Ridiculously heady read. I apologize, but it's worth the slog:
"There are two classes of possible influence on human sexual orientation. One class encompasses the sociocultural influences that may instruct developing children about how they should behave when the grow up (think of all those charming princes wooing girls in Disney movies). The other class of influences includes the endogenous factors - especially differences in fetal exposure to testosterone - that could organize developing brains to be attracted to females or males in adulthood. For the great majority of people who are heterosexual, there's no way to distinguish between these two influences, because they both favor the same outcome. However, people who are homosexual provide a test. Given that they seem to have ignored society's prescription, is there evidence that early hormones are responsible for making some people gay? If so, then maybe hormones play a role in heterosexual development, too.
Certainly, homosexual behavior is seen in other species - mountain sheep, swans, gulls, and dolphins, to name a few (Bagemihl, 1999). Interestingly, homosexual behavior is more common among anthropoid primates - apes and monkeys - than in prosimian primates like lemurs and lorises (Vasey, 1995), so greater complexity of the brain may make homosexual behavior more likely. In the most studied animal model - sheep - some rams consistently refuse to mount females but prefer to mount other rams. There is growing evidence of differences in the POA (preoptic area) of "gay" versus "straight" rams (Roselli, et al., 2004), apparently organized by testosterone acting on the brain via neuronal androgen receptors during fetal development (Roselli and Stormshak, 2009).
Simon LeVay (1991) performed postmortem examinations of the POA in humans and found a nucleus (the third interstitial nucleus of the anterior hypothalamus, or INAH-3) that is larger in men than in women, and larger in heterosexual men than in homosexual men. All but one of the gay men in the study had died of AIDS, but the brain differences could not be due to AIDS pathology, because the straight men with AIDS still had significantly larger INAH-3 than did the gay men. To the press and the public, this finding sounded like strong evidence that sexual orientation is "built in." It's still possible, however, that early social experience affects the development of INAH-3 to determine later sexual orientation. Futhermore, sexual experiences as an adult could affect INAH-3 structure, so the smaller nucleus in some homosexual men may be the result of their homosexuality, rather than the cause, as LeVay himself was careful to point out.
In women, purported markers of fetal androgen exposure - otoacoustic emissions (McFadden and Pasanen, 1998), finger length patterns (T.J. Williams et al., 2000), patterns of eye blinks (Rahman, 2005), and skeletal features (J.T. Martin and Nguyen, 2004) - all indicate that lesbians, as a group, were exposed to slightly more fetal androgen than were heterosexual women. These findings suggest that fetal exposure to androgen increases the likelihood that a girl will grow up to be gay. These studies indicate considerable overlapbetween the two groups, so fetal androgens cannot account for all lesbians. Likewise, the finding that homosexual people are more likely to be left-handed than is the general population (Lalumiere et al., 2000) suggests that androgens cannot be the whole story, because there is no known effect of early androgens on handedness. However, handedness does seem to be established early in life, so this correlation, too, indicated that early events influence adult orientation.
Those same markers of fetal androgen do not provide for a consensus about gay versus straight men; some makers suggest that gay men were exposed to less prenatal testosterone, and others suggest that they were exposed to more prenatal testosterone than straight men. however, another nonsocial factor influences the probability of homosexuality in men: the more older brothers a boy has, the more likely he is to grow up to be gay (Blanchard et al., 2006). Your first guess might be that this is a social influence of older brothers, but it turns out that older stepbrothers that are raised with the boy have no effect, while biological brothers (sharing the same mother) increase the probability of the boy's being gay even if the are raised apart (Bogaert, 2006). Furthermore, this "fraternal birth order effect" is seen in boys who are moderately right-handed, but not in left-handed or extremely right-handed boys (Blanchard et al., 2006; Bogaert, 2007), providing another indication of differences in early development between the two sexual orientation groups. Statistically, the birth order effect is strong enough to estimate that about one in every seven homosexual men in North America - about a million people - are gay because their mother had sons before them (Cantor et al., 2002)
Genetic studies in fruit flies have identified genes that control whether courtship behaviors are directed at same- or opposite-sex individuals (Grosjean et al., 2008l S.D. Zhang and Odenwald, 1995), although no one knows the extent to which similar mechanisms are operation in mammals, including humans. Still, there is evidence that human sexual orientation is at least partly heritable, reinforcing the notion that both biological and social factors have a say. About 50% of variability in human sexual orientation is accounted for by genetic factors, leaving ample room for early social influences. Monozygotic twins, who have exactly the same genes, do not always have the same sexual orientation (J.M. Bailey et al., 1993). In the unusual case of two non-twin brothers who are both homosexual, genetic evidence suggests that they are much more likely than chance would dictate to have both inherited the same X chromosome region (called Xq28) from their mother (Hamer et al., 1993); but again the genetic explanation accounts for only some, not all, of the cases. It seems clear that there are several different pathways to homosexuality.
From a political viewpoint, the controversy - whether sexual orientation is determined before birth or determined by early social influences - is irrelevant. Laws and prejudices against homosexuality are based primarily on religious views that homosexuality is a sin that some people "choose." But almost all homosexual and heterosexual men report that, from the beginning, their interests and romantic attachments matches their adult orientation. So any social influence would have to be acting very early in life and without any conscious awareness (do you remember "choosing" whom to find attractive?). Furthermore, despite extensive efforts, no one has come up with a reliable way to change sexual orientation (LeVay, 1996). These findings, added to evidence that older brothers and prenatal androgens affect the probability of being gay, have convinced most scientists that we do not choose our sexual orientation."
Breedlove, S.M., Watson, N.V., Rosenzweig, M.R. Biological Psychology: An Introduction to Behavioral, Cognitive, and Clinical Neuroscience, 6th Edition. Sinauer Associates, 2010.
