If men define situations as real, they are real in their consequences.

The Thomas theorem is a theory of sociology which was formulated by W. I. Thomas during the year 1928.
In other words, the interpretation of a situation causes the action. This interpretation is not objective. Actions are affected by subjective perceptions of situations. Whether there even is an objectively correct interpretation is not important for the purposes of helping guide individuals' behavior.

1. If many people in one place believe a false rumour that their bank has become bankrupt, and all of them go to the bank to withdraw their money, the bank will become bankrupt in reality, even though the crisis began simply as a rumour. An example of this during the early 21st century was the run on the British bank Northern Rock which contributed partially to its nationalisation by the British government.


2. The 1973 oil crisis resulted in the so-called "toilet paper panic." The rumour of an expected shortage of toilet paper— resulting from a decline in the importation of oil— caused people to stockpile supplies of toilet paper and this caused a shortage. This shortage, seeming to validate the rumour, is also an example of a self-fulfilling prophecy.

William Isaac Thomas (1863 - 1947), was an American sociologist. He is noted for his innovative work on the sociology of migration on which he co-operated with Florian Znaniecki, and for his formulation of what became known as the Thomas theorem, a fundamental principle of sociology:
"If men define situations as real, they are real in their consequences".