Number of words: 392
People aren’t good at dealing with the uncertainties or missing information in a puzzle. Here’s a neat little example that’s been tested in psychological studies and widely discussed. You’ve got four cards on the table. Each card has a letter on one side and a number on the other side. Naturally,you can only see the side that’s faceup:
The puzzle is this: “Identify which card(s) you need to turn over in order to test the rule ‘If there is a vowel on one side of the card, there is an even number on the other side.’ “
I’ll give you two hints (which aren’t normally given). Hint number one is that this isn’t a trick question. There’s nothing underhanded going on. The puzzle is as simple as it looks. Hint number two is that your answer is probably going to be wrong.
Most people say either the A card or the A and 2 cards. Well, A is a vowel, and we don’t know what number is on the other side. There could be an odd number on the back of the A card. That would disprove the rule. You have to turn the A card over to test it. Fair enough.
What about the 2 card? Two is an even number, and the rule says that if there’s a vowel on one side, then there’s got to be an even number on the other side. It doesn’t say that only vowel cards have even numbers. Say there’s a C on the other side of the 2. It wouldn’t disprove the rule. Whether there’s a vowel or consonant, it makes no difference. The 2 card is irrelevant.
So the answer’s just turn over the A card, right? Wrong. You need to turn over the 7 card too. For all we know, there could be a vowel on the other side of the 7. That would disprove the rule.
The correct answer, then, is that you have to turn over the A and the 7. This type of puzzle is known as the Wason selection task, after Peter Wason, the psychologist who described it in 1966. In studies using this type of puzzle, the reported success rate has ranged from about 20 percent all the way down to 0.
Excerpted from page numbers95-97 of ‘How Would You Move Mount Fuji?’ by William Poundstone.