This paper explores young children's motivation for gender-stereotypical preferences by comparing two theories, both based on Kohlberg's stages of cognitive understanding within the cognitive developmental tradition. The first, elaborated by Kohlberg, suggests that gender-stereotypical preferences are the result of the child's cognitive understanding of the constancy of their gender. The second theory suggests that it is precisely the lack of certainty of gender constancy that influences gender-stereotypical behavior. Data from a cross-sectional study of 100 children sampled during their first year of school, and longitudinal case studies of 10 children during the transition to school, are brought to bear on these two theories. The sample was drawn from a range of working class and middle class home backgrounds. The children were mainly White, with 6% Asian-Indian in the cross-sectional sample, and 1 Asian-Indian child in the longitudinal group. The study finds an association between gender-stereotypical behavior and the attainment of gender constancy, suggesting support for the first theory.