Beginning in the 1960s and continuing through the 1980s, the educational psychologists Lee J. Cronbach and Richard E. Snow spearheaded a program of research connecting key ideas from the scientific disciplines of differential psychology, which looks at the differences among individuals and groups, and experimental psychology, which uses empirical principles and procedures. As Cronbach argued in a seminal paper addressed to the American Psychological Association in 1975, research in these two disciplines had heretofore progressed independently, with different foci and traditions, though both were contributing important understandings to the domains of teaching and learning (see also earlier writing on this topic, e.g., Cronbach, 1957, 1967). Snow had begun examining the effects of instructional treatments on performance as a doctoral student at Purdue University. He joined ...