Homosexuality in men and number of older brothers

R. Blanchard, A. Bogaert

Investigated was whether homosexual men have a higher mean birth order than heterosexual men primarily because they have more older brothers or because they have more older siblings of both sexes. 302 homosexual men were individually matched on year of birth with an equal number of heterosexual men. Homosexuality appeared to be positively correlated with the proband's number of older brothers but not with older sisters, younger brothers and sisters, or parental age at the time of his birth. Each additional older brother increased the odds of homosexuality by 33%. The results restrict the range of possible theories of the birth order phenomenon to those that can explain not only why older brothers increase the probability of homosexuality in later-born males but also why older sisters neither enhance this effect nor counteract it.