This review of research conducted on sex differences attempts to answer the following questions: (1) Do the sexes differ in their basic capacities for specific types of mental performance and occupational achievement? (2) If such differences exist, to what extent are they genetically determined? (3) Are sex differences in abilities, interests, and psychological traits determined by genetics, environment, or a combination of both factors? (4) Do they appear in early life, develop, and interact with other factors, and can they be measured and detected at successive ages? (5) What significance do the sex differences possess with regard to the school curricula, the educational programs, and the possibilities for the increased utilization of women in business, industry, the professions, and public life? The review of literature centers areound the following topics: insufficient consideration of sex differences in research, early studies of sex differences, biological sex differences, manifestations of sex differences in the first year of life, sensory discrimination, and sex differences in abilities, interests, and intellectual performance. Conclusions are discussed in terms of scholastic and occupational achievement, practices which disadvantage one sex or the other, and sex-related variables which require a better understanding on the basis of appropriate research. An extensive bibliography is included. (JS)