The cognitive development of 4 subcultural groups (black and white, lower and middle-class) of boys was compared. The subcultural groups, age range and measures used were chosen to test Jensen''s theory. The total design included 4 longitudinal and 2 additional samples covering the ages from 6-9. The measures were conceptually divided into memory, intelligence, learning and transfer. A delayed posttest made it possible to compare 9-yr-old groups on memory for answers on learning and transfer tasks. Analyses of longitudinal data were computed to compare subcultural groups on level of performance and rate of development on each measure. The additional samples were used to determine the importance of age, cohort and practice. It was possible to compute the relationship between performance on measures collected and the California Achievement Test. While the race and social class differences typically found were replicated, the major result was the absence of an age .times. subcultural group interaction in nearly all analyses. There were no systematic effects found for cohort or practice. Apparently, black and lower-class children are behind at 6 yr but continue to progress cognitively at approximately the same rate as middle-class children until at least 9 yr of age. No subcultural group differences were found on the delayed posttest. In most cases the measures predicted achievement test scores equally well for all subcultural groups. Results supported a differential readiness position.