Evidence from Opposite-Sex Twins for the Effects of Prenatal Sex Hormones
Edward M. Miller
The animal literature reports that females that are adjacent to males in the uterus develop more masculine morphological and behavioral traits than do females adjacent only to other females. This can be attributed to hormones transferring between the fetuses and in turn altering the developing brain.
If the effect occurs in humans, twins whose womb mates were of the opposite sex (OS) would be morphologically or behaviorally less sex typical than twins whose womb mates were of the same sex (SS). This chapter will start by reviewing the animal literature. It will then discuss various morphological effects in humans that might be explained by hormone transfer, notably some otherwise inexplicable dental data. Various human behavior and cognitive ability data relating to opposite-sex twins are then presented; opposite-sex twins seem less sex typical in their abilities and emotions. This is followed by research done by the present author, using British and Australian data sets, showing that the sex of the co-twin affects the attitudes of female twins, such that females who shared the womb with males have more masculine attitudes than those who shared the womb with females. The results are interpreted as support for the joint hypotheses that hormones transfer and prenatal hormones affect the extent to which the brain exhibits masculine and feminine typical behaviors and abilities. In closing, a plea is made for more opposite-sex twin research.