W.I. and D.S. Thomas wrote several watershed articles on the influence of social situations on individual behavior. Their work influenced later disciplines and helped segment Social Psychology apart from both Sociological and Psychological studies.
Thomas and Thomas (1928) wrote "If men define situations as real, they are real in their consequences" (page 571-572). W.I. Thomas and D.S. Thomas (1928). The Child in America: Behavior Problems and Programs. New York: Alfred A. Knopf.