Why does pretty much everyone feel that it's good to punish terrorists? Obviously, there are sound practical reasons to punish people who do bad things, and all of us, pressed to articulate our retributive urge, can list them. Still, the urge precedes the articulation. It is an emotional reflex, a part of human nature no less than the feelings of hunger or lust.
And, like them, it has done much to shape human history. It motivated last week's terrorist attack, and it will help guide the American response. That's why it's worth trying to fathom: The better we understand the retributive urge, the less likely we are to be misguided by it.
In a way, "urge" is too crude a label. I'm talking not just about the initial burst of anger and vengefulness, but the conviction, forged in that burst and surviving cool reflection, that retribution is right: the sense that, somewhere up in the heavens, some moral scale will have been balanced when people who have wronged you—or who have wronged other people—suffer.
One of the triumphs of modern Darwinian theory is to show how even feelings this ethereal—moral intuitions, not just raw animal impulses—could be products of natural selection. Evolutionary psychologists now have a pretty clear idea why people in all cultures intuit that just as good deeds should be rewarded, bad deeds should be punished.
The explanation gets back to those practical reasons for retribution that we all can articulate when pressed: If people who have wronged you see that there's a price to pay, they'll be less likely to wrong you in the future; and if bystanders see that people who wrong you wind up suffering, they, too, will be less likely to mess with you. Though our pre-human ancestors weren't smart enough to grasp these principles, these principles nonetheless shaped those ancestors, via natural selection. (Birds don't grasp the laws of aerodynamics, but those laws shaped birds.) Because the retributive impulse helped those who possessed it to survive and thrive in the social jungle, genes conducive to it flourished. Meanwhile, those who lacked a retributive disposition were more likely to get abused—have their mates stolen, their food stolen, their acts of generosity not repaid, and so on. They were in general less likely to survive and reproduce in the rough-and-tumble hunter-gatherer social environment that was the context of human evolution. Over time, their genes tended to fall by the wayside, and genes counseling retribution won out.
You see the legacy of this Darwinian logic in chimpanzees, our nearest relatives and the closest thing we have to a living example of our ancestors circa 5 million B.C. They don't just fight back in self-defense, something any old organism can do. They remember the particular chimp that has wronged them and may retaliate well after the event.
There are occasions when a chimp gets so theatrically angry over being wronged that some primatologists think they see signs of a moralistic outrage. Chimps may possess "the same sense of moral rightness and justice" that humans possess, according to Frans de Waal.
But it's unlikely that in chimps this sense is nearly as well developed as in humans. In the view of evolutionary psychologists Martin Daly and Margo Wilson, this sense got so elaborate in our lineage because of language: We are "designed" by natural selection not only to retaliate, but to justify our retaliation to others—whether to recruit allies to our cause or to keep allies of our enemies from intervening. We naturally go around asserting that people and groups of people have done us wrong and therefore should be punished. And arguments of this form have resonance precisely because the retributive instinct is built into everyone. Even pacifists have to consciously resist it from time to time.
In sum: Natural selection "discovered" the principles of deterrence back when our ancestors were too dumb to discover them and implanted a kind of proxy for them in our lineage—the innate sense that retribution is good. Then, later, we in a sense rediscovered the deterrent logic behind retribution and started babbling about it as if we had discovered it first.
In fact, we not only rediscovered this logic; we enshrined it in law. The various deterrent effects of punishment are explicit rationales for it in American legal doctrine. But—interestingly, in light of Darwinian theory—legal doctrine also deems retribution a good thing in and of itself, regardless of whether it actually has deterrent effects. And why is that? So far as I can tell, after listening to moral philosophers try to justify this principle, the answer is this: because these moral philosophers—and jurists over the ages—were born with the intuition that retribution is good. They are just trusting their instincts. And because this instinct feels loftier than, say, hunger or lust, they accord it a loftier kind of authority.