This research investigated development of cognitive abilities in a normal vs. emotionally disturbed school age population (N = 240) ages 6-12. The Ss had to display skills on the Southern Illinois University Test necessary to show understanding of Piagetian concepts of class inclusion, class exclusion, and complement of set. A three way analysis of variance indicated significant main effects for age, sex, and emotionality factors. There were, however, no significant interactions among these factors. Emotionally disturbed children were not only deficient in the measured cognitive skills, but even the rate of development of these cognitive skills was inferior to that of the normal population.