Disconfirmation Bias

In their paper, “A Disconfirmation Bias in the Evaluation of Arguments,” psychologists Kari Edwards and Edward Smith (Journal of Personality and Social Psychology, 1996, Vol. 71, No. 1, 5-24) explore this tendency. They begin their article as follows:

When evaluating an argument, can one assess its strength independently of one’s prior belief in the conclusion? A good deal f evidence indicates the answer is an emphatic no. This phenomenon, which we refer to as the prior belief effect, has important implications. Given two people, or groups, with opposing beliefs about a social, political, or scientific issue, the degree to which they will view relevant evidence as strong will differ. This difference, in turn, may result in a failure of the opposing parties to converge on any kind of meaningful agreement, and, under some circumstances, they may become more extreme in their beliefs.

Edwards and Smith then summarize a classic experiment by Lord, Ross, and Lopper ( 1979). This study focused on “people’s evaluations of arguments about whether the death penalty is an effective deterrent against murder.”

They selected two groups of participants, one known to believe that the death penalty is an effective deterrent and one known to believe that it is not an effective deterrent. Both groups were presented with two arguments, one that pointed to the deterrent efficacy of the death penalty and one that pointed to its inefficacy as a deterrent. Each argument consisted of a brief description of the design and findings of a study supporting or opposing the death penalty (e.g., a study showing that a state’s murder rate declined after institution of the death penalty) and was followed by criticisms of the study itself, as well as rebuttals of these criticisms. The best-known finding associated with this study is that the pro-death-penalty and anti-death-penalty participants became more polarized in their beliefs– and hence more different from one another–as a result of reading the two arguments. Note, however, that this result is a logical consequence of another more basic finding obtained by Lord et al.: When participants were asked to rate how convincing each study seemed as evidence (i.e., assessments involved participants’ judgment of the argument’s strength rather than their final belief in the conclusion), proponents of the death penalty judged the pro-death-penalty arguments to be more convincing or stronger than the anti-death-penalty arguments, whereas the opponents of the death penalty judged the anti-death-penalty arguments to be more convincing. This is the prior belief effect, and it has as one of its consequences the polarization of belief.

Edwards and Smith then introduce their model:

When faced with evidence contrary to their beliefs, people try to undermine the evidence. That is, there is a bias to disconfirm arguments incompatible with one’s position. This idea can be developed into a disconfirmation model by making the following assumptions.

1. When one is presented an argument to evaluate, there will be some automatic activation in memory of material relevant to the argument. Some of the accessed material will include one’s prior beliefs about the issue.

2. If the argument presented is incompatible with prior beliefs, one will engage in a deliberative search of memory for material that will undermine the argument simply. Hence, “scrutinizing an argument” is implemented as a deliberate memory search, and such a search requires extensive processing.

3. Possible targets of the memory search include stored beliefs and arguments that offer direct evidence against the premises and conclusion of the presented argument.

4. The outputs of the memory search are integrated with other (perhaps unbiased) considerations about the current argument, and the resulting evaluation serves as the basis for judgments of the current argument’s strength.

Edwards and Smith then conducted two experiments that supported this model and also showed that emotional conviction influences the magnitude and form of the disconfirmation bias.

It makes sense that a disconfirmation bias would exist. If the human brain is wired to defend its preconceptions with confirmation bias, attacking beliefs that threaten those preconceptions would likely be part of the same strategy. This undercuts Michael Shermer’s belief that “Skepticism is the antidote for the confirmation bias.” In reality, hyper-skepticism, or selectively applied skepticism, may simply be another facet of the same brain processes that generate confirmation bias.

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