confirmation bias

Confirmation bias refers to a type of selective thinking whereby one tends to notice and 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" (Thomas Gilovich, How We Know What Isn't So: The Fallibility of Human Reason in Everyday Life). It is much easier to see how data support a position than it is to see how they 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.

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. We have to continually remind ourselves of this tendency and actively seek out data contrary to our beliefs. Since this is unnatural, most of us are doomed to die with our biases on.

To counteract the natural tendency to try to confirm our beliefs, science has developed methods of testing claims that involve trying to falsify them, rather than trying to confirm them. Paranormal investigators who set out to prove ghosts haunt some hotel or ancient castle aren't being very scientific, no matter how many electronic gizmos they carry in their toolkit. The scientific paranormal investigator approaches an investigation with an open mind, collects and examines as much relevant evidence as is reasonable for the claim being investigated, develops hypotheses (alternative explanations), and tries to falsify them. Yes, a scientist tries to falsify, not verify, his hypothesis. If you set out to verify your hypothesis you are very likely to be misdirected by confirmation bias. You will look for only those things that confirm what you believe and you will systematically ignore those things that might disconfirm your belief. To keep an open mind, the scientist, like a good detective, must not form hypotheses too early in the investigation, as the tendency of all of us is to confirm, not disconfirm, our hypotheses. Unless you are lucky and your first guess happens to be the right one, you run the risk of building up a convincing case for a false claim. The study of criminal profilers, psychics, and psychic detectives offer examples of how confirmation bias works: colleagues, the media, and gullible law enforcement officers focus on anything that seems to confirm the profile the investigator is working with or the prediction of the psychic, while ignoring all the claims that were irrelevant to that profile or made no sense in light of the prediction. The importance of trying to collect data that is relevant to the investigation in such a way that one's biases don't lead one to ignore important avenues of investigation cannot be overemphasized.

Those who favor their interpretations of personal experience over the results of double-blind, randomized, control group studies are doomed to die with their biases on. One way to counteract confirmation bias is to consciously seek out literature that opposes your beliefs and hang around with people who don't share your cherished opinions. To do so, however, is so unnatural that very few people will do it. I can attest that one of the most tedious tasks I ever set for myself was to read the works of Dean Radin, Gary Schwartz, Charles Tart, L. Ron Hubbard, and the like. On the other hand, my graduate training in philosophy required me to read the likes of St. Thomas Aquinas, St. Augustine, Bishop George Berkeley, and many others whose ideas I could never agree with. Had I been allowed to read only Hume or other philosophers I agreed with, my education would have been an impoverished one.