A COMPARISON OF ADULT FEMALE TWINS FROM OPPOSITE-SEX AND SAME-SEX PAIRS ON VARIABLES RELATED TO REPRODUCTION

In several litter-bearing species, prenatal exposure of a female fetus to hormones from adjacent male fetuses can lead to later effects on v arious anatomical and behavioral characteristics of the female, includ ing a number related to reproduction. To see if such traits are also a ffected in humans, adult female twins from a large Australian sample w ho had male cotwins were compared to females with female cotwins on 90 questionnaire items related to reproductive functions. No substantial effects could be clearly demonstrated, although some weak effects rem ained a possibility. Some variables, such as age at first menstruation , age at first pregnancy, and height, were consistent in direction wit h results from the animal literature, although the effect sizes were s mall and not statistically significant.