The black swan theory or theory of black swan events is a metaphor that describes an event that comes as a surprise, has a major effect, and is often inappropriately rationalized after the fact with the benefit of hindsight.

The theory was developed by Nassim Nicholas Taleb to explain:

  1. The disproportionate role of high-profile, hard-to-predict, and rare events that are beyond the realm of normal expectations in history, science, finance, and technology
  2. The non-computability of the probability of the consequential rare events using scientific methods (owing to the very nature of small probabilities)
  3. The psychological biases that make people individually and collectively blind to uncertainty and unaware of the massive role of the rare event in historical affairs

Unlike the earlier philosophical “black swan problem,” the “black swan theory” refers only to unexpected events of large magnitude and consequence and their dominant role in history. Such events, considered extreme outliers, collectively play vastly larger roles than regular occurrences. More technically, in the scientific monograph Lectures on Probability and Risk in the Real World: Fat Tails (Volume 1), Taleb mathematically defines the black swan problem as “stemming from the use of degenerate metaprobability”.

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