Birth order and sibling sex ratio in homosexual male adolescents and probably prehomosexual feminine boys.

The purpose of this study was to extend the findings, previously limited to adults, that male homo sexuals 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 (MP) group in cluded boys referred to a specialty clinic because of persistent cross-gender behavior plus homosex ual adolescents with or without gender identity problems. The controls were male child and adoles cent patients referred for reasons other than gender identity disorder, homosexuality, or transvestism. Both predicted results were obtained in comparisons of the MP 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