Suppose there are four cards on the table in front of you. These are not regular playing cards - they have a number on one side and a letter on the other.

The cards read: A, D, 4, 7.

Which cards do you need to turn over to make sure this statement is true:

“If a card has a vowel on one side, then it has an even number on the other side.”

If you’re like over 90% of the people that psychologist Peter Wason asked in 1977, you probably got the answer wrong. The correct answer is A and 7. If you answered correctly, good job!

The reasoning is that A could have an odd number on the other side, and on the other side of 7 there could be a vowel. Turning D and 4 can’t prove the statement wrong, because it doesn’t matter what’s on the other side.

What this problem is trying to illustrate is how people use logic inconsistently. Some people assume that the question also meant that consonants must have an odd number on the other side. Other people find the problem too complex - but upon explanation, they generally understand the solution and agree that it’s correct.

What’s also very interesting is how in a different context, the answer to this problem may be more obvious. Psychologists Leda Cosmides and John Tooby created an alternative statement, in the context of social relations:

“If you’re drinking alcohol then you must be over 18.”

If you were presented with cards that have beverages on one side and age on the other:

16, beer, 25, coke

Most people would be able to figure out that the answer is 16 and beer. It doesn’t matter what a 25-year-old is drinking, or who’s drinking coke.

The lesson here is: use logic wisely. In everyday life, it’s easy to misunderstand problems, assume things are true when they may not be, or skip a step in our reasoning.

“To make mistakes is human; to stumble is commonplace; to be able to laugh at yourself is maturity.” - William Arthur Ward