Homosexuality in men and number of older brothers

This study investigated whether homosexual men have a lighter 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 years of birth with an equal number of heterosexual men. Each completed an anonymous questionnaire concerning family background and other biodemographic information. Homosexuality was positively correlated with the proband's number of older brothers but not with older sisters, younger brothers, younger sisters, or paternal age at the time of the proband's birth. Each additional older brother increased the odds of homosexuality by 33%. These results restrict the range of possible theories of the birth order phenomenon. The point is to 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.