Carl Zimmer in his excellent blog, The Loom (do first read the post, then try the tests before scrolling down to the answer):
Evolutionary psychologists argue that we can understand the workings of the human mind by investigating how it evolved. Much of their research focuses on the past two million years of hominid evolution, during which our ancestors lived in small bands, eating meat they either scavenged or hunted as well as tubers and other plants they gathered. Living for so long in this arrangement, certain ways of thinking may have been favored by natural selection. Evolutionary psychologists believe that a lot of puzzling features of the human mind make sense if we keep our heritage in mind.
The classic example of these puzzles is known as the Wason Selection Task. People tend to do well on this task if it is presented in one way, and terribly if it is presented another way. You can try it out for yourself.
You are given four cards. Each card has a number on one side and a letter on the other. Indicate only the card or cards you need to turn over to see whether any of these cards violate the following rule: if a card has a D on one side, it has a 3 on the other side.
Now you’re a bouncer at a bar. You must enforce the rule that if a person is drinking beer, then he must be over 21 years old. The four cards below each represent one customer in your bar. One side shows what the person is drinking, and the other side shows the drinker’s age. Pick only the cards you definitely need to turn over to see if any of these people are breaking the law and need to be thrown out.
Highlight the area between the Xs for the answers:
X The answer to version one is D and 5. The answer to version two is beer and 17. X
If you took these tests, chances are you bombed on version one and got version two right. Studies consistently show that in tests of the first sort, about 25% of people choose the right answer. But 65% of people get test number two right.
This is actually a very weird result. Both tests involve precisely the same logic: If P, then Q. Yet putting this statement in terms of social rules makes it far easier for people to solve than if it is purely descriptive.