A study of emerging self-awareness in children 18-24 months, which suggests that the two-year-old's ""I can't do that"" is a major achievement, the consequence of a significant cognitive maturation. Kagan, the noted Harvard psychologist, discusses the subtle signs of self-awareness at some length, and formally but not drily presents his findings on children in Cambridge, Mass., Winston-Salem, North Carolina, and the Fiji Islands. Around 18 months of age, he observes, children become more alert to adult standards; they show appropriate emotion--the ""mastery smile""--upon successful completion of a chosen task; they give directives to adults to change their behavior; and they use language to describe their own actions. These new competences, which emerge along with sharply improved memory skills, are characterized with Kagan's customary insight and precision. The fact that these capacities appear at the time of important histological changes in the cerebral cortex gives his findings added dimension, though he is quick to dissociate himself from identity theory (""which holds that all statements about behavior will eventually be reduced to sentences that contain only biological concepts"") and to insist that he has not studied the relationship between brain growth and behavioral development. Kagan tends to call on older theorists for support and contrast, and he gives some celebrated contemporaries (Piaget, Mahler) the cold shoulder--which will leave some readers with a slightly battered frame of reference. Nonetheless, Kagan always comes up with new and exciting ways of reviewing familiar behaviors; he consistently reflects on the premises of his and comparable researches; and he generally addresses the questions his material and methodology raise. A thoughtful, often profound study of ""the ordered events in the growth of consciousness.