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.

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.

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