PRENATAL AND PERINATAL ENVIRONMENT
The turning point in our understanding of the effect of environmental factors in sexual orientation was generated by the work of Gunter Dorner. Probably because Dorner was based in East Berlin his pioneering research in the 1970s was not widely noticed (Dorner, 1972; Dorner, 1976; Dorner, 1977). Before Dorner there had been unsuccessful attempts to compare the hormonal profiles of adults expressing different sexual orientations. Dorner’s studies revealed the importance of the critical period when the sexual differentiation of the brain happens. While this critical period may vary slightly from one species to another, it is always around the time of birth.
Dorner started with animal experiments. Male rats were castrated on the first day of life and were injected with male hormones when adults. These male rats expressed a complete inversion of sexual behaviour. In other words, being deprived of testosterone during the critical fetal period of sexual determination produced homosexual behaviour in their adult lives.
What we know now about the hormonal profile of homosexuals fits perfectly with the hypothesis of a transitory lack of testosterone during the critical period. Homosexuals usually have the same level of total testosterone as heterosexuals, but their level of ‘free testosterone’ (testosterone that is not combined with other chemicals) is lower. The levels of pituitary hormones, which control testicular functions, are relatively high and so are the levels of oestrogens. It is important to realize is that if this hormonal profile were to be artificially reproduced in an adult, it would not give rise to homosexual behavior. When a fetus is faced with a lack of testosterone at the end of pregnancy it compensates for this by increasing secretions of pituitary hormones. At the same time as the fetus tries to increase the level of male hormones by a feedback mechanism, it increases in parallel the level of oestrogens. In fact, oestrogens increase the binding capacity of sexual hormones with proteins and lower the level of free testosterone.
This raises the question of how and why some fetuses lack male hormones at the end of pregnancy. The answer could be that certain stressful situations at this time might trigger a high level of activity in the mother’s adrenal glands. The adrenal glands release male hormones the action of which is different from testosterone, but similar enough to compete with testosterone in the fetal brain to lower the amount of free testosterone. Furthermore, a complementary question is raised: Can prenatal stress play a causal role in human male homosexuality?