Gender identity: nature and nurture, but mainly nature

Reiner & Gearhart's NEJM Study on Cloacal Exstrophy - Review by Vernon Rosario, M.D., Ph.D.:

Of their 16 affected subjects, 14 had been reassigned female at birth. At follow-up, between ages 5 to 12, 8 of those reassigned female now identified as boys. The children and their parents completed a battery of questionnaires assessing psychosexual development, sexual identity, and gendered behavior. Follow-up assessments were done at least annually ranging from 34 to 98 months follow-up. All of the subject had moderate-to-marked male-typical attitudes and interests. The two children reared as males continued to identify as males. Of the 14 reassigned female at birth, five still persistently identified as girls, four spontaneously declared a male identity, and four chose to identify as boys after told that they were born male.

While nature can explain why most of them later identified as male, the 5 who didn't suggest a significant role for the environment in the development of gender identity.