Wason selection task  

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"Given the judgments (themselves produced by the non-conscious cognitive machinery in the brain, sometimes correctly, sometimes not so), human beings produce rationales they believe account for their judgments. But the rationales (on this argument) are only ex post rationalizations." Wason, 1969, “Regression in Reasoning?” British Journal of Psychology 60:471–80

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Train wreck at Montparnasse (October 22, 1895) by Studio Lévy and Sons.
Train wreck at Montparnasse (October 22, 1895) by Studio Lévy and Sons.

The Wason selection task (or four-card problem) is a logic puzzle devised by Peter Cathcart Wason in 1966. It is one of the most famous tasks in the study of deductive reasoning. An example of the puzzle is:

You are shown a set of four cards placed on a table, each of which has a number on one side and a colored patch on the other side. The visible faces of the cards show 3, 8, red and brown. Which card(s) must you turn over in order to test the truth of the proposition that if a card shows an even number on one face, then its opposite face is red?

A response that identifies a card that need not be inverted, or that fails to identify a card that needs to be inverted, is incorrect. The original task dealt with numbers (even, odd) and letters (vowels, consonants).

The importance of the experiment is not in justifying one answer of the ambiguous problem, but in demonstrating the inconsistency of applying the logical rules by the people when the problem is set in two different contexts but with very similar connection between the facts.



The correct response is to turn over only the 8 and brown cards.

The rule was "If the card shows an even number on one face, then its opposite face is red." Only a card with both an even number on one face and something other than red on the other face can invalidate this rule:

  • If the 3 card is red (or brown), that doesn't violate the rule.
  • If the 8 card is brown, that violates the rule.
  • If the red card is odd (or even), that doesn't violate the rule.
  • If the brown card is even, that violates the rule.

Use of logic

The interpretation of "if" here is that of the material conditional in classical logic, so this problem can be solved by choosing the cards using modus ponens (all even cards must be checked to ensure they are red) and modus tollens (all non-red cards must be checked to ensure they are non-even).

Explanations of performance on the task

In Wason's study, not even 10% of subjects found the correct solution. This result was replicated in 1993.

Some authors have argued that participants do not read "if... then..." as the material conditional, since the natural language conditional is not the material conditional. (See also the paradoxes of the material conditional for more information.) However one interesting feature of the task is how participants react when the classical logic solution is explained:

A psychologist, not very well disposed toward logic, once confessed to me that despite all problems in short-term inferences like the Wason Card Task, there was also the undeniable fact that he had never met an experimental subject who did not understand the logical solution when it was explained to him, and then agreed that it was correct.

This latter comment is also controversial, since it does not explain whether the subjects regarded their previous solution incorrect, or whether they regarded the problem sufficiently vague to have two interpretations.

Policing social rules

As of 1983, experimenters had identified that success on the Wason selection task was highly content-dependent, but there was no theoretical explanation for which content elicited mostly correct responses and which ones elicited mostly incorrect responses.

Evolutionary psychologists Leda Cosmides and John Tooby (1992) identified that the selection task tends to produce the "correct" response when presented in a context of social relations. For example, if the rule used is "If you are drinking alcohol then you must be over 18", and the cards have an age on one side and beverage on the other, e.g., "16", "drinking beer", "25", "drinking coke", most people have no difficulty in selecting the correct cards ("16" and "beer"). In a series of experiments in different contexts, subjects demonstrated consistent superior performance when asked to police a social rule involving a benefit that was only legitimately available to someone who had qualified for that benefit. Cosmides and Tooby argued that experimenters have ruled out alternative explanations, such as that people learn the rules of social exchange through practice and find it easier to apply these familiar rules than less-familiar rules.

According to Cosmides and Tooby, this experimental evidence supports the hypothesis that a Wason task proves to be easier if the rule to be tested is one of social exchange (in order to receive benefit X you need to fulfill condition Y) and the subject is asked to police the rule, but is more difficult otherwise. They argued that such a distinction, if empirically borne out, would support the contention of evolutionary psychologists that human reasoning is governed by context-sensitive mechanisms that have evolved, through natural selection, to solve specific problems of social interaction, rather than context-free, general-purpose mechanisms. In this case, the module is described as a specialized cheater-detection module.

Davies et al. (1995) have argued that Cosmides and Tooby's argument in favor of context-sensitive, domain-specific reasoning mechanisms as opposed to general-purpose reasoning mechanisms is theoretically incoherent and inferentially unjustified. Cosmides and Tooby have adapted the Wason selection task to test for one aspect of propositional logic while ignoring general-purpose reasoning based on syllogistic logic, predicate logic, modal logic, and inductive logic. They assumed that the domain-specific hypothesis was correct without first ruling out the general-purpose hypothesis. Moreover, the experiments intended to show that reasoning is not governed by general-purpose rules either fail to eliminate the general-purpose hypothesis or else actually provide evidence supporting it, which according to Davies and his colleagues suggests that the argument in favor of domain-specific reasoning might be empirically false. In addition to that, Cosmides and Tooby assume that reasoning is either determined exclusively by domain-specific or general-purpose algorithms, a dichotomy that Davies et al. view as misconceived.

See also

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