Arthur Jensen observed in The g Factor that black pupils with IQ deficiencies often socially integrate well with their brighter peers. They seem quite normal when engaging in non-cerebral activities like play. In contrast, many cognitively impaired white children have difficulty integrating socially and often have physical abnormalities such as flat-footed gaits. Jensen attributes this racial divergence to different etiologies that are transparent to IQ tests. Two types of mental retardation can be differentiated.
Organic retardation is due to a genetic anomaly or brain damage brought about by disease or trauma.
Familial retardation results from normal variations in intelligence. That is, about 2.0 percent of the black population at large is organically retarded. Following Jensen, if we associate most of nonadaptive behavior with organic retardation, then Reference County should classify about 2.0 percent of black children and 0.5 to 1.0 percent of white children as mentally retarded. Blacks in Reference County will be retarded at 2 to 4 times the rate of whites. Table 2 compares the offending counties to Reference County .