title: Fraternal birth order and ratio of heterosexual/homosexual feelings in women and men.
abstract: Studies of the 2-3% of persons who identify as homosexual found men but not women had more older brothers than persons who identify as heterosexual. The present study investigated the birth order in the approximately 20% of men and women who anonymously report some homosexual feelings, few of whom identify as homosexual. The number of older brothers and sisters was investigated in seven cohorts: 319 male twins; and 49, 54, and 61 female and 66, 116, and 50 male medical students. Both women and men who anonymously reported homosexual feelings had a greater mean number of older brothers and sisters than did those who reported no homosexual feelings. The difference was stronger in relation to brothers than sisters. The birth order effect was not related to the strength of the subjects' degree of homosexual compared with heterosexual feelings. Its presence in women could not be accounted for by the widely accepted hypothesis that the birth order effect is due to a maternal immune reaction provoked only by male fetuses. The lack of relationship between the strength of the effect and degree of homosexual feelings in the men and women suggests the influence of birth order on homosexual feelings was not due to a biological, but a social process in the subjects studied. Investigating the neglected significant percentage of predominantly heterosexual men and women who anonymously report some homosexual feelings may aid in understanding the factors influencing sexual orientation, and identity.
authors.affiliation: University of New South Wales, New South Wales, Australia.
authors.forename: Nathaniel
authors.initials: N
authors.lastname: McConaghy
authors.forename: Dusan
authors.initials: D
authors.lastname: Hadzi-Pavlovic
authors.forename: Carol
authors.initials: C
authors.lastname: Stevens
authors.forename: Vijaya
authors.initials: V
authors.lastname: Manicavasagar
authors.forename: Neil
authors.initials: N
authors.lastname: Buhrich
authors.forename: Ute
authors.initials: U
authors.lastname: Vollmer-Conna
mesh_terms: D000328:Adult; D001722:Birth Order; D003219:Confidentiality; D005260:Female; D020010:Heterosexuality; D006716:Homosexuality; D006801:Humans; D008297:Male; D012727:Sex Characteristics; D012744:Sex Ratio; D035781:Siblings; D011795:Surveys and Questionnaires