These are indignant times. Reading newspapers, talking to friends or coworkers, we seem often to live in a state of perpetual moral outrage. The targets of our indignation depend on the particular group, religion, and political party we are associated with. If the Terry Schiavo case does not convince you of this, take the issue of same-sex marriage. Conservatives are furious over the prospect of gays and lesbians marrying, and liberals are furious that conservatives are furious. But has anyone on either side subjected their views to serious scrutiny? What’s the response, for example, when conservatives are asked exactly why gays and lesbians shouldn’t be allowed to marry? “It threatens the institution of marriage.” OK. How? “Marriage is between a man and a woman.” (Democrats give this answer as well.) Right, but why? “It’s unnatural.” Isn’t that true of marriage in general? “Well… look… I mean… it’s just wrong!”

If you are familiar with the work of Jonathan Haidt, it will come as no surprise that our resentment, disgust, and outrage are rarely supported by fully developed arguments and deliberation. A psychologist at the University of Virginia, Haidt has devoted his career to the study of moral judgment and decision-making; his results are revealing and perhaps a bit unflattering. We tend to think of ourselves as arriving at our moral judgments after painstaking rational deliberation, or at least some kind of deliberation anyhow. According to Haidt’s model—which he calls “the social intuitionist model”—the process is just the reverse. We judge and then we reason. What, then, is the point of reasoning if the judgment has already been made? To convince other people (and also ourselves) that we’re right.

To support his model, Haidt has devised a number of ingenious experiments. He presents scenarios designed to evoke strong moral responses (“it’s wrong!”) but ones that are hard to justify rationally. (Examples include: having sex with a chicken carcass you’re about to eat, wiping your toilet with a national flag, and, as we’ll see, brother/sister incest.) Although the goals of these experiments vary, the results all point to the causal importance of emotions and intuitions in our moral life, and to different roles for reason from the ones we might expect or hope for. Haidt’s model has gone against some dominant trends in moral and social psychology, in particular the theories of well-known psychologists Piaget and Kohlberg, whose work appeared to support rationalist models of moral judgment (where reason plays the primary causal role in moral decision-making). But as Haidt himself notes, his own work can be placed within a grand tradition of psychology and philosophy—a return to an emphasis on the emotions which began in full force...