OBJECTIVE: This study investigated 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. METHOD: For the main analyses, 302 heterosexual men were individually matched on year of birth with an equal number of heterosexual men. Each completed a self-administered, anonymous questionnaire concerning family background and other biodemographic information. RESULTS: Logistic regression analysis showed that homosexuality was positively correlated with the proband's number of older brothers but not with older sisters, younger brothers, younger sisters, or parental age at the time of the proband's birth. Each additional older brother increased the odds of homosexuality by 33%. CONCLUSIONS: These 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.Abstract Teaser