ABSTRACT It is proposed that defense mechanisms may be characterized as forming a hierarchy, from least to most complex, and that the lowest level defenses emerge early in life, while the more complex defenses emerge later in development Three defenses–Denial, Projection and Identification–were chosen to test this assumption A method for assessing the use of these defenses in Thematic Apperception Test (TAT) stories was developed and validated in a study of four age groups preschool, elementary school, early adolescent, and late adolescent The results of the study were consistent with the prediction Denial was used most frequently by preschool children, and decreased in use thereafter Identification was used minimally by preschool children but increased steadily through adolescence The use of Projection was most frequent in the two middle age groups Some evidence for sex differences, based on the internal/external orientation of the defense, also was found