A Brain Teaser Illustrating the Importance of Context

Paraphrased from The Tipping Point by Malcolm Gladwell (pp. 159Ė160)

Given a deck of cards with letters on one side and numbers on the other. The rule of the the deck is that a card with a vowel on it always has to have an even number on the other side.

Suppose you are shown four cards on a table labeled with the letters A and D and the numerals 3 and 6. Which of the cards would you have to turn over to verify this rule of the deck?

The answer is two: the A card and the three card. The overwhelming majority or the people given this test, though, donít get it right. They tend to answer just the A card, or the A and the six. Itís a hard question.

Now consider this question. Suppose there are four people at a bar. One is drinking fruit juice. One is clearly a teenager. One is drinking beer and one is clearly over twenty-five.

Given the law that no one under twenty-one is allowed to drink beer, which of those peopleís ID has to be checked to make sure the law is being observed?

Now this answer is easier. In fact, Iím sure that almost everyone will get it right: the beer drinker and the sixteen-year old.

But as the psychologist Leda Cosmides (who dreamt up this example) points out, it is exactly the same puzzle as the A, D, 3, and 6 puzzle. The difference is that it is framed in a way that makes it about people, instead of about numbers, and as human beings we are a lot more sophisticated about each other than we are about the abstract world.