Sexual Orientation, Determinants of
Sexual Orientation, Determinants of
A person’s sexual orientation concerns whether he or she is sexually attracted to people of the same sex, the opposite sex, or both. Starting in the nineteenth century, scientists, physicians, and mental health specialists have offered numerous theories of how people develop sexual orientations. What determines a person’s sexual orientation remains a question of much scientific, psychological, and social scientific controversy. Current biological theories focus on genes, brains, and hormones.
For much of the twentieth century, scientists believed that gay men and lesbians had gender-atypical hormone levels in their bodies; for example, they thought that lesbians had more testosterone and less estrogen in their systems than did heterosexual women. Recent studies have shown that almost all lesbians and gay men have the same circulating hormone levels as their heterosexual counterparts. In response, current hormonal theories propose a link between sexual orientation and prenatal hormone levels. As prenatal hormone levels play a role in the organization of the developing brain, the idea is that different prenatal hormones levels could explain sexual orientation differences among adults. This prenatal hormonal hypothesis is premised largely on observations that early hormone exposure determines both the repertoire of mating behaviors exhibited by laboratory animals and the morphology of particular brain regions believed to modulate those behaviors.
Over the past few decades, confirmed sex differences in the size of several brain structures in various laboratory animals have lead to speculation regarding parallel differences in the human brain associated with sex and sexual orientation. Several of the structural sex differences identified in animals involve specific cell groups in a region of the rodent hypothalamus involved in regulating particular mating behaviors. One such anatomical sex difference in rodents develops in response to sex differences in early exposure to sex hormones and involves a part of a rodent’s hypothalamus called the sexually dimorphic nucleus of the preoptic area (SDN-POA). In the laboratory, damage in the vicinity of the SDN-POA decreases mounting behavior, and electrical stimulation of the region elicits mounting behavior. These observations established the belief that the SDN-POA participates in regulating male sexual behavior. That belief lacks critical support because the destruction of the SDN-POA of male rats does not, however, disrupt mounting behavior. The function of the SDN-POA remains to be elucidated.
The belief that the SDN-POA participates in regulating sex behavior in rats led to the search for a comparable nucleus in humans. The human third interstitial nucleus of the anterior hypothalamus (INAH3) has been identified as the most promising candidate. This nucleus is much larger and contains substantially more neurons in presumed heterosexual men than in women. By extrapolation from animal work, this human sex difference is widely believed to reflect sex differences in early hormone exposure, but this hypothesis is exceedingly difficult to test in humans. The AIDS epidemic has, however, made it possible to study this nucleus in men whose medical records indicate homosexual behavior. These studies suggest that the volume of INAH3 may be smaller in homosexual men than in heterosexual men; however, that finding must be viewed tentatively for various reasons, including that all the homosexual men in these studies died from complications of AIDS.
Researchers have also sought to identify variation with sex and sexual orientation in the bundles of fibers that connect the left and right hemispheres of the brain. These studies have produced conflicting results regarding variation with both sex and sexual orientation.
Inside each person’s cells are chains of DNA that act as a recipe for making that person. Determining the role a portion of genetic material plays in the development of a particular human trait is complicated, although there has been some success for specific anatomical and physiological traits. It is much more difficult to identify genetic material that influences complex psychological traits such as sexual orientations. One way to infer the extent to which a trait is genetic is to study twins. Identical twins share 100 percent of their genetic material. Fraternal twins are genetically only as closely related as non-twin biological siblings, sharing only 50 percent of their DNA. Because identical twins are genetically identical, differences between them are due to their pre- or post-natal environment, not genes. This inference does not work in the other direction. If identical twins have the same trait, it might be because they were raised in the same environment.
Sophisticated studies have assessed sexual orientation in identical twins, same-sex fraternal twins, non-twin biological siblings, and similarly-aged unrelated adopted siblings. The idea is that if sexual orientation is genetic, identical twins should have the same sexual orientation, and the rate of homosexuality among the adopted siblings should be equal to the rate in the general population. If, on the other hand, identical twins and adopted siblings are as likely to have the same sexual orientation, then genetic factors make very little contribution to sexual orientation. Subjects recruited through ads in gay publications looking for volunteers with twins of the same sex were asked to rate their own sexual orientation, the sexual orientation of their relatives, and for permission to contact their siblings. In these studies, identical twins of gay men and lesbians were substantially more likely to share their sibling’s sexual orientation than were fraternal twins.
