He then taught at the University of Minnesota at Minneapolis and later at Indiana University, where he was chair of the psychology department from 1946–1947, before returning to Harvard as a tenured professor in 1948.
Pak učil na Minnesotské univerzitě v Minneapolis a později na Indianské univerzitě, kde byl vedoucím katedry psychologie v letech 1946-1947, než se v roce 1948 vrátil na Harvard, kde se stal dlouholetým profesorem.