irrationality? I'm a sucker for studies which mix economics and biology. Like this one, examining brain scans of women playing Prisoner's Dilemma games:

Using MRI scans, the investigators found that when both players cooperated, the player whose brain was being scanned showed significant activation in brain regions associated with reward. They report their findings in the July 18th issue of Neuron. In an interview with Reuters Health, Berns pointed out that this finding may explain why players opt for cooperating over the more rational option of defecting, "because social cooperation is intrinsically rewarding in the human brain."

Unfortunately, this is plain wrong -- he's trying to solve a problem that doesn't exist. While defecting is the "rational" option for a single game of Prisoner's Dilemma, it is not necessarily the best strategy for repeated interactions. (The best strategy for repeated interactions depends on your opponent's strategy.) And if you believe that our evolutionary history is full of repeated interactions (which seems a pretty reasonable assumption), then there's nothing "irrational" about a brain which gets pleasure out of cooperating. That said, the result is kind of neat. Update: I'm not perfectly happy with what I just wrote. Here's the point I was trying to make: I could have told you "people behave 'irrationally' in a one-shot prisoner's dilemma because cooperating brings them brain happiness" without ever having read the study. It's almost a tautology. The study demonstrates a mechanism for the brain happiness. But calling that an "explanation" is deeply unsatisfactory to me. Why would our brains direct us to behave in such an "irrational" way? Because (possible explanation) our brains evolved to deal with (among other things) repeated prisoner's dilemma situations, in which case cooperation might well be part of a "rational" strategy.