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Michigan State UniversityBreedlove Jordan lab at MSU

Do Fetal Androgens Affect Human Sexual Orientation?

In animal models of sex differences in behavior, early exposure to testosterone and other androgens masculinizes the brain so that, in adulthood, the animal displays male-typical behaviors. There are many sex differences in human behavior, but it is not clear whether these are due to the sex difference in fetal exposure to testosterone (as in animal models), or due to the sex difference in social stimulation provided to children. It is not easy to disentangle these two hypotheses in people who show gender-typical behaviors because males are exposed to both high levels of fetal androgens and male socialization pressures, while the females are exposed to both low levels of fetal androgen and female socialization pressures. But in those minority of humans who do not show all gender-typical behaviors, one can ask whether differences in social rearing and/or fetal testosterone plays a role.

Androgens Figure 1. In males, the index finger tends to be shorter than the ring finger. In female, the index finger tends to be as long or longer than the ring finger.

There is a sex difference in the ratio of the length of the index finger divided by the length of the ring finger (2D:4D). The 2D:4D tends to be smaller in males than in females (Figure 8). This sex difference has been reported to be present in two year old children, which suggests that it is due to the effects of fetal androgen reducing 2D:4D. We have found support for that notion, as the 2D:4D of people with congenital adrenal hyperplasia (CAH) are lower than that of same-sex controls. We would like to learn more about how fetal androgen affects the development of finger lengths.

Androgens Figure 2

We have found that the 2D:4D of homosexual women is, on average, lower than that of heterosexual women (Figure 9), a finding that has been replicated at least three times by other laboratories. We have also found evidence that 2D:4D is lower in males with older brothers than in males without older brothers. We would like to learn how older brothers influence finger development in subsequent brothers.