Kluge: The Haphazard Construction of the Human Mind (Houghton Mifflin; 2008)

An even more disquieting study asked a group of subjects to play a game known as "prisoner's dilemma," which requires pairs of people to choose to either cooperate with each other or "defect" (act uncooperatively). You get the bigger payoff if you and the other person both cooperate (say, $10), an intermediate reward (say, $3) if you defect and your opponent cooperates, and no reward if you both defect. The general procedure is a staple in psychology research; the catch in this particular study was that before people began to play the game, they sat in a waiting room where an ostensibly unrelated news broadcast was playing in the background. Some subjects heard prosocial news (about a clergyman donating a kidney to a needy patient); others, by contrast, heard a broadcast about a clergyman committing murder. What happened? You guessed it: people who heard about the good clergyman were a lot more cooperative than those who heard about the bad clergyman.

In all these studies, emotions of one sort or another prime memories, and those memories in turn shape choice. A different sort of illustration comes from what economist George Loewenstein calls "the attraction of the visceral." It's one thing to turn down chocolate cheesecake in the abstract, another when the waiter brings in the dessert cart. College students who are asked whether they'd risk wasting 30 minutes in exchange for a chance to win all the freshly baked chocolate chip cookies they could eat are more likely to say yes if they actually see (and smell) the cookies than if they are merely told about them.

Hunger, however, is nothing compared to lust. A follow-up study exposed young men to either a written or a (more visceral) filmed scenario depicting a couple who had met earlier in the evening and are now discussing the possibility of (imminently) having sex. Both are in favor, but neither party has a condom, and there is no store nearby. The woman reports that she is taking a contraceptive pill and is disease-free; she leaves it up to the man to decide whether to proceed, unprotected. Subjects were then asked to rate their own probability of having unprotected sex if they were in the male character's shoes. Guess which group of men — readers or video watchers — was more likely to throw caution to the wind? (Undergraduate men are also apparently able to persuade themselves that their risk of contracting a sexually transmitted disease goes down precisely as the attractiveness of their potential partner goes up.) The notion that men might think with organs below the brain is not new, but the experimental evidence highlights rather vividly the degree to which our choices don't necessarily follow from purely "rational" considerations. Hunger, lust, happiness, and sadness are all factors that most of us would say shouldn't enter into rational thought. Yet evolution's progressive overlay of technology has guaranteed that each wields an influence, even when we insist otherwise.

The clumsiness of our decision-making ability becomes especially clear when we consider moral choices. Suppose, for example, that a runaway trolley is about to run over and kill five people. You (and you alone) are in a position such that you can hit a switch to divert the trolley onto a different set of tracks, where it would kill only one person instead of five. Do you hit the switch?

Now, suppose instead that you are on a footbridge, standing above the track that bears the runaway trolley. This time, saving the five people would require you to push a rather large person (considerably bigger than you, so don't bother to volunteer yourself) off the footbridge and into the oncoming trolley. The large person in question would, should you toss him over, die, allowing the other five to survive. Would thatbe okay? Although most people answer yes to the scenario involving the switch, most people say no to pushing someone off the footbridge — even though in both cases five lives are saved at the cost of one.

Why the difference? Nobody knows for sure, but part of the answer seems to be that there is something more visceral about the second scenario; it's one thing to flip a switch, which is inanimate and somewhat removed from the actual collision, and another to forcibly send someone to his death.