Human gender development begins before birth. Sex chromosomes and sex hormones are the two major sources of prenatal influence on lifelong neurobehavioral gender phenotypes. Differential expression of genes during the embryonic stage of life begins the processes by which sex differences develop. Variation in exposure to prenatal hormones, particularly testosterone, is an important contributor to behavioral and cognitive differences between the sexes, as well as an important explanatory factor for why certain individuals are more sex typical than others. In addition, males and females adjust their neurobehavioral developmental trajectories differentially in response to the same signals.