This reanalysis of a classic survey leads to inferences about design of genetic studies, resolution of heterogeneity, and the role of autosomal and sex linked genes in mental retardation, which is no longer refractory to segregation analysis. By discriminating between sociofamilial and biological types we estimate that at least 351 autosomal loci can produce mental retardation, with an inbred load of 0.83 detrimental equivalents and a mutation rate of 0.008 per gamete, or less than 2.4x10-5 per locus. The distribution of probands was estimated as: 7% medical, 60% sociofamilial, and 33% biological. Simple genetic mechanisms account for virtually all the biological category. Within the sociofamilial group cultural inheritance and polygenes could not be resolved.