The purpose of this study was to extend the findings, previously limited to adults, that male homosexuals have a greater than average proportion of male siblings and a later than average birth order. There were 2 matched groups of 156 probands. The homosexual-prehomosexual (HP) group included boys referred to a specialty clinic because of persistent cross-gender behavior plus homosexual adolescents with or without gender identity problems. The controls were male child and adolescent patients referred for reasons other than gender identity disorder, homosexuality, or transvestism. Both predicted results were obtained in comparisons of the HP group with the controls and with expected values for the general population. Psychosocial and biological theories have been advanced to explain why male homosexuals have later births and more brothers, but none are well established.Previous investigations of the etiology of male homosexuality may be divided into two classes: studies showing that the erotic preference for male partners tends to be associated with other traits typical of the opposite sex, and studies relating male homosexuality to epidemiological and demographic variables of traditional interest in psychology and medicine.Investigations of the first class include prospective and retrospective studies showing that prehomosexual boys have a stronger interest in girls' games and toys, female playmates, feminine roles in fantasy play, and so on than do preheterosexual boys (