A developing body of research suggests that there are few sex differences in the rate and severity of problem behavior in early childhood, but clear sex differences emerge at about 4 years of age. The authors explore 2 hypotheses to further the understanding of emerging sex differences in problem behavior across the first 5 years of life. The first posits that the change in girls' problem behavior from infancy to school entry represents a channeling of early problem behavior into predominantly internalizing problems as a result of socialization. The second hypothesis is that the change in girls' early problem behavior during the preschool period results from the more rapid biological, cognitive, and social-emotional development of girls relative to boys. The authors review research on the influence of parents, teachers, and peers on girls' behavior from infancy to preschool regarding the first hypothesis, whereas they review studies of sex differences in developmental processes to test the second. They find moderate support for both hypotheses and present a comprehensive theory of girls' developmental psychopathology that integrates social and developmental influences.