THE RELATIONSHIP BETWEEN PARENTAL AGE, BIRTH ORDER and THE SECONDARY SEX RATIO IN HUMANS*

Abstract

SUMMARY An analysis is made of the change in secondary sex ratio in humans as the ages of the parents increase, and also as the age of the mother and birth order increases. It is shown that both age of the father and birth order are significantly correlated with this change in sex ratio, whereas the age of the mother is not. The changes with increasing birth order and with increasing age of the father are so similar when properly compared, that they appear quite likely to be caused by the same underlying factor.