2D:4D finger-length ratios in children and adults with gender identity disorder

Previous research suggests that prenatal testosterone affects the 2D:4D finger ratio in humans, and it has been speculated that prenatal testosterone also affects gender identity differentiation. If both things are true, then one would expect to find an association between the 2D:4D ratio and gender identity. We measured 2D:4D in two samples of patients with gender identity disorder (CID). In Study 1, we compared the 2D:4D ratios of 96 adult male and 51 female patients with CID to that of 90 heterosexual male and 112 heterosexual female controls. In Study 2, we compared the 2D:4D ratios of 67 boys and 34 girls with CID to that of 74 control boys and 72 control girls. In the sample of adults with CID, we classified their sexual orientation as either homosexual or non-homosexual (in relation to their birth sex) to examine whether or not there were any within-group differences as a function of sexual orientation. In the sample of adult men with CID (both homosexual and non-homosexual) and children with CID, we found no evidence of an altered 2D:4D ratio relative to same-sex controls. However, women with CID had a significantly more masculinized ratio compared to the control women. This last finding was consistent with the prediction that a variance in prenatal hormone exposure contributes to a departure from a sex-typical gender identity in women. (C) 2008 Elsevier Inc. All rights reserved.