Sexual orientation refers to the object of a person's sexual desires, whether the person is sexually attracted to those of the same sex or of a different sex. Although not all personality researchers consider individual differences in sexual orientation to be part of personality, a reasonable case can be made that this is an important way in which individuals differ from each other. And these differences tend to be relatively stable over time. Moreover, these differences are associated with a host of important life outcomes, such as the social groups with which one affiliates, the leisure activities one pursues, and the lifestyle one adopts. By the definition of personality provided in Chapter 1, sexual orientation clearly falls well within the scope of personality.
Behavioral genetic studies of sexual orientation have been in the newspaper headlines. Is homosexuality inherited? Psychologist Michael Bailey has conducted the most extensive studies of this issue. Bailey and his colleagues examined the twin brothers of a sample of homosexuals, as well as the adoptive brothers of another sample of homosexuals. Heritability estimates from all studies, depending on various assumptions, ranged from 30 percent to a strikingly high 70 percent. Similar heritabilities were found in a sample of lesbians and their adoptive sisters (Bailey et al., 1993).
These heritability findings come on the heels of another startling discovery, which was published in Science magazine (LeVay, 1991). Brain researcher Simon LeVay discovered that homosexual and heterosexual men differ in a specific area of the brain known as the hypothalamus. One area of the hypothalamus, the medial preoptic region, appears to be partially responsible for regulating male-typical sexual behavior (LeVay, 1993, 1996). LeVay obtained the brains of gay men who had died of AIDS and compared them with the brains of heterosexual men who had died of AIDS or other causes. He found that the size of the medial preoptic region of the hypothalamus—the region believed to regulate male-typical sexual behavior—to be two to three times smaller in the gay men, compared with that of the heterosexual men. Unfortunately, given the extremely expensive nature of brain research, the samples in this study were quite small. Moreover, no one has yet replicated these findings.
Behavioral geneticist Dean Hamer has published some evidence that male sexual orientation is influenced by a gene on the X chromosome (Hamer & Copeland, 1994). However, this finding also needs to be replicated, and several researchers have debated its validity (e.g., see Bailey, Dunne, & Martin, 2000).
Obviously, this research area is controversial, and the findings are hotly debated. Moreover, the genetic studies of homosexuality have attracted their share of critics. The studies have been challenged on the grounds that the samples, which were secured from advertisements in lesbian and gay publications, were unrepresentative (Baron, 1993). For example, gays are probably more likely to respond to an advertisement looking for gays with twins only if each is actually gay, inflating the estimate of heritability.
Another weakness in past studies was a neglect of the correlates of sexual orientation. For example, childhood gender nonconformity is strongly related to adult sexual orientation. Gay men as adults recall having been feminine boys, and lesbian women as adults recall being masculine girls. This association is strong and has been established with many sources of data (e.g., using peer reports of childhood gender nonconformity). Regarding the importance of gender nonconformity in childhood, a leading researcher has remarked that "it is difficult to think of other individual differences that so reliably and so strongly predict socially significant outcomes across the life span, and for both sexes, too" (Bem, 1995, p. 323). In fact, Bem has proposed his own theory of the source of adult sexual orientation, that biological factors may cause childhood gender nonconformity and that early gender nonconformity causes children to feel different from other children of their own sex and, as a result, to be attracted to people who are "different" from themselves (even though they are of the same gender).
Bailey and his colleagues set out to clear up these two weaknesses— unrepresentative samples and lack of accounting for childhood gender nonconformity—by conducting one of the largest twin studies of adult sexual orientation to date (Bailey et al., 2000). The participants were from a sample of almost 25,000 twin pairs in Australia, out of which approximately 1,000 MZ and 1,000 DZ twins participated. Their average age at time of participation was 29 years. The participants completed a questionnaire about childhood (before age 12) participation in a variety of sex-stereotyped activities and games. They also completed a detailed questionnaire on adult sexual orientation and activity, such as "when you have sexual daydreams, how often is your sexual partner male? how often female?"
Results showed that approximately 92 percent of the men and 92 percent of the women were exclusively heterosexual in orientation. An interesting sex difference was found, however, in the distribution of sexual behaviors. The women were more likely than the men to have slight homosexual feelings without being exclusively homosexual, whereas the men tended to be more either exclusively heterosexual or exclusively homosexual. Just over 3 percent of the men, but only 1 percent of the women, were predominantly or exclusively homosexual in sexual attraction and sexual fantasy. This finding suggests that sexual behavior and orientation should be analyzed separately for men and for women, with researchers prepared to develop a different theoretical account for each group.
Regarding whether homosexual orientation runs in families, this study found lower rates than previous studies, at 20 percent concordance for the identical twin men and 24 percent concordance for the identical twin women. Concordance is the probability that one twin is gay if the other is also gay. Previous studies typically found concordance rates ranging between 40 and 50 percent. Bailey argues that previous studies overestimate genetic contributions due to selecting participants by advertising in gay and lesbian magazines.
In the Bailey et al. (2000) study, participants were randomly selected from a large pool of twins, so there was no selection bias. It seems likely that the real rate of genetic contribution to sexual orientation is much lower than previously thought. Childhood gender nonconformity did, however, show significant heri-tability for both men (50 percent heritability) and women (37 percent heritability). This finding provides some support for Bem's (1995) theory that childhood gender nonconformity may be the inherited component of adult sexual orientation. And the link from gender nonconformity in childhood to adult homosexual orientation, although statistically significant, is far from perfect. Clearly, the most recent evidence suggests that genes provide a relatively modest and indirect influence on adult sexual orientation.
A recent twin study explored a phenomenon known as gender identity disorder (GID) (Coolidge, Thede, & Young, 2002). A diagnosis of GID requires that two aspects be present simultaneously:
(1) cross-gender identification that is strong and persists over time, and
(2) persistent psychological discomfort with one's biological sex (American Psychological Association, 1994). In the twin study, clinically significant GID was
present in roughly 2.3 percent of his sample of 314 twins. The results showed a strong genetic component in whether or not the individuals were diagnosed with GID—62 percent of the variance was due to heritability. The authors conclude that "gender identity may be much less a matter of choice and much more a matter of biology" (Coolidge etal., 2002, p. 251).
In summary, the findings from behavioral genetics and brain research point to the fascinating possibility that sexual orientation—an individual difference that is linked with the social groups one associates with, the leisure activities one pursues, and the lifestyle one adopts—may be partly heritable. However, exactly which part is heritable and how this indirectly affects adult sexual orientation are questions for future research.
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