Piaget, in describing the sequence of classificatory development, describes class inclusion as composed of two processes; hierarchical classification and post-whole comparisons. In the experiment reported here, elementary school children, trained in the concept of sets in first grade mathematics were given a task where they were required to assess a relationship between majority subclass and its superordinate class. Second and third graders were significantly more capable than were first graders, but no sex differences were indicated. Performance on the relational assessment task, (Hierarchical classification of subsets and superordinate sets) was better than performance on the traditional task where subjects compared subordinate and superordinate sets. (KS)