A self-fulfilling prophecy is a prediction that directly or indirectly causes itself to become true, by the very terms of the prophecy itself, due to positive feedback between belief and behavior. Although examples of such prophecies can be found in literature as far back as ancient Greece and ancient India, it is 20th-century sociologist Robert K. Merton who is credited with coining the expression "self-fulfilling prophecy" and formalizing its structure and consequences. In his 1948 article Self-Fulfilling Prophecy, Merton defines it in the following terms:
The self-fulfilling prophecy is, in the beginning, a false definition of the situation evoking a new behavior which makes the original false conception come true. This specious validity of the self-fulfilling prophecy perpetuates a reign of error. For the prophet will cite the actual course of events as proof that he was right from the very beginning.
In other words, a positive or negative prophecy, strongly held belief, or delusion—declared as truth when it is actually false—may sufficiently influence people so that their reactions ultimately fulfill the once-false prophecy.
Self-fulfilling prophecy are effects in behavioral confirmation effect, in which behavior, influenced by expectations, causes those expectations to come true. It is complementary to the self-defeating prophecy.