The result of such twin studies are difficult to interpret because gays and lesbians seem more likely to volunteer themselves and their twin siblings for such studies when they have gay or lesbian siblings. When the proportions of identical and fraternal twins who are concordant for homosexuality are examined in databases of twins who were not selected on the basis of sexual orientation, the estimates for heritability of homosexuality are much lower than when subjects are selected from newspaper ads recruiting on the basis of sexual orientation.
Building on the twin studies, scientists have tried to isolate the portion of the human genome that hypothetically influences sexual orientation. One such study began with the observation that some studies had found that homosexuality in men seems to follow the maternal line of the family. Some have interpreted this pattern among gay men’s families, which has only sometimes been observed, as suggesting that male homosexuality, similar to color blindness, is inherited from one’s mother on the X chromosome. To test the hypothesis that the X chromosomes contains genes that influence sexual orientation, families with at least two gay brothers were recruited from a sample of families in which homosexuality appeared to follow the maternal line. DNA samples were obtained from two gay brothers in each subject families. Samples were then analyzed using linkage analysis to determine how frequently the gay brothers had inherited markers from the same X chromosome of their mother. Because mothers have two X chromosomes but only pass one to their sons, one would expect two sons to have a 50 percent chance of sharing markers from the same maternal X chromosome. One research team, however, reported that when both sons were gay, 64 percent appeared to have inherited a particular portion of the same maternal chromosome (known as the Xq28 region). This finding was interpreted as suggesting that genes influencing sexual orientation in men may reside on the Xq28 region. That suggestion must be viewed cautiously because it has been criticized on the basis of multiple technical and statistical concerns, and two independent research teams have failed to replicate it.
More generally, linkage analysis is best suited for discovering the genetic basis of traits determined in a genetically simple manner rather than traits that are influenced by several genes working in concert. This technique has mistakenly indicated that a specific genetic sequence plays a role in the development of a particular trait. Such mistakes are especially likely in the case of genetically complex traits, cognitively mediated psychological/behavioral traits, or those traits strongly affected by environmental factors.
In addition to linkage analysis, some studies have looked for differences related to sexual orientation in candidate genes chosen on the basis of their involvement in the hormonal mechanisms of sexual differentiation and development. To date, these studies have not produced positive findings.
In any event, genes in themselves cannot directly cause a behavior or a psychological phenomenon. Genes direct RNA synthesis that in turn leads to the production of a protein that may influence the development of psychological dispositions and particular behaviors. There are many intervening pathways between a gene and a behavior or a behavioral disposition and even more intervening variables between a gene and a cognitively mediated behavior.
None of the most popular scientific theories of the determinants of sexual orientations are well supported. This does not mean, however, that sexual orientations are not biological in the sense that all psychological phenomena require the activity of a living brain. Rather than asking whether or not sexual orientations are biological, the more salient question is what role biology plays in the development of sexual orientations. Biology would still be a factor in sexual orientation if there are multiple pathways to the development of a particular sexual orientation. Biological factors may also play a role if they influence temperamental or personality factors that shape how a person interacts with the environment and experiences of it, which in turn affects the development of one’s sexual orientation.
SEE ALSO AIDS; Gender; Gender, Alternatives to Binary; Sexual Orientation, Social and Economic Consequences; Sexuality; Twin Studies
Byne, William, et al. 2001. The Interstitial Nuclei of the Human Anterior Hypothalamus: An Investigation of Variation with Sex, Sexual Orientation and HIV Status. Hormones and Behavior 40: 86-92.
Hamer, Dean, and Peter Copeland. 1994. The Science of Desire: The Search for the Gay Gene and the Biology of Behavior. New York: Simon and Schuster.
LeVay, Simon. 1996. Queer Science: The Use and Abuse of Research into Homosexuality. Cambridge, MA: MIT Press.
Mustanski, B. S., M. L. Chivers, and Bailey J. Michael. 2002. A Critical Review of Recent Biological Research on Human Sexual Orientation. Annual Review of Sex Research 13: 89-140.