The Wason selection task was devised by Peter Cathcart in 1966 examining the use of formal logic in human reasoning. It constitutes one of the most famous experiments in psychology, after the stroop effect. In the selection task, participants are asked to solve the following problem using the smallest number of steps:

Firstly, you are shown a set of 4 cards, all of which contains a number on one side and a letter on the other. There are 4 characters visible to you because they are facing up: E, K, 4, 7 (as shown on right). You are asked to turn over one or more cards in order to test the validity of the proposition that if there is a vowel on one side, there must be an even number on the other. Which card would you flip over first. Remember, you are asked to use the smallest number of steps.

Would you turn over A or 4 first? If you answer yes, then you agree with the majority of the participants in the experiment who argue that turning over A and 4 alone are sufficient in establishing the validity of the proposition.

However, the correct answer is neither A or 4. Instead, it is 7 because 7 is the only card that can invalidate the proposition and thereby establishing the truth. Turning over A to find an even number or 4 to find a vowel cannot invalidate the proposition. It merely establish the fact that the rule has been correct up until this point. But what if one turns over the odd number 7 and find a vowel on the other side? This observation would immediately invalidate the proposition because the rule stipulates that if there's a vowel on one side, there must be an even number on the other. However, as one can see, this rule does not hold for the card with the number of 7, thus the proposition is rendered false. Therefore, turning over 7 is the most effecient way of examining the validity of the proposition.

However, as Cathcart found out, the majority of individuals did not use formal logic when solving the problem. Instead, more than 90% of the participants chose either A or 4. Only 5% of the participants chose the correct answer 7. This finding would suggest that individuals do not readily use formal logic in reasoning.

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