THE RELATION OF BIRTH-ORDER TO SEXUAL ORIENTATION IN MEN AND WOMEN

Homosexual men have a higher mean birth order than do heterosexual men , primarily because they have a greater number of older brothers. The purpose of this study was to determine whether the same difference occ urs in homosexual vs heterosexual women. The probands were 964 homosex ual and heterosexual, male and female volunteers, from whom birth orde r data were collected with self-administered questionnaires. The homos exual men had more older brothers than the heterosexual men, but they did not have more older sisters, younger brothers, or younger sisters. The homosexual women did not differ from the heterosexual women with regard to any class of sibling. These results are consistent with the hypothesis that the high birth order of homosexual men reflects the pr ogressive immunization of certain mothers to H-Y antigen by succeeding male fetuses, and the increasing effects of H-Y antibodies on sexual differentiation of the brain in succeeding male fetuses.