One of the big challenges to the perfectly rational value maximizer beloved of economic models is the existence of altruistic punishers--people who will, in experiments, spend their own money to punish people htey perceive as having acted unfairly. These people do so even though there is no particular likelihood of seeing those people again, suggesting that this is some sort of genetically programmed response, found in a significant fraction of the population, that helps quell opportunistic behaviour. Now there's some evidence for the genetic explanation: apparently, identical twins tend to play the same strategy in ultimatum games.

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