ATTITUDES AND SOCIAL COGNITION A Disconfirmation Bias in the Evaluation of Arguments Kari Edwards Edward E. Smith Brown University University of Michigan (Ann Arbor) Two experiments provided evidence for a disconfirmation bias in argument evaluation such that arguments incompatible with prior beliefs are scrutinized longer, subjected to more extensive refu- tational analyses, and consequently are judged to be weaker than arguments compatible with prior beliefs. The idea that people are unable to evaluate evidence independently of prior beliefs has been documented elsewhere, including in the classic study by C. G. Lord, L. Ross, and M. R. Lepper (1979). The present findings contribute to this literature by specifying the processes by which prior beliefs affect the evaluation of evidence. The authors compare the disconflrmation model to several other models of how prior beliefs influence current judgments and present data that provide support for the disconfirmation model. Results indicate that whether a person's prior belief is accompanied by emotional conviction affects the magnitude and form of the disconfirmation bias. When evaluating an argument, can one assess its strength in- whether the death penalty is an effective deterrent against mur- dependently of one's prior belief in the conclusion? A good deal der. They selected two groups of participants, one known to be- of evidence indicates the answer is an emphatic no (e.g., Batson, lieve that the death penalty is an effective deterrent and one 1975; Chapman & Chapman, 1959; Darley & Gross, 1983; known to believe that it is not an effective deterrent. Both Geller & Pitz, 1968; Nisbett & Ross, 1980; Sherif & Hovland, groups were presented with two arguments, one that pointed to 1961). This phenomenon, which we refer to as the prior belief the deterrent efficacy of the death penalty and one that pointed effect, has important implications. Given two people, or groups, to its inefficacy as a deterrent. Each argument consisted of a with opposing beliefs about a social, political, or scientific issue, brief description of the design and findings of a study support- the degree to which they will view relevant evidence as strong ing or opposing the death penalty (e.g., a study showing that a will differ. This difference, in turn, may result in a failure of the state's murder rate declined after institution of the death opposing parties to converge on any kind of meaningful penalty) and was followed by criticisms of the study itself, as agreement, and, under some circumstances, they may become well as rebuttals of these criticisms. The best-known finding as- more extreme in their beliefs. sociated with this study is that the pro-death-penalty and anti- Perhaps the most renowned study documenting the prior be- death-penalty participants became more polarized in their be- lief effect is one conducted by Lord, Ross, and Lepper (1979); liefs—and hence more different from one another—as a result this study served as the starting point for our work. Lord et al. of reading the two arguments. Note, however, that this result is were concerned with people's evaluations of arguments about a logical consequence of another more basic finding obtained by Lord et al.: When participants were asked to rate how con- vincing each study seemed as evidence (i.e., assessments in- volved participants' judgment of the argument's strength rather Kari Edwards, Department of Psychology, Brown University; Edward than their final belief in the conclusion), proponents of the E. Smith, Department of Psychology, University of Michigan (Ann death penalty judged the pro-death-penalty arguments to be Arbor). more convincing or stronger than the anti-death-penalty argu- The research reported in this article was supported by funds provided ments, whereas the opponents of the death penalty judged the by Brown University and by US. Air Force Contract AFOSR-93-0265. We are grateful to Patrick Maher, whose ideas were instrumental to our anti-death-penalty arguments to be more convincing. This is thinking about the normative status of the prior belief effect. We would the prior belief effect, and it has as one of its consequences the also like to thank Thane Pittman and Liz Tighe for their helpful com- polarization of belief. ments on an earlier version of this article, Jon Baron for additional in- Given the importance of the prior belief effect, it is important sights about the normative questions, J. B. Smeltzer for programming to identify the mechanisms that underlie it. Lord et al. suggested assistance, and Hilary Ammazzalorso, Ray Ashare, Craig Malina, and that the effect arises because people tend to accept at face value Silvio Menzano for assistance in running the experiments and coding the protocols. those arguments that are compatible with their prior beliefs but Correspondence concerning this article should be addressed to Kari tend to scrutinize those arguments that are incompatible with Edwards, Department of Psychology, Brown University, Providence, their prior beliefs. This idea has been proposed by other inves- Rhode Island 02912. Electronic mail may be sent via the Internet to tigators as well (e.g., Ditto & Lopez, 1992; Koehler, 1993; kari [email protected]. Kunda, 1990; Ross & Lepper, 1980). Our objective in the pres- Journal of Personality and Social Psychology. 