The hormonal theory of sexuality holds that, just as exposure to certain hormones plays a role in fetal sex differentiation, such exposure also influences the sexual orientation that emerges later in the adult. Fetal hormones may be seen as the primary determiner of adult sexual orientation, or a co-factor with genes and/or environmental and social conditions.
Physiological factors present at birth may produce behaviours in children that are interpreted as gender difference. Such gendered behaviour may be an indicator of adult sexual orientation - a 1987 longitudinal study suggested that 75% of boys who exhibit cross-gender behaviour grow up to be gay or bisexual adults (Green 1987).
Cornell psychologist Daryl Bem has proposed a theory called "the exotic becomes erotic". In it he suggests children with childhood gender nonconformity (CGN), a biologically-caused attribute, are often ostracized by their own sex for their atypical behavior and this leads to physiological arousal of fear and anger in their presence, arousal that eventually is transformed from exotic to erotic.
William Reiner, a psychiatrist and urologist with the University of Oklahoma has evaluated more than a hundred cases of children born with sexual differentiation disorders. In the 1960s and 70s, it was common in developed countries for doctors to castrate boys born with a micropenis and have them raised as girls. However, this practice has come under attack, because even though these boys were raised as girls, they nearly all report as adults that they are sexually attracted to women. This suggests that their sexual orientation was determined at birth. The only cases Reiner found where children born with a X and Y chromosome are attracted to males as adults were those where testosterone receptors were absent, which prevented the male sex hormones from masculinizing the fetus.
There is evidence of a correlation between sexual orientation and traits that are determined in utero. Williams et al. (2000) found that finger length ratio, a characteristic controlled by prenatal hormones, is different in people of distinct sexual orientations. Another study by McFadden in 1998 found that auditory systems in the brain, another physical trait influenced by prenatal hormones is different in those of differing orientations, likewise the suprachiasmatic nucleus of homosexual men was found by Swaab and Hopffman to be larger in homosexual men than in heterosexual men, , the suprachiasmatic nucleus is also known to be larger in men than in women . Gay men have also been shown to have higher levels of circulating androgens and larger penises, on average, than straight men.
Gay men have more older brothers on average, a phenomenon known as the fraternal birth order effect. It has been suggested that the greater the number of older male siblings the higher the level of androgen fetuses are exposed to .
In a 1991 study, Simon LeVay demonstrated that a tiny clump of neurons of the anterior hypothalamus-which is believed to control sexual behavior and linked to prenatal hormones-was on average more than twice the size in heterosexual men when contrasted to homosexual men. Initially he could not rule out that this may be due to AIDS since all of his homosexual male subjects had died from it before the autopsies were performed. However in 2003 scientists at Oregon State University announced that it replicated his findings in sheep.
Girls with congenital adrenal hyperplasia (an autosomal recessive condition which results in high androgen levels during fetal development) have more masculinized sex role identities and are more likely to have a homosexual sexual orientation as adults than controls (Dittmann et al. 1990ab, 1992; Zucker et al., 1996; Hines et al., 2004). An alternative explanation for this effect is that the fact that girls with this condition are born with masculinized external genitalia leads their parents to raise them in a more masculine manner which then influences their sexual orientation as adults. However, the degree to which the girls' genitals are masculinized does not correlate with their sexual orientation, suggesting that prenatal hormones are the causal factor, not parental influence.
The alleged evolutionary disadvantage of homosexuality has led Amherst College biologist Paul W. Ewald to argue that it might be caused by an as-yet undetected virus working in utero that triggers hormonal responses.
In response to recent findings, the National Institute of Health in 2005 announced a 2.5 million dollar five-year study of sexual orientation.
The perceived causes of sexual orientation have a significant bearing on the status of sexual minorities such as lesbians and gays. The Family Research Council, a conservative Christian think tank in Washington, D.C., argues in the book Getting It Straight that finding people are born gay "would advance the idea that sexual orientation is an innate characteristic, like race; that homosexuals, like African-Americans, should be legally protected against 'discrimination;' and that disapproval of homosexuality should be as socially stigmatized as racism. However, it is not true."
One prominent evangelical leader, Rev. Rob Schenck, who used to advocate reparative therapy, came to believe that homosexuality is not a choice after speaking with scientists, and that conservative Christians need to drop the choice argument in order to continue opposing homosexual sex. He was quoted in the Boston Globe as saying "if it's inevitable that this scientific evidence is coming, we have to be prepared with a loving response. If we don't have one, we won't have any credibility."
Some advocates for the rights of sexual minorities also resist the idea that sexuality is biologically determined, or fixed at birth. They point out that sexual orientation can shift over the course of one's life. Many resist any attempts to pathologise or medicalise 'deviant' sexuality, and choose to fight for acceptance in a moral or social realm. Others fear that discoveries about medical causes of sexuality may be used by doctors and parents to eradicate non-heterosexual orientations.
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