That most psychological research is conducted with students led to concerns that psychological laws apply only to this population. These fears are based on Campbell and Stanley's concept of external validity that specifies the extent to which research findings can be generalized. This concept is based on an inductivist philosophy. As philosophers of science have argued since Hume, one cannot derive general laws from singular observations. Instead, one develops theories and uses empirical studies to test these theories. This solves the problem of generalization because the domain of applicability is specified by the theory. Reports that studies result in different findings when conducted in different cultures are unproblematic as long as these differences can be explained with psychological theories.