Sexual Orientation

Twin studies are a classic tool for examining the role of genes. Twins brought up together share a similar environment. Monozygotic twins share all their genes, while dizygotic twins share only half their genes. Early twin studies by Franz Kallmann in 1952 and Leonard Heston in 1968 reported that if one monozygotic twin was homosexual, there was a greater chance the other twin would be homosexual. The likelihood of this was greater than for dizygotic twins. These studies were potentially biased. They recruited homosexual subjects and had relatively small sample sizes. Recent twin studies have examined all twins in a community without regard to sexual orientation, providing large, less biased sample sizes. In 2000 Kenneth Kendler and colleagues evaluated genetic and environmental factors in a large U.S. sample of twin and nontwin sibling pairs. Sexual orientation was classified as heterosexual or nonheterosexual (bisexual or homosexual) and was determined by a single item on a self-report questionnaire. There was a greater chance for both monozygotic twins to be nonheterosexual than for dizygotic twins or sibling pairs. Results suggested that sexual orientation was greatly influenced by genetic factors, but family environment might also play a role. One problem with this study is that a single item was used to assess the complexity of sexual orientation.

Katherine Kirk's study in 2000 involved a community sample of almost 5,000 adult Australian twins who answered an anonymous questionnaire on sexual behavior and attitudes. Multiple measures of sexual orientation (behaviors, attitudes, feelings) provided stronger evidence for additive genetic influences on sexual orientation. Heritability estimates of homosexuality in this sample were 50 to 60 percent in females and 30 percent in males. In 1999 J. Michael Bailey found that if a man was homosexual, the percentage of his siblings who were homosexual or bisexual was 7 to 10 percent for brothers and 3 to 4 percent for sisters, higher than would be due to chance.

Some family studies have reported more homosexuals had homosexual maternal relatives but not paternal relatives. This might support a genetic factor on the X chromosome and/or environmental influences. Other, similar studies did not find this. Thus, evidence exists for both genetic and environmental determinants of sexual orientation which may be different for men and women.

A 2000 study examined whether sexual orientation is fixed or changes with time through environmental influence or the effects of aging. J. Michael Bailey recruited a community sample of twins from the Australian Twin Registry and assessed sexual orientation, childhood gender nonconformity (atypical gender behavior), and continuous gender identity (an individual's self-identification as "male" or "female"). Familial factors were important for all traits, but less successful in distinguishing genetic from shared environmental influences. Only childhood gender nonconformity was significantly heritable for both men and women. Statistical tests suggested that causal factors differed between men and women, and for women provided significant evidence for the importance of genetics factors.

Birth-order studies found homosexual males were not usually first born, having older siblings. Extremely feminine homosexual men had a higher than expected proportion of brothers, not an equal numbers of brothers and sisters.