ABSTRACT
The frequency of retardation, or IQ under 70, was determined among the relatives of the severely and mildly retarded children. Siblings, half siblings, cousins, and twins were identified in the study cohort. In families with two affected individuals, each was counted as both an index and secondary case. This procedure is unbiased in studies such as the Collaborative Project, in which families with more than one affected child were not more likely to be identified than families with only one affected child.
