This document discusses research on the influence of hormones on gender identity and behavior. Several studies are reviewed that show prenatal testosterone exposure increases male-typical behavior in both males and females. Individuals with congenital adrenal hyperplasia or androgen insensitivity syndrome also demonstrate more male-typical traits and identities than controls. However, the findings are mixed, and social and medical factors complicate determining the role of biology alone. Overall, gender appears to be influenced by a complex interplay of biological and environmental factors.