This paper examines sex differences in achievement expectancies from a cognitive-developmental point of view. It has been reported consistently that females have lower initial expectancies for success than males. The antecedents of this difference are considered: (a) by examining when expectancy differences develop, and (b) by examining cognitive factors which may underlie these differential expectancies. The discussion of the first question is primarily empirical in nature; previous developmental studies of expectancy are reviewed and original data presented. Discussion of the second question is more theoretical and speculative. Based on the Weiner attributional model of achievement, the role of subjective perceptions in mediating expectancy differences is considered.