According to Plutarch, Aristotle gave the following advice to Alexander the Great: "to have regard for the Greeks as for friends and kindred, but to conduct himself toward other peoples as though they were plants or animals." Believe it or not, this attitude was, in its day, morally progressive.

After all, not so long ago the Greeks had treated one another as plants and animals. A tombstone from the fifth century B.C. recounts how a Greek fighting for Athens had killed seven non-Athenian Greeks yet had "brought sorrow to no one among all men who dwell on earth." In those days, Athenian armies had a habit of celebrating victories over another Greek city by executing all its male citizens (though there was always the temptation to enslave them instead).

So when Aristotle--like his mentor, Plato--counseled mutual respect among Greeks, he was urging an expansion of the compass of moral consideration. And that was just the beginning. Today, two and a half millennia later, humanity's moral compass has reached planetary breadth.

Athenian armies had a habit of celebrating victories over another Greek city by executing all its male citizens.

By that I don't mean that everyone treats everyone well, or that there is no bigotry or oppression or slaughter. I just mean that in most of the world--and certainly in the most economically developed nations--the consensual, publicly stated morality deems all people worthy of decent treatment, regardless of race, religion, or nationality. Even if that ideal is sometimes honored in the breach, we've still come a long way since Aristotle.

Why the progress? This question was taken up two decades ago by the philosopher Peter Singer in his book "The Expanding Circle." (More recently, Singer has become controversial for arguing that parents with severely disabled newborns should have the option of euthanasia.)

Singer's answer to the riddle of moral progress starts with the fact that people naturally defend their personal interests in objective-sounding language. If you ask your boss for a raise, you don't say, "I deserve it because I'm me." Rather, you claim that anyone with your strong work ethic, steadfast devotion, and so on, would deserve a raise. If you kill someone, you don't claim that you alone are entitled to kill people. Rather, you appeal to some particular circumstance--self-defense, temporary insanity--that, you argue, would exonerate anyone who shared it.

In Singer's view, once we buy into the premise of this rhetorical strategy--that what people deserve depends on their objective circumstances, not on who they are--it drags us toward moral enlightenment. "The idea of a disinterested defense of one's conduct" may have arisen as a tool of self-interest, "but in the thought of reasoning beings, it takes on a logic of its own which leads to its extension beyond the bounds of group."

There is probably some truth to this, but I have a different--or, at least, a supplementary--explanation. A recurring theme in history is growing interdependence. Once those Greek cities united to fight the Persian Empire, they depended on one another for their security. They would prevail together or fall together; their fates were positively correlated.

It isn't just war that puts previously separate peoples in the same boat. As transportation and communications technologies have advanced--ever since the invention of the wheel and of writing--the realm of economic exchange has grown. And, as we're constantly reminded in this age of globalization, economic linkage means interdependence. Today, if any big nation sinks into economic depression, that's bad news for other nations. By the same token, prosperity is contagious; economic advance in one place spurs advance elsewhere.

Game theorists describe interdependence as a "non-zero-sum" game. Most games--badminton, boxing, chess--are "zero-sum," with one winner and one loser. In non-zero-sum games, the outcome can be "win-win"--or, if the game is played badly, "lose-lose."

In a sense, human history is the story of more and more people playing non-zero-sum games over greater and greater distances. This expansion of "non-zero-sumness," I think, has much to do with the expansion of our moral compass. In a non-zero-sum game, your fate is correlated with the fates of other players, so it doesn't make sense to wish them ill. You need them too much to hate them.

The Greeks could afford to hate the Persians, but Americans can't afford to hate the Russians--or, at least, they can't afford to act on such hatred.