Named in honour of Peter Cathcart Wason, who first described the task, the Wason selection task is a logical 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 coloured patch on the other side. The visible faces of the cards show 3, 8, red and brown. Which cards should you turn over in order to test the truth of the proposition that if a card shows an even number, then its opposite face shows a primary colour?

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.

Solution

The correct response is that the cards showing 8 and brown must be inverted, but no other card. Remember how the question was stated: "If the card shows an even number, Then its opposite face shows a primary colour." 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, and does not have a primary colour.

Experiments have shown that, presented with a Wason task as an uncontextualised logic puzzle, people perform very poorly. Furthermore, even of those who respond correctly, some obtain the correct result by conscious application of the contrapositive rule. By contrast, some (though not all) Wason tasks prove much easier when they are presented in a context of social relations. For example, if the rule "Only people over 18 are allowed to drink alcohol" is tested, most people have no difficult in selecting the correct subjects (those under 18 and those that are drinking) that must be tested.

Adherents of evolutionary psychology have argued that a simple rule distinguishes Wason tasks which people find easy from those that they find difficult. The suggested rule is that a Wason task proves to be easy if the rule to be tested is one of social exchange (in order to receive benefit X you fulfill condition Y) and the subject is asked to police the rule, but is difficult otherwise. If this classification is accepted, then it supports 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.

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