Greene doesn’t neglect these sorts of impulses and biases. Indeed, he explains the dark side of our cooperative machinery and of human nature generally. This all feeds into his belief that “our brains are wired for tribalism.” But he doesn’t seem to reflect on the import of that observation. If indeed we’re wired for tribalism, then maybe much of the problem has less to do with differing moral visions than with the simple fact that my tribe is my tribe and your tribe is your tribe. Both Greene and Bloom cite studies in which people were randomly divided into two groups and immediately favored members of their own group in allocating resources—even when they knew the assignment was random.

Of course, for things to get really nasty, you need more than just the existence of two groups. The most common explosive additive is the perception that relations between the groups are zero-sum—that one group’s win is the other group’s loss. In a classic 1950s study mentioned by Bloom (a study that couldn’t be performed today, given prevailing ethical strictures), experimenters created deep hostility between two groups of boys at summer camp by pitting them against each other in a series of zero-sum games. The rift was mended by putting the boys in non-zero-sum situations—giving them a common peril, such as a disruption in the water supply, that they could best confront together.

When you’re in zero-sum mode and derogating your rival group, any of its values that seem different from yours may share in the derogation. Meanwhile, you’ll point to your own tribe’s distinctive, and clearly superior, values as a way of shoring up its solidarity. So outsiders may assume there’s a big argument over values. But that doesn’t mean values are the root of the problem.

The question of how large a role differing value systems play in human conflict hovers over some of the world’s most salient tensions. Many Americans see Muslim terrorists as motivated by an alien “jihadist” ideology that compels militants to either kill infidels or bring them under the banner of Islam. But what the “jihadists” actually say when justifying their attacks has pretty much nothing to do with bringing Sharia law to America. It’s about the perception that America is at war with Islam.

Just look at the best-known terrorist bombers and would-be terrorist bombers who have targeted the United States since 9/11: the Boston Marathon bombers, the would-be “underwear bomber,” the wouldbe Times Square bomber, and the would-be New York subway bombers. All have explicitly cited as their motivating grievance one or more of the following: the wars in Iraq and Afghanistan, drone strikes in various Muslim countries, and American support for Israeli policies toward Palestinians.

There’s no big difference over ethical principle here. Americans and “jihadists” agree that if you’re attacked, retaliation is justified (an extension of the sense of justice, and a belief for which you could mount a plausible utilitarian rationale, if forced). The disagreement is over the facts of the case—whether America has launched a war on Islam. And so it is with most of the world’s gravest conflicts. The problem isn’t the lack of, as Greene puts it, a “moral language that members of all tribes can speak.” Retributive justice, for better or worse, is a moral language spoken around the world—but it is paired with a stubborn and lethal bias about who should be on the receiving end of the retribution.