Piagetian causality was studied with a male and a female chimpanzee (Pan troglodytes), whose ages were 2:6 and 1:10 years respectively at the beginning of the study. Tasks derived from Piaget

The relation between class inclusion developmental stages and the logical necessity of the judgments provided by the children is not yet strongly established. 192 children from grades 2 to 6 showing three different levels of class-inclusion answers (failure, correct answer based upon counting, correct answer based upon logical reasons) were submitted to one out of four different necessity tasks. In these tasks, children were required (1) to solve class-inclusion problems when the use of empirical means of verification was prevented; (2) to maintain their inclusion judgment despite a modification of the initial display; (3) to discover the B = A relationship in initial displays or through modification; and (4) to understand the impossible (B <A) and possible relationships (B > A, B = A). Results indicate (1) that subjects using logical reasons differ from those using counting in only one specific situation, the ability to reject the impossible case, while subjects who fail class-inclusion problems also fail all necessity tasks, (2) that the four necessity tasks are not equally difficult, suggesting a gradual mastery of these problems, and (3) that the necessity tasks are more difficult than the usual class-inclusion problems. Results are discussed with reference to the two kinds of generalization process (inductive and constructive) hypothesized by Piaget and Henriques (1978).

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Sixty‐two French‐Canadian children, seven to ten years old; 64 schooled Rwandese children, ten to thirteen years old; and 32 unschooled Rwandese subjects aged 15 and 17 were tested on four piagetian logico‐mathematical tasks: hierarchical classification (CA), multiple classification (CM), seriation of weight (RA) and double seriation (RM). Due to the effect of order, separate analyses were performed for tasks done during the same test session. When the results of tasks CA and RM, on the one hand, and of RA and CM, on the other hand, are combined, it is observed, firstly, that the items alternate in order of difficulty and, secondly, that the number of coherent patterns of responses exceeds the null hypothesis in French‐Canadian and schooled Rwandese populations, supporting the hypothesis of a complementary and partially synchronous modality of structuration of logical abilities. From another point of view, when the items from all four tasks are combined in a single scale of difficulty, the very close similarity observed between the ranks of the items in the three populations strengthens the hypothesis of a universally determined cognitive development based mainly on the equilibration process.