Upon returning to the United States, he became a researcher and professor at the University of California, Berkeley, where he focused on individual differences in learning, especially the influences of culture, development, and genetics on intelligence and learning.
Ta sai uurijaks ja professoriks California Ülikoolis, Berkeleys, kus ta keskendus individuaalsetele erinevustele õppimises, eriti kultuuri, arengu ja geneetika mõjudele intelligentsusele ja õppimisele.