This research is, in part, a study of a method of differentiating between two different types of retardation (the exogenous or brain injured and the endogenous or non-brain injured). However, this paper is also an example of the evaluation of a research problem, for it incorporates data and information from three different studies, each of which leads to the next study. The first study concerned the establishment of a profile of abilities of the retarded child as measured by the Wechsler Intelligence Scale for Children (WISC). The Vocabulary and Arithmetic subtests were found to be significantly low, while the Picture Completion, Object Assembly and Similarities subtests were significantly higher than the mean. A followup study was done to determine the causes for the extreme scores found in the previous study. A tentative conclusion was that the brain injured child contributed more variability to the subtest scores than did the non-brain injured. More indications of brain injury and a lower correlation between intelligence and achievement was found for the hypothesized exogenous group. The final study (presently in the proposal stage) will attempt to show the differences which exist between the exogenous and the endogenous mental defective as measured by the WISC. (Author)