A cross-sectional test of the similar-trajectory hypothesis among adults with mental retardation

The similar-sequence and the similar-structure hypotheses are the two mainstays of the developmental approach to mental retardation. In the present study, a third way, the similar-trajectory hypothesis, is described and illustrated using the WAIS-R results of adults with and without mental retardation aged from 20 to 54 years. The whole sample (N = 633) comprised 306 participants with mental retardation and 327 without mental retardation. Hierarchical regression analyses comparing the two groups showed similar evolutions of scores with increasing age for verbal and performance scales. These results seem to validate the similar-trajectory hypothesis, at least for the present samples and for the aspects of cognitive development considered here. Some weaknesses and implications of the study are considered in the discussion.