Individual differences in learning and memory abilities have fascinated people since they began thinking about how their minds work. In discussing his wax metaphor of memory, Plato (1953) noted that memories made of "pure and clear [wax] … easily learn and easily retain," whereas those made of "muddy and of impure wax [have] … a corresponding defect in the mind." Plato realized that people differ in what they learn and remember and in how well they do both. This is certainly true at the extremes, but how relevant is it over the normal range of memory abilities?

The psychological research supports four main conclusions about individual differences in learning and memory (Bors and MacLeod, 1996). First, people differ in what they know, their knowledge base. Second, people differ in their working memory capacity, the ability to hold information in consciously accessible memory...