Many of the experiments Hauser tells us about are intended to delimit stages in child development. Three-year-olds already know, for example, that “if an act causes harm, but the intention was good, then the act is judged less severely.” Hauser takes this fact to support the claim that “rather than a learned capacity ... our ability to detect cheaters who violate social norms is one of nature’s gifts.” But do such facts as that children learn to use expressions like “didn’t mean to do it” at roughly the same time as they learn “shouldn’t have done it” help us draw a line between nature and nurture? Hauser does not spell out the relevance of data about child development to the question of whether internalizing a moral code requires a dedicated area of the brain.

Image Credit... Boris Kulikov

To convince us that such an organ exists, Hauser would have to start by drawing a bright line separating what he calls “the moral domain” — one that nonhuman species cannot enter — from other domains. But he never does. The closest he comes is saying things like “a central difference between social conventions and moral rules is the seriousness of an infraction.” He takes this to suggest “that moral rules consist of two ingredients: a prescriptive theory or body of knowledge about what one ought to do, and an anchoring set of emotions.” Apparently both rules of etiquette and moral rules embody knowledge about what ought to be done. All that is distinctive about morality is added emotional freight. But, as Hauser tells us, many nonhuman species obey social conventions. (For example, “Do not start tearing at the carcass before the alpha male has eaten his fill.”) It is hard to see why evolution had to carve out a new, specialized organ just to generate the extra emotional intensity that differentiates guilt from chagrin.

Perhaps Hauser does not mean to say that greater seriousness is the only, or the most important, mark of the moral domain. But the reader is left guessing about how he proposes to distinguish morality not just from etiquette, but also from prudential calculation, mindless conformity to peer pressure and various other things. This makes it hard to figure out what exactly his moral module is supposed to do. It also makes it difficult to envisage experiments that would help us decide between his hypothesis and the view that all we need to internalize a moral code is general-purpose learning-from-experience circuitry — the same circuitry that lets us internalize, say, the rules of baseball.

Hauser thinks that Noam Chomsky has shown that in at least one area — learning how to produce grammatical sentences — the latter sort of circuitry will not do the job. We need, Hauser says, a “radical rethinking of our ideas on morality, which is based on the analogy to language.” But the analogy seems fragile. Chomsky has argued, powerfully if not conclusively, that simple trial-and-error imitation of adult speakers cannot explain the speed and confidence with which children learn to talk: some special, dedicated mechanism must be at work. But is a parallel argument available to Hauser? For one thing, moral codes are not assimilated with any special rapidity. For another, the grammaticality of a sentence is rarely a matter of doubt or controversy, whereas moral dilemmas pull us in opposite directions and leave us uncertain. (Is it O.K. to kill a perfectly healthy but morally despicable person if her harvested organs would save the lives of five admirable people who need transplants? Ten people? Dozens?)

Hauser hopes that his book will convince us that “morality is grounded in our biology.” Once we have grasped this fact, he thinks, “inquiry into our moral nature will no longer be the proprietary province of the humanities and social sciences, but a shared journey with the natural sciences.” But by “grounded in” he does not mean that facts about what is right and wrong can be inferred from facts about neurons. The “grounding” relation in question is not like that between axioms and theorems. It is more like the relation between your computer’s hardware and the programs you run on it. If your hardware were of the wrong sort, or if it got damaged, you could not run some of those programs.