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Suppose that I have a pack of cards each of which has a letter written on one side and a number written on the other side. Suppose in addition that I claim the following rule is true:

If a card has a vowel on one side, then it has an even number on the other side. Imagine that I now show you four cards from the pack:

```      E    T    4    7
```

Which card or cards should you turn over in order to decide whether the rule is true or false?

Answer: Justification: Only about 5% of the educated population give the correct answer, which is E and 7. If you said (only) E, you saw that the rule in question would be overthrown were there to be an odd number on the other side of this card -- but you failed to note that if the 7 card has a vowel on the other side, this too is a case that shoots down the rule.

Devised in 1966 by Peter Cathcart Wason, the Wason selection task, one of the most famous tasks in the psychology of reasoning, is a logic puzzle which is formally equivalent to the following question:

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) should 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 which identifies a card which need not be inverted, or a response which fails to identify a card which needs to be inverted are both incorrect. Note that the original task dealt with numbers (even, odd) and letters (vowels, consonants).

The response Wason considered correct was to turn the cards showing 8 and brown, but no other card. Remember how the question was stated: "If the card shows an even number, then its opposite face is red." If we turn over the card labelled "3" and find that it is red, this does not invalidate the rule. Likewise, if we turn over the red card and find that it has the label "3", this also does not make the rule false. On the other hand, if the brown card has the label "4", this invalidates the rule: it has an even number, but is not red. The interpretation of "if" here is that of the material conditional in classical logic.

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. 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., "17", "beer", "22", "coke", most people have no difficulty in selecting the correct cards ("17" and "beer").

This suggests a principle to distinguish Wason tasks which people find easy from those that they find difficult: namely, 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. Such a distinction, if empirically borne out, would support the contention of evolutionary psychologists that certain features of human psychology may be mechanisms that have evolved, through natural selection, to solve specific problems of social interaction, rather than expressions of general intelligence.