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Confirmation Bias: A Ubiquitous Phenomenon in Many Guises

I would argue that the confirmation bias is of utmost importance to everyone. It invades our lives (e.g., brand loyalty, stereotyping, halo effect, availability bias, …) and, unfortunately, many people exploit this bias tilting the odds in their favor.

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If one were to attempt to identify a single problematic aspect of human reasoning that deserves attention above all others, the confirmation bias would have to be among the candidates for consideration.


Confirmation bias, as the term is typically used in the psychological literature, connotes the seeking or interpreting of evidence in ways that are partial to existing beliefs, expectations, or a hypothesis in hand. The author reviews evidence of such a bias in a variety of guises and gives examples of its operation in several practical contexts. Possible explanations are considered, and the question of its utility or disutility is discussed.

Some Interesting points:

  • There is an obvious difference between impartially evaluating evidence in order to come to an unbiased conclusion and building a case to justify a conclusion already drawn.
  • A great deal of empirical evidence supports the idea that the confirmation bias is extensive and strong and that it appears in many guises. The evidence also supports the view that once one has taken a position on an issue, one's primary purpose becomes that of defending or justifying that position. This is to say that regardless of whether one's treatment of evidence was evenhanded before the stand was taken, it can become highly biased afterward.
  • People tend to seek information that they consider supportive of favored hypotheses or existing beliefs and to interpret information in ways that are partial to those hypotheses or beliefs. Conversely, they tend not to seek and perhaps even to avoid information that would be considered counter-indicative with respect to those hypotheses or beliefs and supportive of alternative possibilities (Farnam Street here: Munger praises Darwin because he was always looking for evidence that refuted his thoughts.)
  • people seek a specific type of information that they would expect to find, assuming the hypothesis is true.
  • Preferential treatment of evidence supporting existing beliefs or opinions is seen in the tendency of people to recall or produce reasons supporting the side they favor—my-side bias—on a controversial issue and not to recall or produce reasons supporting the other side. It could be either that how well people remember a reason depends on whether it supports their position, or that people hold a position because they can think of more reasons to support it. (Farnam Street: Think about availability bias here as well)
  • When children and young adults were given evidence that was inconsistent with a theory they favored, they often “either failed to acknowledge discrepant evidence or attended to it in a selective, distorting manner.
  • Many people fail to understand falsification.
  • People can exploit others' tendency to over- weight (psychologically) confirming evidence and underweight disconfirming evidence for many purposes. When the mind reader, for example, describes one's character in more-or- less universally valid terms, individuals who want to believe that their minds are being read will have little difficulty finding substantiating evidence in what the mind reader says if they focus on what fits and discount what does not and if they fail to consider the possibility that equally accurate descriptions can be produced if their minds are not being read 
  • People sometimes see in data the patterns for which they are looking, regardless of whether the patterns are really there (a form of Bias from insensitivity to sample size)
  • Several studies by Snyder and his colleagues involving the judgment of personality traits lend credence to the idea that the degree to which what people see or remember corresponds to what they are looking for exceeds the correspondence as objectively assessed.
  • Numerous studies have reported evidence of participants seeing or remembering behavior that they expect. Sometimes the effects occur under conditions in which observers interact with the people observed and sometimes under conditions in which they do not.
  • The results of experiments using such abstract tasks as estimating the proportions of beads of different colors in a bag after observing the color(s) of one or a few beads shows that data can be interpreted as favorable to a working hypothesis even when the data convey no diagnostic information. 
  • After one has formed a preference for one brand of a commercial product over another, receiving information about an additional feature that is common to the two brands may strengthen one's preexisting preference

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