Evidence of the similar-structure hypothesis controlling for organicity.

The similar-structure hypothesis and the two-group approach to mental retardation, proposed by Zigler (1967, 1969) and his colleagues, was investigated. Nonretarded, socioculturally mildly retarded, organically impaired retarded, and organically impaired moderately retarded children matched for MA were compared on moral and cognitive reasoning tasks. No differences were found between the nonretarded and socioculturally retarded subjects or between the two organically impaired groups. Differences were found between the organically impaired groups and the nonretarded and socioculturally retarded groups. When the effect of cognitive reasoning was partialled out of the moral reasoning scores, however, no differences were found among the four groups. The findings supported Ellis' (1963, 1969) difference theory as well as the similar-structure hypothesis but did not totally support Zigler's two-group position.