Mental retardation or mental disability, is a generalized disorder characterized by impaired cognitive functioning. The affected individuals with mental retardation have an IQ below 70 (between 50 and 70 = mild mental retardation; Below 50 = moderate mental retardation).
The races differ in IQ (70 to Sub-Saharan Africans, 85 African Americans, 100 whites) but also in the prevalence of mental retardation, after correcting for socio-economic status. This is what Yeargin-Allsopp et al. (1995, pp. 326-327) have found : the prevalence of mild mental retardation was twice as high among black Americans, controlling for SES.