NCBI Bookshelf. Sex differences of importance to health and human disease occur throughout the life span, although the specific expression of these differences varies at different stages of life. Some differences originate in events occurring in the intrauterine environment, where developmental processes differentially organize tissues for later activation in the male or female. In the prenatal period, sex determination and differentiation occur in a series of sequential processes governed by genetic and environmental factors. During the pubertal period, behavioral and hormonal changes manifest the secondary sexual characteristics that reinforce the sexual identity of the individual through adolescence and into adulthood.