Shaun Nichols’ "Rational Rules" is the most creative and interesting response to moral nativism to appear since the naturalistic turn in moral psychology that began several decades ago. Nichols accepts the basic nativist observation that the moral rules children acquire are surprisingly rich and complex in relation to the relevant evidence, but he maintains that the acquisition process can be explained by statistical learning, rather than innate endowment. His key idea is that the same principles that underpin statistical learning in other domains can explain how children acquire their moral rules. After summarizing this fascinating book, this review raises some doubts and concerns about its main arguments. First, as Nichols admits, his empiricist learning theory does not explain how complex concepts like agent, intention, cause, and wrong are acquired on the basis of experience. Instead, it simply presupposes these concepts, along with the hypothesis space and statistical techniques on which it depends. Second, Nichols’ empiricism does not set out to explain the actual transition that occurs when children who initially do not possess rule-like elements of moral cognition acquire them on the basis of certain evidence. Instead, its aim is to show how that transition might occur by means of statistical learning. This explanation is weaker than it appears, however, because nativists typically argue that non-nativist learning theories are empirically improbable, not logically impossible, in light of the facts of moral development. Finally, Nichols’ account of the learning target in moral theory is far too crude and simplistic to do justice to the argument from the poverty of the stimulus. The moral grammars acquired by all normal children include (but are not limited to) prohibitions of familiar crimes and torts, such as murder, assault, theft, and fraud; principles of justification and excuse, such as self-defense, mistake of fact, duress, and provocation; and a variety of even more complex rules, standards, and principles, such as the duty to rescue, the prohibition of unreasonable risk (i.e. negligence), and the principle of double effect, all of which require comparing acts and omissions with their alternatives. Does Nichols' empiricist theory explain how any of these complex deontic rules could be learned by means of statistical inferences from examples in the child’s environment? A careful review of this book and the experiments on which it relies makes clear that it does not
