A masculinizing effect on the auditory systems of human females having male co-twins.

Spontaneous otoacoustic emissions (SOAEs) are continuous, essentially tonal sounds that are produced by many normal-hearing cochleas. In humans, females generally exhibit more SOAEs than males, a sex difference that exists from birth. However, it is shown here that females having male co-twins [opposite-sex dizygotic (OSDZ) females] exhibit about half the average number of SOAEs per ear observed in same-sex female twins or female non-twins. Indeed, the average in OSDZ females is about the same as that seen in males--twins or non-twins. The explanation offered here is that prenatal exposure to high levels of androgens has produced a masculinizing effect on the auditory systems of these OSDZ females. Prenatal masculinizing effects have long been recognized in certain litter-bearing mammals, but their existence in humans is not well-studied.