Fraternal birth order and ratio of heterosexual/homosexual feelings in women and men.

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.