Demonstrated is a covariance principle that causes the observer to assume that if one aspect of a two-dimensional figure (its perimeter or its area) is conserved, the other aspect must also be conserved (pseudo-conservation). Mentally retarded individuals, assuming no such fixed relationship, correctly judged the changed state of the nonconserved…

These authors cite their own study of the relationship between traditional measures of intelligence and Piagetian measures of cognitive development in support of Glass and Stephens' contention that there are important qualitative differences. They question Humphreys' and Parsons' conclusions on both substantive and factor theoretical grounds. (CTM)