In this article we review information-processing studies applicable to the similar structure hypothesis. This hypothesis holds that when nonorganically impaired retarded and nonretarded persons are similar in general developmental level, they are also similar in the cognitive processes and concepts by which they reason. Results of this review were strikingly different from those of an earlier review focusing on Piagetian investigations. A meta-analysis revealed that the performance of retarded groups was significantly inferior to that of their nonretarded psychometric mental age peers. We also found a significant relation between the mental age of the subjects and the probability of finding a significant difference between retarded and nonretarded subjects. A number of alternative explanations are offered for these findings.