Update on the Biology of Transgender Identity

Numerous studies have attempted to isolate biological factors in the development of transgender identities through research into genetics, prenatal hormone exposure, neuroanatomy, and cognitive processing. Genetic studies demonstrate that chromosomal variations are uncommon but may occur at higher rates than in the general population. Candidate genes have been investigated, with some positive results, though these have yet to be replicated. Investigations into the effects of hormone exposure on the developing fetus have focused on gender identity in intersex people, which is often unpredictable, and proxy markers for prenatal hormone exposure such as finger length ratio and birth order, which do not show clear trends in transgender groups. A few small neuroanatomical studies show distinctions in transgender people, but results are limited in their scope due to small sample sizes and confounding variables such as adult hormone exposure. Numerous studies demonstrate that male-to-female (MTF) transgender people have higher rates of left-handedness, but the theoretical basis for this difference is not well described. Accumulating evidence indicates that prenatal biology likely contributes to transgender identity, but that its role may be interactive, rather than deterministic.