Sex differences in activity selection or choice appear by age 12 or 18 months. These choices are one of the earliest indicators of sex differences in the behavior of young children. Differences in activity participation or toy choices are evident long before the emergence of sex differences in personality characteristics like passivity or sex-typed behavior like aggression. Some theorists view sex-typed participation in activities as an outcome of an already developed gender identity or as an outcome of differential reinforcement or modeling. Sex-typed behaviors can be learned in at least two ways. In the first, antecedent events such as participation in sex-typed activities and toy preference lead to sex-typed behaviors of passivity and aggression. In the second, sex-typed behavior like aggression leads to a preference for sex-typed activities, toy preferences, or dress-style. Assuming that participation in sex-typed activities is an antecedent of sex-typed behavior, there is evidence that changes in sex-typed behavior can be expected as a function of exposure to or participation in activities. (Author/HLM)