Teachers of retarded classes claimed that far more of their black pupils seem to look and act less retarded than the white pupils with comparable IQ. This was especially apparent in social interactions and playground activities observed by teachers during recess and recreation periods.
Their observations were indeed accurate, as I later confirmed by my own observation and testing of pupils in special classes.
In social and outdoor play activities, black children with IQ below seventy seldom appeared as other than quite normal youngsters--energetic, sociable, active, motorically well coordinated, and generally indistinguishable from their age-mates in regular classes.
But this was not so for as many of the white children with IQ below seventy. More of them were somehow "different" from their white age-mates in regular classes. They appeared less competent in social interactions with their classmates and were motorically clumsy or awkward, or walked with a flat-footed gait.
The retarded white children more often looked and acted generally retarded in their development than the black children of comparable IQ.
In most of the cognitive tasks of the classroom that call for conceptual learning and problem solving, however, black and white retarded children of the same IQ do not differ measurably either in classroom performance or on objective achievement tests.
The explanation for these differences cannot be that the IQ test is biased in its predictive validity for the children's general scholastic learning, because predictive validity scarcely differs between black and white groups. Nor is the test biased according to any of the other standard criteria of bias.
There are two distinguishable types of mental retardation, referred to as familial and organic. The lower tail (IQ < 70) of the normal distribution of IQ in the population comprises both of these types of retardation.
In familial retardation, there are no detectable causes of retardation other than the normal polygenic and microenvironmental sources of IQ variation that account for IQ differences throughout the entire range of IQ.
Although persons with familial retardation are, on average, lower in IQ than their parents and siblings, they are no lower than would be expected for a trait with normal polygenic inheritance.
In other words, the familial retarded are biologically normal individuals who deviate statistically from the population mean because of the same factors that cause IQ variation among all other biologically normal individuals in the population.
Organic retardation, on the other hand, comprises over 350 identified etiologies, including specific chromosomal and genetic anomalies and environmental prenatal, perinatal, and postanatal brain damage due to disease or trauma that affects brain development.
Nearly all of these conditions, when severe enough to cause mental retardation, also have other, more general, neurological and physical manifestations of varying degree.
The IQ of organically retarded children is scarcely correlated with the IQ of their first-order relatives, and they typically stand out as deviant in other ways as well.