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"It is the peculiar and perpetual error of the human understanding to be more moved and excited by affirmatives than by negatives." --Francis Bacon
Confirmation bias refers to a type of selective thinking whereby one tends to notice and to look for what confirms one's beliefs, and to ignore, not look for, or undervalue the relevance of what contradicts one's beliefs. For example, if you believe that during a full moon there is an increase in admissions to the emergency room where you work, you will take notice of admissions during a full moon, but be inattentive to the moon when admissions occur during other nights of the month. A tendency to do this over time unjustifiably strengthens your belief in the relationship between the full moon and accidents and other lunar effects.
This tendency to give more attention and weight to data that support our beliefs than we do to contrary data is especially pernicious when our beliefs are little more than prejudices. If our beliefs are firmly established on solid evidence and valid confirmatory experiments, the tendency to give more attention and weight to data that fit with our beliefs should not lead us astray as a rule. Of course, if we become blinded to evidence truly refuting a favored hypothesis, we have crossed the line from reasonableness to closed-mindedness.
Numerous studies have demonstrated that people generally give an excessive amount of value to confirmatory information, that is, to positive or supportive data. The "most likely reason for the excessive influence of confirmatory information is that it is easier to deal with cognitively" (Gilovich 1993). It is much easier to see how a piece of data supports a position than it is to see how it might count against the position. Consider a typical ESP experiment or a seemingly clairvoyant dream: Successes are often unambiguous or data are easily massaged to count as successes, while negative instances require intellectual effort to even see them as negative or to consider them as significant. The tendency to give more attention and weight to the positive and the confirmatory has been shown to influence memory. When digging into our memories for data relevant to a position, we are more likely to recall data that confirms the position (ibid.).
Researchers are sometimes guilty of confirmation bias by setting up experiments or framing their data in ways that will tend to confirm their hypotheses. They compound the problem by proceeding in ways that avoid dealing with data that would contradict their hypotheses. For example, some parapsychologists used to engage in optional starting and stopping in their ESP research. Experimenters might avoid or reduce confirmation bias by collaborating in experimental design with colleagues who hold contrary hypotheses, as Richard Wiseman (skeptic) and Marilyn Schlitz (proponent) have done.* Individuals have to continually remind themselves of this tendency and actively seek out data contrary to their beliefs. Since this is unnatural, it appears that the ordinary person is doomed to bias.
For examples of confirmation bias in action, see "alternative" health practice, curse, ESP, intuitive, lunar effect, personology, plant perception, the Sokal hoax, therapeutic touch, and thought field therapy.
To see confirmation bias at work, review the conspiracy theories offered for the JFK assassination or the 9/11 conspiracy theories. It is a good lesson to observe how easily intelligent people can see intricate connections and patterns that support their viewpoint and how easily they can see the faults in viewpoints contrary to their own. As long as one ignores certain facts and accepts speculation as fact, one can prove just about anything to one's own satisfaction. It is much harder cognitively, but a requirement of good science, to try to falsify a pet hypothesis.
books and articles
Coincidences: Remarkable or Random? by Bruce Martin
Schlitz, M., Wiseman, R., Radin, D., & Watt, C. (2005). Of two minds: Skeptic-proponent collaboration within parapsychology. Proceedings of the 48th Annual Convention of the Parapsychological Association. USA. 171-177.