Birth order in girls with gender identity disorder

This study examined the birth order of girls with gender identity disorder (N=22). Each proband was matched to 3–7 clinical control girls for age at assessment and number of siblings (the mode number of controls per proband was 7) (total N=147). The number of older brothers, older sisters, younger brothers, and younger sisters was recorded. Slater’s birth order index showed that the probands were significantly more likely to be early born than were the controls. A modified Slater’s index also compared the birth order of the probands and the controls only to their brothers (when they had one or more) and only to their sisters (when they had one or more). Compared to the controls, the probands were born early compared to their sisters, but not to their brothers. These findings are the inverse of two previous studies of boys with gender identity disorder, who were later born relative to clinical control boys (11), an effect that appeared to be accounted for primarily by being born later relative to older brothers, but not to older sisters (46).