Joshua Greene, who teaches psychology at Harvard, is a leading contributor to the recently salient field of empirical moral psychology. This very readable book presents his comprehensive view of the subject, and what we should make of it. The grounds for the empirical hypotheses that he offers about human morality are of three types: psychological experiments, observations of brain activity, and evolutionary theory. The third, in application to the psychological properties of human beings, is necessarily speculative, but the first and second are backed up by contemporary data, including many experiments that Greene and his associates have

carried out themselves.

But Greene does not limit himself to factual claims. He also asks how our moral beliefs and attitudes should be affected by these psychological findings. Greene began his training and research as a doctoral student in philosophy, so he is familiar from the inside with the enterprise of ethical theory conceived not as a part of empirical psychology but as a direct first-order investigation of moral questions, and a quest for systematic answers to them. His book is intended as a radical challenge to the assumptions of that philosophical enterprise. It benefits from his familiarity with the field, even if his grasp of the views that he discusses is not always accurate.

The book is framed as the search for a solution to a global problem that cannot be solved by the kinds of moral standards that command intuitive assent and work well within particular communities. Greene calls this problem the “tragedy of commonsense morality.” In a nutshell, it is the tragedy that moralities that help members of particular communities to cooperate peacefully do not foster a comparable harmony among members of different communities.

Morality evolved to enable cooperation, but this conclusion comes with an important caveat. Biologically speaking, humans were designed for cooperation, but only with some people. Our moral brains evolved for cooperation within groups, and perhaps only within the context of personal relationships. Our moral brains did not evolve for cooperation between groups (at least not all groups).... As with the evolution of faster carnivores, competition is essential for the evolution of cooperation.

The tragedy of commonsense morality is conceived by analogy with the familiar tragedy of the commons, to which commonsense morality does provide a solution. In the tragedy of the commons, the pursuit of private self-interest leads a collection of individuals to a result that is contrary to the interest of all of them (like over-grazing the commons or over-fishing the ocean). If they learn to limit their individual self-interest by agreeing to follow certain rules and sticking to them, the commons will not be destroyed and they will all do well. As Greene puts it, commonsense morality requires that we sometimes put Us ahead of Me; but the same disposition also leads us to put Us ahead of Them. We feel obligations to fellow members of our community but not to outsiders. So the solution to the tragedy of the commons has generated a new tragedy, which we can see wherever the values and the interests of different communities conflict, not only on an international scale but also more locally, within pluralistic societies that contain multiple moral communities.