Many people believe that sexual orientation (homosexuality vs. heterosexuality) is determined by education and social constraints. There are, however, a large number of studies indicating that prenatal factors have an important influence on this critical feature of human sexuality. Sexual orientation is a sexually differentiated trait (over 90% of men are attracted to women and vice versa). In animals and men, many sexually differentiated characteristics are organized during early life by sex steroids, and one can wonder whether the same mechanism also affects human sexual orientation. Two types of evidence support this notion.
It has been shown in several animal species that experimental manipulations of the hormonal environment during the prenatal or early postnatal life can reverse sexual partner preferences so that a male will now try to mate with another male and a female with another female. Sexual orientation is thus determined in animals by the perinatal endocrine environment.
This type of experimental manipulation is impossible in humans for obvious ethical reasons but two types of correlative evidence indicate that the mechanisms identified in animals are still functional in humans.
On the one hand, multiple sexually differentiated behavioral, physiological, or even morphological traits are significantly different in homosexual and heterosexual populations. Because some of these traits are known to be organized by prenatal steroids, including testosterone, these differences suggest that homosexual subjects were, on average, exposed to atypical endocrine conditions during development.
On the other hand, clinical conditions associated with significant endocrine changes during embryonic life result in an increased incidence of homosexuality. It seems therefore that the prenatal endocrine environment has a significant influence on human sexual orientation but a large fraction of the variance in this behavioral characteristic remains unexplained to date. Genetic differences affecting behavior either in a direct manner or by changing embryonic hormone secretion or action are also clearly involved. How these biological prenatal factors interact with postnatal social factors to determine life-long sexual orientation remains to be determined but current evidence clearly points to a very significant role of hormonal and genetic factors in the control of human sexual orientation.
These data have also been summarized on the Reflexions web site of the University of Liège at
http://www.reflexions.uliege.be/cms/c_415441/en/sexual-orientation-in-all-its-aspects
