Recent reports from Sweden and other European countries have shown a sharp disparity between their incidence and prevalence figures on mild mental retardation and similar figures in the U.S. (Grunewald, 1979; Martin, Blodgett, Edwards, Geer, & Melcher, 1974; Robinson & Robinson, 1976). The purpose of this paper is to provide a model of intellectual performance that might explain such disparities and predict future levels of occurrence of mild mental retardation. Such explanations should have relevance for our views on the development of intellectual ability as well as, on a practical level, planning the allocation of treatment resources. Such planning clearly depends upon the number of children having the condition in question. Over the past few decades we have progressed substantially in our ability to describe mental retardation and to differentiate it from conditions that superficially resemble it; such as, autism, learning disabilities, schizophrenia, etc., (Baroff, 1974; Robinson & Robinson, 1976). Our ability to identify variables that relate to mild mental retardation also has developed substantially, but correlation does not necessarily lead to understanding. One of the large set of variables that often is associated with mild mental retardation in the U.S. is poverty. But “poverty” is too broad a variable to be explanatory. Such a dimension is a surrogate factor standing in the place of other variables, not yet understood, that control, in part, the actual process, by which the child develops the condition of mild mental retardation (Kirk & Gallagher, 1983). A series of papers has been presented by psychologists in Sweden that allow us to address this issue. The ability to observe two cultures, Sweden and the U.S., that have both substantial similarities and differences provides a natural comparison of the prevalence of mental retardation that would be impossible to achieve through experimental manipulation. Prevalence is used here as the total number of cases present in a population group during a specific interval of time (Kramer, 1975).