Cumulative deficit in IQ of Blacks in the rural South.

Abstract

The cumulative deficit hypothesis with respect to age decrement in IQ between the ages of 5 and 18 was investigated in large samples of white and black school children in rural Georgia. Age decrement in verbal and nonverbal IQ was measured by the average IQ difference between younger and older siblings. It was found that blacks (but not whites) showed significant and substantial decrements in both verbal and nonverbal IQs as a linear function of age in the rank from about 5 to 16 years of age. An environmental interpretation of the age decrement in IQ seems reasonable in view of the comparative lack of such a decrement in a parallel study of California blacks whose environmental circumstances are markedly better than those of the black sample from rural Georgia. The cumulative deficit hypothesis is in- in a California school district and found a tended to explain the increasing decrement in mental test scores, relative to population norms, as a function of age in groups considered environmentally deprived. According to the hypothesis, the decrement is a result of the cumulative effects of environmental disadvantages on mental development. The history of the cumulative deficit hypothesis and its theoretical and methodological problems have been reviewed by Jensen (1974a). It was concluded that most of the studies of the phenomenon are seriously flawed by methodologica l deficiencies. The majority of studies have found no evidence of an age-related IQ decrement in blacks. Jensen (1974a) proposed investigating IQ decrement by the sibling method; that is, using the difference in standardized test scores between younger and older siblings within the same family as an indicator of IQ decrement. If there is a true IQ decrement, older siblings should obtain lower test scores than their younger siblings, and there should be a positive correlation between sibling age difference and IQ difference. Jensen applied the sibling method to large samples of whites and blacks of ages 5 to 12