This Card Trick Shows If You’re Logical

You sit down at a table and the man in tweed smiles. He introduces himself, lights his pipe, then lays four cards down carefully in front of you. The first card shows the number five, and the second card shows the number eight. The third card is simply blue, and the fourth card green. The man looks at you, and speaks bluntly.

“If there is an even number on one side of a card,” he says, “then the opposite side is always blue.”

Here’s the thing: He’s probably lying. You know this, but now you have to prove it. Without turning any unnecessary cards, which cards on the table would you have to flip over in order to prove whether the man’s statement is true or false?

This game is called the “Wason selection task,” named for English psychologist Peter Wason (the kind gent who hypothetically just dealt the cards to you). And while it may seem easy, this card trick has been endlessly analyzed and replicated since its 1964 debut as a perfect example of just how illogical most humans actually are. The correct answer, it turns out, eluded 90% of test subjects in Wason’s original experiments—even when those subjects were given a second or third chance to try and solve it.

Do you have a logical mind?

How did you do? If you didn’t get the correct answer  on your first try, don’t worry: the setup of the test is intentionally misleading, encouraging you to take mental shortcuts and answer before really thinking about the problem.

But what if the cards weren’t so abstract? What if they actually referenced a selection problem you might encounter in real life—say, enforcing a law as an officer of the peace? Later researchers wondered this same thing, and came up with this tweak to the puzzle in the ‘80s:

You are a police officer who walks into a bar and must determine whether four patrons are obeying the law, “if a person is drinking beer, that person must be 21 or older.” Four cards are dealt as before, but this time the first card says “drinking beer,” the second says “drinking Coke,” the third says “16 years old,” and the fourth says “21 years old.” Which cards must you flip over to determine if the law was being observed in this bar?

The underlying logic of this puzzle is exactly the same as Wason’s original brain-teaser—yet this time, thanks to the average person’s intimate understanding of social contracts, 75% of subjects correctly chose to turn over only the “16 years old” card and the “drinking beer” card to find out whether the law was in effect. Amazing how a little change of context can make a big difference in our crazy, brilliant brains.