The idea that all people have an innate “gender identity” recently has been endorsed by many health-care professionals and mainstream medical organizations. This term commonly is defined to mean the “internal, deeply held” sense of whether one is a man or a woman (or, in the case of children, a boy or a girl), both, or neither. It also has become common to claim that this sense of identity may be reliably articulated by children as young as three years old. While these claims about gender identity did not attract systematic scrutiny at first, they now have become the subject of criticism from a growing number of scientists, philosophers and health workers. Developmental studies show that young children have only a superficial understanding of sex and gender (at best). For instance, up until age 7, many children often believe that if a boy puts on a dress, he becomes a girl. This gives us reason to doubt whether a coherent concept of gender identity exists at all in young children.