It has been known for some time that men with older brothers are more likely to be homosexual than men without older brothers. One explanation for this effect has been that this is a phenomenon of socialization related to the environment in which males are raised.

Bogaert (2006) investigated this possibility by studying 944 people and looking at the number of older brothers and sisters an individual had and whether those were biological siblings or step-siblings. With older biological brothers the probability of homosexuality increased from 3% to around 5%, but there was no similar increase with older stepbrothers or with biological sisters or stepsisters. How should Bogaert analyze these data?