The present study was designed to (1) investigate the changes in the structure and organization of mental abilities during childhood and adolescence of 514 students as sampled by the Wechsler Intelligence Scale for Children (5 to 15 years) and the Wechsler Adult Intelligence Scale (16 to 18 Years) and (2) interpret, and possibly modify, the current theories of cognitive and psychomotor development. The findings of the study are summarized and an overview of intellectual developmental theories, with reference to the differentiation hypothesis, is presented. (Author/CK)