A birth order and sexual orientation relationship has been demonstrated numerous times in men, but a related variable, parental age (i.e. age of parents when the participant was born), has been less studied and has demonstrated contradictory results. In this research, the relations among birth order, parental age and sexual orientation were examined in a national probability sample of the US (Kessler, 1994; Kessler
et al., 1994) and in a Canadian sample of homosexual and heterosexual men closely matched on demographic characteristics (Blanchard & Bogaert, 1996a). In both studies, an interaction between birth order and parental age was observed in men, such that there was positive association between number of older siblings and the likelihood of homosexuality, but this association weakened with increasing parental age. No significant effects were observed for women. The results are discussed in relation to recent theories of the birth order/sexual orientation relationship.