Concept
Thomas theorem
If men define situations as real, they are real in their consequences (W. I. and D. S. Thomas, 1928); the belief need not be true.
The Thomas theorem is the sentence in William I. Thomas and Dorothy Swaine Thomas’s The Child in America (1928): “If men define situations as real, they are real in their consequences.” That is, when people take a situation to be real, the consequences are real even if the definition is wrong. In 1948 Robert K. Merton named the sentence the “Thomas theorem” and made it the starting point of his analysis of the “self-fulfilling prophecy”.
The root of the idea is older: the methodological note of Thomas and Znaniecki’s 1918 The Polish Peasant counts the “definition of the situation” as one of three elements of a social act. Its importance for communication is that rumours, headlines and conversation are the material through which people come to define a situation. Merton’s Last National Bank is a fictional parable: when enough depositors believe a rumour that a sound bank will fail, the bank really does fail.
Example
A prisoner who thought people talking to themselves in the street were insulting him had killed them; Thomas and Thomas tell this case just before the theorem.