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Prenatal hormones

The prenatal hormonal theory of sexual orientation suggests that people who are exposed to excess levels of sex hormones have masculinized brains and show increased homosexuality or bisexuality. Studies providing evidence for the masculinization of the brain have, however, not been conducted to date. Research on special conditions such as congenital adrenal hyperplasia (CAH) and exposure to diethylstilbestrol (DES) indicate that prenatal exposure to, respectively, excess testosterone and estrogens are associated with female–female sex fantasies in adults. Both effects are associated with bisexuality rather than homosexuality.

There is research evidence that the digit ratio of the length of the 2nd and 4th digits (index finger and ring finger) is somewhat negatively related to prenatal testosterone and positively to estrogen. Studies measuring the fingers found a statistically significant skew in the 2D:4D ratio (long ring finger) towards homosexuality with an even lower ratio in bisexuals. It is suggested that exposure to high prenatal testosterone and low prenatal estrogen concentrations is one cause of homosexuality whereas exposure to very high testosterone levels may be associated with bisexuality. Because testosterone in general is important for sexual differentiation, this view offers an alternative to the suggestion that male homosexuality is genetic.

The prenatal hormonal theory suggests that a homosexual orientation results from exposure to excessive testosterone causing an over-masculinized brain. This is contradictory to another hypothesis that homosexual preferences may be due to a feminized brain in males. However, it has also been suggested that homosexuality may be due to high prenatal levels of unbound testosterone that results from a lack of receptors at particular brain sites. Therefore, the brain could be feminized while other features, such as the 2D:4D ratio could be over-masculinized.