A cognitive bias is a pattern of deviation in judgment that occurs in particular situations. Implicit in the concept of a "pattern of deviation" is a standard of comparison; this may be the judgment of people outside those particular situations, or may be a set of independently verifiable facts.
Cognitive biases are instances of evolved mental behavior. Some are presumably adaptive, for example, because they lead to more effective actions in given contexts or enable faster decisions when faster decisions are of greater value. Others presumably result from a lack of appropriate mental mechanisms, or from the misapplication of a mechanism that is adaptive under different circumstances.
Cognitive bias is a general term that is used to describe many observer effects in the human mind, some of which can lead to perceptual distortion, inaccurate judgment, or illogical interpretation. It is a phenomenon studied in cognitive science and social psychology.
Bias arises from various processes that are sometimes difficult to distinguish. These include information-processing shortcuts (heuristics), motivational factors and social influence.
The notion of cognitive biases was introduced by Amos Tversky and Daniel Kahneman in 1972 and grew out of their experience of people's innumeracy, or inability to reason intuitively with the greater orders of magnitude. They and their colleagues demonstrated several replicable ways in which human judgments and decisions differ from rational choice theory. They explained these differences in terms of heuristics, rules which are simple for the brain to compute but introduce systematic errors. For instance the Availability heuristic, when the ease with which something comes to mind is used to indicate how often (or how recently) it has been encountered.
These experiments grew into the heuristics and biases research program which spread beyond academic psychology into other disciplines including medicine and political science. It was a major factor in the emergence of behavioral economics, earning Kahneman a Nobel Prize in 2002. Tversky and Kahneman developed prospect theory as a more realistic alternative to rational choice theory.
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