Brad DeLong points to an article by Virginia Postrel about the nascent science of neuroeconomics. There’s a lot of experimental work showing that people typically trust each other much more than homo economicus would. In standard bargaining games, where it’s rational to stiff the other guy but better (in terms of your prize money) if you both trust each other for a bit, people tend to trust each other. This holds even where experimenters create exchange conditions that positively encourage you to be an asshole—by removing face-to-face interaction, ensuring anonymity, making the games one-shot deals, and so on.

The new studies look at what happens to the brain chemistry of people playing the game. They find that people who trust show higher levels of the hormone oxytocin. In a different set of studies, the article says researchers ' receiving low-ball offers stimulates the part of the brain associated with disgust. “They can predict with good reliability, from looking at the brain, what a person will do,” said Colin F. Camerer, an economist at the California Institute of Technology. “People whose brains are showing lots of disgust will reject offers."'

There’s an interesting question here about what, exactly, is being explained here. On the one hand, people are of course unaware of their current oxytocin levels and fMRI scan patterns, so it’s interesting that we can do a blood test or a brain scan and predict their choice. On the other hand, they’re well aware of when they’re feeling trusting or disgusted. Those feelings have some biological substrate (it’s all happening inside your brain, after all) but how much does it add to say “People whose brains are showing lots of disgust will reject offers”? They’re showing lots of disgust because they’re disgusted.

This is the sort of levels of explanation/ reductionism/ supervenience stuff that the philosophers are good at clearing up. There’s never one around when you need one.