Sex and gender are key to people's lives, and are the focus of scientific and popular interest and controversy. Sex-related psychological characteristics reflect more than socialization; they are influenced by sex hormones present during sensitive periods of development, particularly prenatal androgens. Studies of females with congenital adrenal hyperplasia (CAH) show how prenatal androgens affect behavior across the life span, with large effects on gendered activity interests and engagement, moderate effects on spatial abilities, and relatively small (or no) effects on gender identity, gender cognitions, and gendered peer involvement. In addition to showing the complexity of androgen effects on gendered behavior, studies of females with CAH provide an opportunity to test theories of gender development, understand how nature and nurture work together, and examine mechanisms of development. The implications of this work have often been misunderstood, so we consider what it means - and does not mean - for biology to influence gender-related behavior.