Self-fulfilling prophecies

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.

Merton's concept of the self-fulfilling prophecy stems from the Thomas theorem, which states that "If men define situations as real, they are real in their consequences". According to Thomas, people react not only to the situations they are in, but also, and often primarily, to the way they perceive the situations and to the meaning they assign to their perceptions. Therefore, their behaviour is determined in part by their perception and the meaning they ascribe to the situations they are in, rather than by the situations themselves. Once people convince themselves that a situation really has a certain meaning, regardless of whether it actually does, they will take very real actions in consequence.

Merton took the concept a step further and applied it to recent social phenomena. In his book Social Theory and Social Structure, he conceives of a bank run at the fictional Last National Bank, over which Cartwright Millingville presides. It is a typical bank, and Millingville has run it honestly and quite properly. As a result, like all banks, it has some liquid assets (cash), but most of its assets are invested in various ventures. Then one day, a large number of customers comes to the bank at once—the exact reason is never made clear. Customers, seeing so many others at the bank, begin to worry. False rumours spread that something is wrong with the bank and more customers rush to the bank to try to get some of their money out while they still can. The number of customers at the bank increases, as does their annoyance and excitement, which in turn fuels the false rumours of the bank's insolvency and upcoming bankruptcy, causing more customers to come and try to withdraw their money. At the beginning of the day—the last one for Millingville's bank—the bank was not insolvent. But the rumour of insolvency caused a sudden demand of withdrawal of too many customers, which could not be answered, causing the bank to become insolvent and declare bankruptcy. Merton concludes this example with the following analysis:

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