Confirmation Bias: A Ubiquitous Phenomenon in Many Guises

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184 RAYMOND S. NICKERSON fit what one sees into the taxonomic bins at hand. In accordance with the confirmation bias, people are more likely to look for, and find, confirmation of the adequacy of a taxonomy than to seek and discover evidence of its limitations. Formal Reasoning and the Selection Task I have already considered a task invented by Wason and much used in rule-discovery experiments that revealed a tendency for people to test a hypothesized rule primarily by considering instances that are consistent with it. Wason (1966, 1968) invented another task that also has been widely used to study formal reasoning. In a well-known version of this task, participants see an array of cards and are told that each card has a letter on one side and a number on the other. Each of the cards they see shows either a vowel, a consonant, an even number, or an odd number, and participants are asked to indicate which cards one would have to turn over in order to determine the truth or falsity of the following statement: If a card has a vowel on one side then it has an even number on the other side. Suppose the array that people performing this task see is as follows: Given this set of cards, experimenters have generally considered selection of those showing A and 7 to be correct, because finding an odd number on the other side of the A or finding a vowel behind the 7 would reveal the statement to be false. The cards showing B and 4 have been considered incorrect selections, because whatever is on their other sides is consistent with the statement. In short, one can determine the claim to be false by finding either the card showing the A or the card showing the 7 to be inconsistent with it, or one can determine the claim to be true by finding both of these cards to be consistent with it. Wason found that people performing this task are most likely to select only the card showing a vowel or the card showing a vowel and the one showing an even number; people seldom select either the card showing a consonant or the one showing an odd number. Numerous investigators have obtained essentially the same result. Experiments with this task and variants of it have been reviewed many times (Cosmides, 1989; Evans, 1982; Evans, Newstead, & Byrne, 1993; Tweney & Doherty, 1983; Wason & Johnson-Laird, 1972). The logic of Wason's selection task is that of the conditional: if P then Q. In the case of the above example, P is there is a vowel on one side, and Q is there is an even number on the other. Selecting the card showing the 7 is analogous to checking to see if the not-Q case is accompanied by not-P, as it must be if the conditional is true. The basic finding of experiments with the task lends support to the hypothesis—which is supported also by other research (Evans, Newstead, & Byrne, 1993; Hollis, 1970)—that people find the modus tollens argument (not-Q, therefore not-P) to be less natural than the modus ponens form (P, therefore Q). And it is consistent with the idea that, given the objective of assessing the credibility of a conditional assertion, people are more likely to look for the presence of the consequent given the presence of the antecedent than for the absence of the antecedent given the absence of the consequent. Several experiments have shown that performance of the selection task tends to be considerably better when the problem is couched in familiar situational terms rather than abstractly (Johnson-Laird, Legrenzi, & Legrenzi, 1972; Wason & Shapiro, 1971), although it is by no means always perfect in the former case (Einhorn & Hogarth, 1978). People also generally do better when the task is couched in terms that require deontic reasoning (deciding whether a rule of behavior—e.g., permission, obligation, promise—has been violated) rather than indicative reasoning (determining whether a hypothesis is true or false; Cheng & Holyoak, 1985; Cosmides, 1989; Cosmides & Tooby, 1992; Gigerenzer & Hug, 1992; Griggs & Cox, 1993; Kroger, Cheng, & Holyoak, 1993; Manktelow & Over, 1990, 1991; Markovits & Savary, 1992; Valentine, 1985; Yachanin & Tweney, 1982). In relating confirmation bias to Wason's selection task, it is important to make three distinctions. The first is the distinction between the objective of specifying which of four cards in view must be turned over in order to determine the truth or falsity of the assertion with respect to those four cards and the objective of saying which of the four types of cards represented should be turned over in order to determine the plausibility of the assertion
more generally. I believe that in most of the earlier experiments, the first of these tasks was the intended one, although instructions have not always made it explicit that this was the case. The distinction is critical because what one should do when given the first objective is clear and not controversial. However the answer to what one should do when given the second objective is considerably more complicated and debatable. When the task is understood in the first way, the only correct answer is the card showing the vowel and the one showing the odd number. However, when one believes one is being asked how one should go about obtaining evidence regarding the truth or falsity of an assertion of the form if P then Q where P and Q can be understood to be proxies for larger sets, it is no longer so clear that the cards showing P and ~Q are the best choices. Depending on the specifics of the properties represented by P and Q and what is known or assumed about their prevalence in the population of interest, P may be the only selection that is likely to be very informative, and selection of ~Q may be essentially pointless (Kirby, 1994b; Nickerson, 1996; Oaksford & Chater, 1994; Over & Evans, 1994). Although experimenters have often, perhaps more often than not, intended that the selection task be interpreted in the first of the two ways described, the second interpretation seems to me more representative of the kind of conditional reasoning that people are required to do in everyday life; it is hard to think of everyday examples of needing to determine the truth or falsity of a claim about four entities like the cards in the selection task, all of which are immediately available for inspection. Perhaps a tendency to carry over to the task, uncritically, an effective approach in the world of everyday hypothesis testing that is not always effective in the contrived world of the laboratory contributes to performance on the selection task. The second and third distinctions pertain to instances when the task is understood to involve only the four cards in view. They are the distinctions that were made in the context of the discussion of how confirmation bias relates to "find-the-rule" tasks and other tasks involving conditional syllogisms, namely the distinction between evidence that is confirmatory by virtue of the presence of two entities rather than by their joint absence and the distinction between CONFIRMATION BIAS 185 logical and psychological confirmation. As applied to the selection task, confirmation bias has typically connoted a tendency to look for the joint occurrence of the items specified in the task description. In the original version of the task, this means looking for instances of the joint occurrence of a vowel and an even number, and thus the turning over of the cards showing A and 4, inasmuch as these are the only cards of the four shown that could have both of the named properties. Selection of evidence that can only be confirmatory of the hypothesis under consideration—or avoidance of evidence that could possibly falsify the hypothesis—is one interpretation of the confirmation bias that is not consistent with the selection of A and 4 in Wason's task. A is a potentially falsifying card; if the rule that cards that have a vowel on one side have an even number on the other is false, turning over the card showing A could reveal the fact. So in selecting A one is not avoiding the possibility of falsification; one is performing an appropriate test, even in the strictest Popperian sense. The only way to avoid the possibility of disconfirming the hypothesized rule in Wason's task is to select either or both of the cards showing B and 4, inasmuch as nothing that is on the other sides of these cards could be inconsistent with it. The fact that, when given the selection task, people are at least as likely to select A as they are to select 4 weighs against the idea that their behavior can be attributed to a logically based desire to seek only confirming evidence and to avoid potentially disconfirming evidence. I say logically based because it may be that people select A because they are seeking confirming evidence, even though their selection also provides the opportunity to acquire falsifying evidence. From a strictly logical point of view, although selecting A is part of the correct solution, selecting only A, or A and 4, not only fails to ensure the discovery that the hypothesized rule is false if it is false, it also fails to establish its truth if it is true. The only way to demonstrate its truth is to rule out the possibility that it is false, and this means checking all (both) cases (A and 7) that could possibly show it to be false if it is. So again, behavior that has been interpreted as evidence of a confirmation bias is not strongly confirmatory in a logical sense, al-
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Page 39 and 40: Volume 12: Linguistics and adjacent
Page 41 and 42: Hoch, S. J. (1985). Counterfactual
Page 43 and 44: Matlin, M. W., & Stang, D. J. (1978
Page 45 and 46: pyramid (5th ed.). New York: Randol

Confirmation Bias: A Ubiquitous Phenomenon in Many Guises

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