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.
The answer to version one is D and 5. The answer to version two is beer and 17.
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.
Leda Cosmides and John Tooby of the University of California at Santa Barbara have argued that the difference reveals some of our evolutionary history. Small bands of hominids could only hold together if their members obeyed social rules. If people started cheating on one another--taking other people's gifts of food, for example, without giving gifts of their own--the band might well fall apart. Under these conditions, natural selection produced a cheating detection system in the brain. On the other hand, our hominid ancestors did not live or die based on their performance on abstract logic tests. Rather than being a general-purpose problem-solver, the human brain became adapted to solving the problems that our ancestors regularly faced in life.
The Wason Selection Task has become the center of the debate over evolutionary psychology. Some critics, such as the French psychologist Dan Sperber, claim that Cosmides and Tooby can't make such strong statements about human reasoning from the Wason Selection Task. Others claim that the brain can't be sliced up into modules so nicely.
The controversy has taken a very interesting turn now, thanks to brain imaging. A team of Italian psychologists had people lie in an MRI scanner and work their way through a set of puzzles that followed the same line of logic as the ones I presented above. They then compared how the brain responded to the challenges to see if indeed the brain works differently when it is solving problems in terms of social exchange than when the problem is more abstract.
The psychologists didn't use a conventional Wason Selection Task like the ones above, because they wanted to make the problems as similar as possible, except that one dealt with social exchanges. Brain imaging requires this sort of strict experimental design, because it's very easy to see differences in brain activity that aren't actually relevant to the question a scientist wants to answer. For example, if one puzzle just so happens to involve picturing an object, some of the brain's visual processing may become active. So the researchers told their subjects that the puzzles would involve a hypothetical tribe. A purely descriptive puzzle might require subjects to consider the rule, "If a person cracks walnut shells, then he drinks pond water." The subjects might then see a set of cards that read, "He didn't drink pond water," "He didn't crack walnut shells," He cracked walnut shells," and "He drank pond water." The researchers also had their subjects solve puzzles that involved social exchanges. The rule in these cases might be, "If you give me sunflower-seeds, then I give you poppy petals."
The psychologists report the results of the test in a paper in press at the journal Human Brain Mapping (click the html link to get the whole paper for free). The results are fascinating--although the researchers don't claim to have settled the debate over the cheater module. Both the social exchange and descriptive version of the puzzle activated the same network of regions on the left side of the brain. One region (the angular gyrus) is considered important for semantic tasks. A second region is located near the left temple (the dorsolateral prefrontal cortex). It's essential for considering many different pieces of information at once. The third region, the medial prefrontal cortex, becomes active when people need to bear in mind a larger goal while they solve the many small problems it poses. Previous studies have shown that the left side of the brain plays a much more important role than the right in reasoning and coming up with explanations for how the world works in general.
Now here's the kicker: the social exchange version of the problem doesn't just activate this left-brain network. It also activates the same regions in the right side of the brain. Many studies in which people have thought about social situations have tended to turn on the right side of the brain more than the left, and so in one sense this result isn't too surprising. But it is surprising when you consider that the descriptive version of the puzzle that only switch on parts of the left side of the brain involved thinking about other people and their actions. You might think that that would be social enough to engage any parts of the brain specializing in social thinking. Apparently not. Only when the puzzle involved rules for social exchanges did the right-brain network come on line.
Is this the cheater module? It's conceivable that the Italian psychologists tapped into some social brain circuit that isn't specifically adapted for enforcing social rules, but for some somewhat broader group of social problems. It would be interesting if a test other than the Wason Selection Task could trigger the same left versus left-right patterns. The precise evolutionary forces that shaped this feature of the mind may not be clear yet. But this experiment is an important step towards working out the biology between the strange results of the Wason test. Clearly, our brains throw a lot more neurons at logic problems when they concern our social lives instead of abstractions. Analytic philosophers are made, you could say, but political philosophers are born.
Update: 7:15 pm-- I decided to change the first version of the test to avoid ambiguity.
Update: Tuesday, 8:15 am-- Some commenters have argued that people do better with the bar version of the puzzle because people have more experience with it than with abstract logic. Actually, many variations of the puzzle have been tested out, and the same results emerge. Notice, for example, that the Italian scientists who did the most recent study put the puzzles in terms of a hypothetical tribe, with which the subjects had no experience at all. Despite this different format, almost precisely the same fraction of the subjects got the different versions write as in more familiar versions of the test, such as the bartending example.
Thanks also to the sharp readers who pointed out that the puzzles need to be If-Then propositions.