Cognitive development in retarded and nonretarded persons: Piagetian tests of the similar sequence hypothesis.

From the debate over developmental "universals" in Piagetian theory and the controversy between developmental and difference theories of mental retardation, an important hypothesis emerges—one that is testable via cognitivedevelopmental comparisons between retarded and nonretarded persons. This similar sequence hypothesis holds that retarded and nonretarded persons traverse the same stages of cognitive development in the same order, differing only in the rate at which they progress and in the ultimate developmental ceiling they attain. Current evidence relevant to this hypothesis is drawn from 3 longitudinal and 28 cross-sectional studies of developmental phenomena described by Piaget. The great preponderance of this evidence supports the hypothesis with respect to every subject group, with the possible exception of individuals suffering from pronounced electroencephalogram abnormalities. The quality of current evidence is critically evaluated, and procedures by which more precise tests of the hypothesis might be fashioned are proposed. Overall, the review illustrates that developmental research with atypical populations can be a potent tool in testing general developmental theory. Conversely, it illustrates the power of general developmental theory to enrich our understanding of atypical development.