to an event by one of the twins and not the other.

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 41 cadavers. (LeVay, Simon. “A Difference in Hypothalamic Structure Between Heterosexual and Homosexual Men,” Science, vol. 253, 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, William and Parsons, Bruce. “Human Sexual Orientation: The Biologic Theories Reappraised,” Archives of General Psychiatry 50, Mar. 1993, pp. 228–39.)
  • The sample size of this study was small (only 41). 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, Paul. Medical Consequences of What Homosexuals Do, Family Research Institute, Washington, DC, 1993 and “The Longevity of Homosexuals: Before and After the AIDS Epidemic,” Paul Cameron, William Playfair, and Stephen Wellum, Omega Journal of Death and Dying, vol. 29, no. 3, 1994, Baywood Publishing, Amityville, NY, 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, C. “Are Gay Men Born That Way?,” Time, 9 Sep. 1991, 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, Dean, et. al. “A Linkage Between DNA Markers on the X Chromosome and Male Sexual Orientation,” Science 261, 16 Jul. 1993, 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, Dean and Copeland, P. The Science of Desire, Simon & Schuster, New York, 1994, pp. 145–46.) Scientists have since questioned the validity of these findings and what they purport to show. (“The Biological Evidence Challenged,” May 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, “Gay Genes, Revisited,” Nov. 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, 31 Oct. 1994, 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, A.A. and Meyer-Bahlburg, H.F.L. “Effects of Prenatal Hormones on Gender-Related Behavior,” Science, vol. 211, 20 Mar. 1981, p. 1316.) Dr. John Money also states there is no evidence that prenatal hormonalization alone determines sexual orientation. (Money, John. “Sin, Sickness or Status? Homosexual Gender Identity and Psychoneuroendocrinology,” American Psychologist 42, no. 4, Apr. 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, C. A., The Homosexual Matrix, McGraw-Hill, New York, 1975, p. 12.)

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, Dallin H. “Same-gender Attraction,” Ensign, Salt Lake City, Utah, Oct. 1995, 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 R. C. and Downey, J. “Neurobiology and Sexual Orientation: Current Relationships,” Journal of Neuropsychiatry 5, 1993,  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, William and Parsons, Bruce. “Human Sexual Orientation: The Biologic Theories Reappraised,” Archives of General Psychiatry 50, Mar. 1993, p. 228.) No study suggests that a simple cause–effect relationship exists. (Hubbard, Ruth and Wald, Elijah. Exploding the Gene Myth, Beacon Press, Boston, 1993 and “Human Sexual Orientation: The Biologic Theories Reappraised,” Byne, William and Parsons, Bruce, Archives of General Psychiatry 50, Mar. 1993, p. 228.) And Dr. Earl Wilson wrote, “the disputed evidence for physical causes of male homosexuality is even weaker when it comes to lesbianism.” (Wilson, Earl D. Counseling and Homosexuality, Word Books, Waco, TX, 1988, 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, 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 the causes of same-sex attraction.