Genderless at conception, the brain becomes more masculine in the embryo and then, during infancy, is further altered by a veritable tsunami of hormones. One of the subject's finest specialists lucidly details the exact extent to which genes and sexual hormones, in concert with the environment, influence boys' behavior, demonstrating how the male brain changes over time. Jacques Balthazart is a neuroendocrinologist and emeritus researcher with the Behavioral Neuroendocrinology Research Group at the University of Liège.