Two articles in the early 1990s reported differences between the brains of homosexual and heterosexual men (S. LeVay, 1991; Swaab & Hofman, 1990). Both studies found that certain areas of the hypothalamus, known to play a strong role in sexual urges, were either larger or smaller in homosexual men than in heterosexual men. Even though this research has been replicated (Kinnunen et al., 2004; Swaab, 2004), it has not yet been determined whether the differences were there from birth or developed later in life, and the research cannot prove that the differences were due primarily to the men’s sexual orientation. Other studies that examined differences in ear structure and hearing in heterosexual, lesbian, and bisexual women found differences in these areas of the brain (Jensen, 1998).
Physiology studies have also looked at amount of facial hair, size of external genitalia, the ratio of shoulder width to hip width (Schuklenk et al., 1997), handedness (Lippa, 2003; Lalumiere & Blanchard, 2000), and finger length (Rahman, 2005; Bailey & Hurd, 2005). There has been limited scientific support for some of these physiological findings. For example, lesbian women have been found to have a male-type finger length pattern on the right hand (which is a shorter ring, than index, finger; Williams et al., 2000).
In summary, although there have been some biological differences found among homosexuals, heterosexuals, and bisexuals, findings are inconsistent, and in many cases the evidence is weak. Given the complexity of biological factors, it is impossible to make ac
curate individual predications because of the randomness of neural connections during development (Pillard, 1998). Because of this, it appears that sexual orientation is the result of a combination of genetic, biological, and social influences (Schuklenk et al.,
1997) . We will now examine some of the developmental and sociological theories about sexual orientation.
Question: Is homosexuality found only in humans, or do some animals also exhibit homosexual behavior?
Same-sex activity has been found in 450 species of birds and mammals. In the summer months, killer whales spend one-tenth of their time engaging in homosexual activity (Mackay, 2000). Many mammal species, from rats to lions to cows to monkeys, exhibit same-sex mounting behavior. Males mount other males, and females mount other females (though they rarely do it when a male is present). Female rhesus monkeys probably mount each other to establish dominance hierarchies, and cows may mount to coordinate their reproductive cycles. Bonobo chimpanzees have been found to engage in all types of sexual behaviors, including same – and other-sex behaviors (Waal, 1995). Even so, no one has reliably reported on cases in which individual animals display exclusively homosexual behavior; that seems to be restricted to human beings. However, we should be very careful in extending animal analogies to humans.