1 »6, Vol. 71, No. 1.5-24 Copyright 1996 by the American Psychological Association. Inc. 0022-3514/96/53.00 EDWARDS AND SMITH ent article is to move beyond the straightforward and now 1. Because memory searches are time consuming, partici- widely accepted notion that prior beliefs affect the extent to pants should take longer to evaluate arguments that are incom- which relevant information is scrutinized and to specify the patible with their prior beliefs than arguments that are compat- processes by which they do so. We sketch an explicit model of ible with these beliefs. how such differential scrutiny comes about, generate predic- 2. If participants are asked to report what they are thinking tions from this model, and present two experiments on argu- while evaluating an argument, they should report more mate- ment evaluation that support the predictions.1 rial when the argument is incompatible with their beliefs than when it is compatible. This is because there will be substantially Disconfirmation Model more output from the memory search in the case of incompati- ble arguments. Our central thesis is the same as Lord et al.'s (1979): When 3. If participants are asked to report what they are thinking faced with evidence contrary to their beliefs, people try to un- while evaluating an argument, most of the reported material dermine the evidence. That is, there is a bias to disconfirm ar- will be compatible with their prior beliefs. This is because the guments incompatible with one's position. This idea can be de- initial activation of memory presumably retrieves mainly prior veloped into a disconfirmation model by making the following beliefs, whereas the deliberate memory search retrieves mate- assumptions. rial refuting arguments that are themselves incompatible with 1. When one is presented an argument to evaluate, there will the participants* prior beliefs (i.e., the deliberative search re- be some automatic activation in memory of material relevant trieves material supportive of one's beliefs). to the argument. Some of the accessed material will include This set of predictions does not follow from some familiar one's prior beliefs about the issue. alternative models of how prior beliefs influence current judg- 2. If the argument presented is incompatible with prior be- ments. One such alternative, sometimes alluded to but rarely liefs, one will engage in a deliberative search of memory for ma- developed, hinges on the notion that people with different atti- terial that will undermine the argument simply. Hence, "scruti- tudes about an issue have stored different beliefs (e.g., see Nis- nizing an argument" is implemented as a deliberate memory 2 bett & Ross, 1980). When applied to the Lord et al. (1979) search, and such a search requires extensive processing. study, the differential storage account would proceed as follows. 3. Possible targets of the memory search include stored beliefs When presented an argument, all participants search their and arguments that offer direct evidence against the premises memory for relevant material to bring to bear on the evaluation and conclusion of the presented argument. of this argument. Because pro-death-penalty participants have 4. The outputs of the memory search are integrated with mainly pro-death-penalty material stored and anti-death-pen- other (perhaps unbiased) considerations about the current ar- alty participants have mainly anti-death-penalty material gument, and the resulting evaluation serves as the basis for judg- stored, most of the retrieved material will be consistent with ments of the current argument's strength. participants' prior beliefs. In turn, because this material will These four assumptions are embodied in the simple box enter into the evaluation of the current argument, a prior belief model diagrammed in Figure 1. The model readily explains the effect should arise. This account further predicts that if partici- prior belief effect. Specifically, the evaluation of arguments that are incompatible with one's prior beliefs is biased by counter- evidence retrieved during the memory search, whereas the eval- ' The prior belief effect investigated in this article arises in the context uation of arguments compatible with prior beliefs is not biased of inductive reasoning and should be distinguished from a seemingly related effect obtained in studies of deductive reasoning. In the latter by such counterevidence (see Figure 1). case (e.g., Oakhill & Johnson-Laird, 1985), participants are presented In addition to explaining the prior belief effect, the discon- complex arguments (often syllogisms) and asked to determine whether firmation model leads to the following three predictions.