Review of General Psychology Copyright 1998 by the Educational Publishing Foundation 1998, Vol. 2, No. 2, 175-220 1089-2680«8/$3.00 Confirmation Bias: A Ubiquitous Phenomenon in Many Guises Raymond S. Nickerson Tufts University 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. When men wish to construct or support a theory, how question, evaluates it as objectively as one can, they torture facts into their service! (Mackay, 1852/ and draws the conclusion that the evidence, in 1932, p. 552) the aggregate, seems to dictate. In the second, Confirmation bias is perhaps the best known and most one selectively gathers, or gives undue weight widely accepted notion of inferential error to come out to, evidence that supports one's position while of the literature on human reasoning. (Evans, 1989, p. 41) neglecting to gather, or discounting, evidence If one were to attempt to identify a single that would tell against it. problematic aspect of human reasoning that There is a perhaps less obvious, but also deserves attention above all others, the confirma- important, difference between building a case tion bias would have to be among the candidates consciously and deliberately and engaging in for consideration. Many have written about this case-building without being aware of doing so. bias, and it appears to be sufficiently strong and The first type of case-building is illustrated by pervasive that one is led to wonder whether the what attorneys and debaters do. An attorney's bias, by itself, might account for a significant job is to make a case for one or the other side of fraction of the disputes, altercations, and misun- a legal dispute. The prosecutor tries to marshal derstandings that occur among individuals, evidence to support the contention that a crime groups, and nations. has been committed; the defense attorney tries Confirmation bias has been used in the to present evidence that will support the psychological literature to refer to a variety of presumption that the defendant is innocent. phenomena. Here I take the term to represent a Neither is committed to an unbiased weighing of generic concept that subsumes several more all the evidence at hand, but each is motivated to specific ideas that connote the inappropriate confirm a particular position. Debaters also bolstering of hypotheses or beliefs whose truth would be expected to give primary attention to is in question. arguments that support the positions they are defending; they might present counterargu- ments, but would do so only for the purpose of Deliberate Versus Spontaneous pointing out their weaknesses. Case Building As the term is used in this article and, I There is an obvious difference between believe, generally by psychologists, confirma- impartially evaluating evidence in order to come tion bias connotes a less explicit, less con- to an unbiased conclusion and building a case to sciously one-sided case-building process. It justify a conclusion already drawn. In the first refers usually to unwitting selectivity in the instance one seeks evidence on all sides of a acquisition and use of evidence. The line between deliberate selectivity in the use of evidence and unwitting molding of facts to fit Correspondence concerning this article should be ad- hypotheses or beliefs is a difficult one to draw in dressed to Raymond S. Nickerson, Department of Psychol- ogy, Paige Hall, Tufts University, Medford, Massachusetts practice, but the distinction is meaningful 02155. Electronic mail may be sent to mickerson@infonet. conceptually, and confirmation bias has more to tufts.edu. do with the latter than with the former. The 175 176 RAYMOND S. NICKERSON assumption that people can and do engage in one may seek evidence of that fact or give undue case-building unwittingly, without intending to weight to such evidence. But in such cases, the treat evidence in a biased way or even being hypothesis in question is someone else's belief. aware of doing so, is fundamental to the For the individual who seeks to disconfirm such concept. a hypothesis, a confirmation bias would be a The question of what constitutes confirmation bias to confirm the individual's own belief, of a hypothesis has been a controversial matter namely that the hypothesis in question is false. among philosophers and logicians for a long time (Salmon, 1973). The controversy is exem- plified by Hempel's (1945) famous argument A Long-Recognized Phenomenon that the observation of a white shoe is confirmatory for the hypothesis "All ravens are Motivated confirmation bias has long been black," which can equally well be expressed in believed by philosophers to be an important contrapositive form as "All nonblack things are determinant of thought and behavior. Francis nonravens." Goodman's (1966) claim that Bacon (1620/1939) had this to say about it, for evidence that something is green is equally good example: evidence that it is "grue"—grue being defined The human understanding when it has once adopted an as green before a specified future date and blue opinion (either as being the received opinion or as thereafter—also provides an example. A large being agreeable to itself) draws all things else to literature has grown up around these and similar support and agree with it. And though there be a greater puzzles and paradoxes. Here this controversy is number and weight of instances to be found on the other side, yet these it either neglects and despises, or largely ignored. It is sufficiently clear for the else by some distinction sets aside and rejects; in order purposes of this discussion that, as used in that by this great and pernicious predetermination the everyday language, confirmation connotes evi- authority of its former conclusions may remain dence that is perceived to support—to increase inviolate.. And such is the way of all superstitions, the credibility of—a hypothesis. whether in astrology, dreams, omens, divine judg- ments, or the like; wherein men, having a delight in I also make a distinction between what might such vanities, mark the events where they are fulfilled, but where they fail, although this happened much be called motivated and unmotivated forms of oftener, neglect and pass them by. (p. 36) confirmation bias. People may treat evidence in a biased way when they are motivated by the Bacon noted that philosophy and the sciences desire to defend beliefs that they wish to do not escape this tendency. maintain. (As already noted, this is not to The idea that people are prone to treat suggest intentional mistreatment of evidence; evidence in biased ways if the issue in question one may be selective in seeking or interpreting matters to them is an old one among psycholo- evidence that pertains to a belief without being gists also: deliberately so, or even necessarily being aware If we have nothing personally at stake in a dispute of the selectivity.) But people also may proceed between people who are strangers to us, we are in a biased fashion even in the testing of remarkably intelligent about weighing the evidence and hypotheses or claims in which they have no in reaching a rational conclusion. We can be convinced material stake or obvious personal interest. The in favor of either of the fighting parties on the basis of former case is easier to understand in common- good evidence. But let the fight be our own, or let our own friends, relatives, fraternity brothers, be parties to sense terms than the latter because one can the fight, and we lose our ability to see any other side of appreciate the tendency to treat evidence the issue than our own. .. The more urgent the selectively when a valued belief is at risk. But it impulse, or the closer it comes to the maintenance of is less apparent why people should be partial in our own selves, the more difficult it becomes to be their uses of evidence when they are indifferent rational and intelligent. (Thurstone, 1924, p. 101) to the answer to a question in hand. An adequate The data that I consider in what follows do account of the confirmation bias must encom- not challenge either the notion that people pass both cases because the existence of each is generally like to avoid personally disquieting well documented. information or the belief that the strength of a There are, of course, instances of one wishing bias in the interpretation of evidence increases to disconfirm a particular hypothesis. If, for with the degree to which the evidence relates example, one believes a hypothesis to be untrue, directly to a dispute in which one has a personal CONFIRMATION BIAS 177 stake. They are difficult to reconcile, however, to it. These generalizations are illustrated by with the view that evidence is treated in a totally several of the following experimental findings. unbiased way if only one has no personal Restriction of attention to a favored hypoth- interest in that to which it pertains. esis. If one entertains only a single possible The following discussion of this widely explanation of some event or phenomenon, one recognized bias is organized in four major precludes the possibility of interpreting data as sections. In the first, I review experimental supportive of any alternative explanation. Even evidence of the operation of a confirmation bias. if one recognizes the possibility of other In the second, I provide examples of the bias at hypotheses or beliefs, perhaps being aware that work in practical situations.