In a seven-thousand word investigation into humanity's moral instincts, Steven Pinker essentially endorses Jonathan Haidt's view that our moral impulses can be grouped into five categories, two "liberal" (harm/care, and fairness/reciprocity) and three "conservative" (ingroup/loyalty, authority/respect, and purity/sanctity). Then, near the close of the essay, he takes up the question of whether any of these impulses ought to be obeyed, and if so, why:
Two features of reality point any rational, self-preserving social agent in a moral direction. And they could provide a benchmark for determining when the judgments of our moral sense are aligned with morality itself.
One is the prevalence of nonzero-sum games. In many arenas of life, two parties are objectively better off if they both act in a nonselfish way than if each of them acts selfishly. You and I are both better off if we share our surpluses, rescue each other’s children in danger and refrain from shooting at each other, compared with hoarding our surpluses while they rot, letting the other’s child drown while we file our nails or feuding like the Hatfields and McCoys. Granted, I might be a bit better off if I acted selfishly at your expense and you played the sucker, but the same is true for you with me, so if each of us tried for these advantages, we’d both end up worse off. Any neutral observer, and you and I if we could talk it over rationally, would have to conclude that the state we should aim for is the one in which we both are unselfish. These spreadsheet projections are not quirks of brain wiring, nor are they dictated by a supernatural power; they are in the nature of things.
The other external support for morality is a feature of rationality itself: that it cannot depend on the egocentric vantage point of the reasoner. If I appeal to you to do anything that affects me — to get off my foot, or tell me the time or not run me over with your car — then I can’t do it in a way that privileges my interests over yours (say, retaining my right to run you over with my car) if I want you to take me seriously. Unless I am Galactic Overlord, I have to state my case in a way that would force me to treat you in kind. I can’t act as if my interests are special just because I’m me and you’re not, any more than I can persuade you that the spot I am standing on is a special place in the universe just because I happen to be standing on it.
So it turns out that the "features of reality" militate in favor of a moral system that emphasizes harm, fairness, and individual rights - which is to say, reality is a liberal! Of course, as Will Wilkinson notes, this argument swipes a few bases. For one thing, the liberal instincts are "rational" only if you assume the liberal premise that the primary goal of human life is material flourishing. (As Will writes: "I simply don’t see how this stands as an adequate reply to someone who says that it is better that millions suffer and/or die for the greater glory of the tribe, or the Prophet, or to prevent the defilement of the blood of the Motherland.") For another, even if you set material flourishing as your highest good, it's still possible to make a case on rational, self-interested grounds for the usefulness of the illiberal impulses, because human nature is such that many people may be happier, longer-lived, more prosperous and so forth in societies shaped at least in part by hierarchy, purity, in-group solidarity, and so forth (what Haidt terms the "beehive" instincts) than in societies that recognize "do as you will, harm no one" as the only moral principle there is. (I make roughly that case here, albeit while repeatedly misspelling Haidt's name.)
Moreover, as a guide to individual moral action - as opposed to a description of the impulses most consonant with the goals of a liberal society - Pinker's argument is incredibly weak stuff. Certainly, in a stable, lawbound society, it’s generally rational to deal fairly with your friends and neighbors and co-workers, because you want them to deal fairly with you. But that "generally" excludes all the hard cases, in which doing the right thing isn’t in a person’s rational self-interest, and those hard cases are the essence of what separates morally-impressive behavior from the reverse. Pinker's "rational actor" calculus makes sense in a landscape of equality, where if your neighbor is going hungry today you could easily be going hungry tomorrow, and in a landscape of transparency, in which your neighbor (or your spouse or friend or business partner) will have perfect knowledge of the wrongs you've done them. But most serious moral dilemmas arrive from power differentials on the one hand - situations in which a stronger person has the opportunity to do something for a weaker person, but at a real cost to themselves and with little chance that they'll suffer if they don't - and secret temptations on the other, where you have a chance to commit a wrong that will be known only to yourself (and God). And Pinker's argument that morality should be based on rational self-interest, and that as a general rule, it's in your rational self-interest to treat people as you'd wish to be treated, tells us nothing about why it's wrong in a particular instance for someone to refrain from cheating on his taxes - or on his wife - if he knows he won't get caught. Or why it's wrong in a particular instance for a Hutu family to deny refuge to their Tutsi neighbors if they know that offering the Tutsis sanctuary will put their own lives at risk.
You can fill in your own example, obviously. The point is that Pinker's argument for why our moral instincts aren't just as arbitrary as, say, the color of the sky or the taste of an apple bails out precisely at the moment when any argument for morality needs to kick in - when doing the "wrong" thing will have no obvious cost, or when doing the "right" thing has the chance to do real, palpable damage to the interests (or life) of the person doing it.