Imagine that killers have invaded your neighborhood. They're in your house, and you and your neighbors are hiding in the cellar. Your baby starts to cry. If you had to press your hand over its face till it stopped fighting -- if you had to smother it to save everyone else -- would you do it?

If you're normal, you wouldn't, according to a study published last month in Nature. But if part of your brain -- the ventromedial prefrontal cortex (VMPC) -- were damaged, you would. In the study, people were given hypothetical dilemmas: Would you throw a fatally injured person off a lifeboat to save everyone else? Would you kill a healthy hostage? Most normal people said no. Most people with VMPC damage said yes.

It's easy to dismiss the damaged people as freaks. But the study isn't really about them. It's about us. Neuroscience is discovering that the brain isn't a single organ. It's an assembly of modules that sometimes cooperate and sometimes compete. If you often feel as though two parts of your brain are fighting it out, that's because, in fact, they are.

Some of those fights are about morality. Maybe abortion grosses you out, but you'd rather keep it safe and legal. Or maybe homosexuality sounds icky, but you figure it's nobody's business. Emotion tells you one thing, reason another. Often, the reasoning side makes calculations: Throwing the wounded guy off the lifeboat feels bad, but if it will save everyone else, do it.

Philosophers have a name for this calculating logic: utilitarianism. They've been debating it for 200 years. Some say it's sensible; others say it's ruthless. Lately, however, the debate has been overrun by neuroscience. According to the neuroscientists, philosophers on both sides are wrong, because morality doesn't come from God or transcendent reason. It comes from the brain.

Three years ago in the journal Neuron, the neuroscientists illustrated their point. They showed that utilitarian decisions involved "increased activity in brain regions associated with cognitive control." From this and other data, they surmised that the moral debate reflects "tension between competing subsystems in the brain." On one side are "the social-emotional responses that we've inherited from our primate ancestors." On the other is a utilitarian calculus "made possible by more recently evolved structures in the frontal lobes." The war of ideas is a war of neurons.

That's where the new study comes in. The idea was to find out what happens when the emotional side, through the VMPC, gets knocked out. As predicted, calculation takes over. Take a kidney? Push a guy in front of a trolley? If it'll save more lives, sure.

Some of the study's authors think this finding vindicates emotions. Since people with VMPC damage are "abnormally 'utilitarian,' " they argue, emotions are necessary to produce "normal judgments of right and wrong." In fact, the authors add, "By showing that humans are neurologically unfit for strict utilitarian thinking, the study suggests that neuroscience may be able to test different philosophies for compatibility with human nature."

So brain science has discredited religion and philosophy, but don't worry: Morality won't disappear. Brain science is offering itself as the new authority. What's moral in the new world is what's normal, natural, necessary and neurologically fit.

The catch is that what's normal, natural, necessary and neurologically fit can change. In fact, it has been changing throughout history. As our ancestors adapted from small, kin-based groups to elaborate nation-states, the brain evolved from reflexive emotions toward the abstract reasoning power that gave birth to utilitarianism. The full story is a lot more complicated, but that's the rough outline.

And evolution doesn't stop here. Look around you. The world of touch, tribe and taboo is fading. Acceptance of homosexuality is spreading at an amazing pace. Trade is supplanting war. Democracy and communications technology are forcing governments to promote the general welfare. Utilitarians welcome these changes, and so do I.

But utility unchecked can become a monster. The Internet is liberating us from physical contact. Economic globalization is crushing resistance to the bottom line. Companies are sending employees abroad for cheap medical care. Brokers are buying organs from slum-dwellers. In a utilitarian world, you do what it takes. It's all about helping people.

If you're out of step with this world -- too squeamish to slash the payroll or pull the plug -- we can help. Books by neuroscientists will teach you the appeal of utilitarianism and the illogic of your aversion to it. If that doesn't work, maybe we can tweak your brain. Two months ago, when research showed that damage to another area of the brain could help people quit smoking, scientists inferred that therapy in that area might achieve the same happy result without the damage. Why not target the VMPC in a similar way? We won't even need drugs. Last year, psychologists proved they could boost willingness to kill in a utilitarian dilemma just by showing people a clip from "Saturday Night Live."

Not that we want you to go around killing people. At least, not until you join the military. Five years ago, in a government report, scientists proposed using microscopic technology to screen soldiers' brains for emotional interference. Today, the Neurotechnology Industry Organization is lobbying for a federal initiative to study the ethics as well as the mechanics of brain science. "Right now, we're discovering the seat of morality," warns NIO Director Zack Lynch. "In 10 to 15 years, we'll have the technologies to manipulate it."

But there's the other catch: Once technology manipulates ethics, ethics can no longer judge technology. Nor can human nature discredit the mentality that shapes human nature. In a utilitarian world, what's neurologically fit is utilitarianism. It'll become the norm, the standard of right and wrong. Sure, a few mental relics of our primate ancestry will be lost. But it'll be worth it. I think.

human@slate.com

William Saletan covers science and technology for Slate, the online magazine at www.slate.com.