"Given that the monkeys aren't very smart in the first place, you might assume that any gambling strategy was well beyond their capabilities. In that case, you'd expect them to prefer it when a researcher initially offered them two grapes instead of one. But precisely the opposite happened! Once the monkeys figured out that the two-grape researcher sometimes withheld the second grape and that the one-grape researcher sometimes added a bonus grape, the monkeys strongly preferred the one-grape researcher. A rational monkey wouldn't have cared, but these irrational monkeys suffered from what psychologists call "loss aversion." They behaved as if the pain from losing a grape was greater than the pleasure from gaining one. Up to now, the monkeys appeared to be as rational as humans in their use of money. But surely this last experiment showed the vast gulf that lay between monkey and man."