In human social interactions, rationality is not in the driver's seat. Imagine an economic game, called the Ultimatum Game, where you are given $100 to split between yourself and another person. You have to give some of it away but get to decide the actual amount. The catch is that the other person can refuse the whole transaction. If he or she says no, neither of you gets any money. There is only one rational strategy, from a purely cold-cognition perspective: given $100 to split, you should offer $0.01, keeping $99.99 for yourself, and the other person should accept, since both of you end up better off financially than before. In reality, no one makes such low offers (would you?), and people on the receiving end will resentfully reject any deal that seems extremely unfair, the usual rejection threshold hovering around 20 to 30 percent. Neuroimaging studies of subjects playing the Ultimatum Game show that powerful emotion drives the rejection of unfair offers.