Sibling sex ratio of boys with gender identity disorder.

Sibling sex ratio (the ratio of brothers to sisters) was calculated for 444 boys with gender identity disorder (or with behaviors consistent with this diagnosis). The probands were ascertained from several researchers with expertise with this disorder and from the English language case report literature between 1938 and 1995. Among the probands with at least one sibling (N = 333), the results showed that boys with gender identity disorder had a significant excess of brothers to sisters, 131.1:100, when compared with the expected secondary sex ratio of 106:100. The excess of brothers replicated a previous study by Blanchard, Zucker, Bradley, and Hume (1995), in which the sibling sex ratio was 140.6:100. Further analyses showed that the probands were born later relative to their brothers than they were relative to their sisters. These findings are amenable to several psychosocial and biological explanations, which require further investigation.