This study was planned to examine differences in the ability to stop and think, and knowledge of comparative concepts, among children who succeeded or failed in solving verbally presented class inclusion problems. Thirty third-grade boys were classified as non-Inclusion subjects, and 30 others as Inclusion subjects. The children were classified as average on estimates of verbal ability and socioeconomic status. Analysis of the data supported the hypothesis of a significant difference in the ability to stop and think, among children who were successful and unsuccessful in solving class inclusion problems. The two groups did not differ in their knowledge of comparative concepts. Class inclusion response patterns indicated a roughly bimodal distribution of this variable. The study raised questions about the nature of impulsivity as a multidimensional trait, and about alternative explanations for the positive finding, including the possible roles of memory, attention, and differences in individual strategic approaches to cognitive problem solving. (Author/EB)