Biological theories are essentialist—that is, they claim that differing sexual orientations are due to differences in physiology. This difference can be due to genetics, hormones, birth order, or simple physical traits.

Genetics

In 1952, Franz Kallman tried to show that there was a genetic component to homosex­uality. Kallman compared identical twins (who come from one zygote and have the same genes (see Chapter 3); with fraternal twins (who come from two zygotes and have about 50% of the same genes). Though Kallman found a strong genetic component to homo­sexuality, his study had a number of problems and is unreliable.

Bailey and his colleagues have done a number of studies of twins to determine the genetic basis of homosexuality. They report that in homosexual males, 52% of identical twins, 22% of fraternal twins, and 11% of adoptive brothers were also homosexuals, showing that the more closely genetically related two siblings were, the more likely they were to share a sexual orientation (J. M. Bailey & Pillard, 1993). Among females, 48% of identical twins, 16% of fraternal twins, and 6% of adoptive siblings of lesbians were also lesbians (J. M. Bailey et al., 1993). However, identical twins share much more than genetics. They also share many more experiences than do other kinds of siblings. So the studies cannot tell how much of the concordance is due to genetic factors and how much is due to the identical twins having grown up under similar environmental influences.

A more interesting finding is the one by Hamer and colleagues (1993) of the National Cancer Institute. Hamer found that homosexual males tended to have more homosexual relatives on their mother’s side, and he traced that to the existence of a gene, passed through the mother, that he found in 33 of 40 gay brothers. Gay men have more gay brothers than lesbian sisters, whereas lesbians have more lesbian sisters than gay brothers (Pattatucci, 1998). This research has also found evidence of a “gay” gene on the X chromosome but not a “lesbian” gene. Newer genetic research has revealed that the frequency of homosexuality in twins may be much lower than previously reported (Kirk et al., 1999).

If homosexuality were solely a genetic trait, it should have disappeared long ago. Because homosexuals have been less likely than heterosexuals to have children, each successive generation of homosexuals should have become smaller, until genes for ho­mosexuality disappeared from the gene pool. Yet rates of homosexuality have remained constant. Concordance rates for siblings, twins, and adoptees reveal that genes account for at least half of the variance in sexual orientation (Pillard & Bailey, 1998). Even so, Bailey and his colleagues agree that environmental factors are also very important.

Hormones

Hormonal theories can concentrate either on hormonal imbalances before birth or on hormone levels in adults. Here we look at both prenatal and adult hormonal levels.

Prenatal Factors When certain hormones are injected into pregnant animals, such as rats or guinea pigs, at critical periods of fetal development, the offspring can be made to exhibit homosexual behavior (Dorner, 1976). Some researchers have found ev­idence that sexual orientation may be influenced by levels of prenatal hormones in hu­man beings as well (Cohen-Bendahan et al., 2005; Rahman, 2005). (For more informa­tion about hormones, see Chapter 3.) In a retrospective study, L. Ellis and colleagues
(1988) suggested that stress during pregnancy (which can influence hormonal levels) in­creased the chances of a homosexual offspring. Early hormone levels have also been found to influence both sexual orientation and related childhood sex-typed behaviors (Berenbaum & Snyder, 1995; Swaab, 2004).

However, other researchers have concluded that the evidence for the effect of pre­natal hormones on both male and female homosexuality is weak (Whalen et al., 1990). A study of female rhesus monkeys who were given masculine hormones before birth re­vealed that their environment after birth was as important to their sexual behavior as the hormones (Money, 1987). In other words, even if prenatal hormones are a factor in sexual orientation, environmental factors may be equally important.

Adult Hormone Levels Many studies have compared blood androgen levels in adult male homosexuals with those in adult male heterosexuals, and most have found no significant differences (Green, 1988). Of five studies comparing hormone levels in les­bians and straight women, three found no differences between the two groups in either testosterone, estrogen, or other hormones, and the other two found higher levels of testosterone in lesbians (and one found lower levels of estrogen; Dancey, 1990). Thus, studies so far do not support the idea of adult hormone involvement.