Recent advances in theory and research imply that additional distinctions are needed in terms of peer relationships. Theoretically, peers traditionally have been considered to be any group of children who are interacting with one another in a definable place and at a particular time. Recent advances in theory, however, have led to a redefinition of "peers" as children who display behaviors of a similar level of complexity and who hold similar social status in their group (Hartup, 1978; Lewis & Rosenblum, 1975). Although written in terms of similarities among children, this new definition implies that children are not a homogeneous group. Obvious though this point is, it was not a part of earlier definitions. Recent theory also suggests that children form relationships by selecting playmates according to a number of dimensions, the most relevant of which for purposes of the present study are age and sex (Lewis & Feiring, 1979). Although the fact that preschool-age children prefer playmates similar in age and sex to themselves has long been known (Challman, 1932; McCandless & Hoyt, 1961), recent research clearly demonstrates that general social interaction behaviors among young children vary as a function of whether playmates are of the same or different ages. For example, when preschool-age children were paired with familiar, samesex peers who were either similar (i.e., 2 months of age) or different (i.e., 16 months) in age, Lougee, Gruenich, and Hartup (1977) found