The cumulative deficit hypothesis with respect to age decrement in IQ was investigated in large samples of white and black school children in rural Georgia between the ages of 5 and 18. The cumulative deficits hypothesis suggests that the increasing decrement in mental test scores, relative to population norms, is 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. About 1,300 school children, approximately 49% white and 51% black, were tested. Subjects were tested on the California Test of Mental Maturity, a standardized test of general intelligence, which yields IQ's for verbal and nonverbal abilities from kindergarten through grade 12. Age decrement in verbal and nonverbal IQ is measured in this study 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 IQ's as a linear function of age in the range from about 5 to 16 years of age. The results of this study on Georgia blacks seem to favor an environmental interpretation of the progressive IQ decrement 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. (Author/AM)