Processing speed, working memory, and fluid intelligence: Evidence for a developmental cascade

Processing speed, working memory capacity, and fluid intelligence were assessed in a large sample (N = 214) of children, adolescents, and young adults (ages 7 to 19 years). Results of path analyses revealed that almost half of the age-related increase in fluid intelligence was mediated by developmental changes in processing speed and working memory, and near ly three fourths of the improvement in working memory was mediated by developmental changes in processing speed. Moreover, even when age-related differences in speed, working memory, and fluid intelligence were statistically controlled, individual differences in speed had a direct effect on working memory capacity, which, in turn, was a direct determinant of individual differences in fluid intelligence.