Amid the continuing conflicts symbolized by events such as the Charlie Hebdo massacre, few issues can be more important than the overarching question of this excellent collection: “Is religion a cause of tolerance, is it a cause of intolerance, or do some aspects of religion cause tolerance while others cause intolerance?” (3). The editors promise a survey of “empirically informed approaches to understanding the ways in which religion increases or decreases tolerance, social cohesion, violent conflict, and political compromise” (v). The contributions are drawn from evolutionary anthropology, experimental psychology, and philosophy.

Given the complexity and controversial nature of the enterprise, the editors could be forgiven for avoiding any very definite conclusion, but they are willing to identify a “consistent theme” as emerging from the various papers: religion in general “promotes social cohesion and tolerance within particular social groups” and “intolerance and hostility between different social groups” (v). Although some religious movements (e.g., the Quakers) have tried to advance inter-group toleration, these efforts have been “outweighed by the tendency of many other religious groups to promote intolerance between differing social groups” (vi). The net finding of the collection connects religion more strongly to intolerance than tolerance. Even the more positive, intra-group aspects of religion have more to do with cohesion and co-operation than tolerance.

Of course, such an overarching statement greatly simplifies the subtleties of a collection than includes contributions from Harvey Whitehouse on “Religion, Cohesion and Hostility,” R. I. M. Dunbar on “The Origin of Religion as a Small-Scale Phenomenon,” Will Gervais and Ara Norenzayan on “Religion and the origins of Anti-Atheist Prejudice,” Owen Flanagan on “The View from the East Pole: Buddhist and Confucian Tolerance,” and C. A. J. Coady on “Religious Disagreement and Religious Accommodation,” among others. Nevertheless, I'll take three chapters as indicative of the editors' “consistent theme.” In the first of these, Dominic Johnson and Zoey Reeve argue from an evolutionary perspective that, despite the commendation of peace in some religious doctrines, religion is “an adaptation for war.” Evolutionary adaptation is the natural selection of advantages for individuals, groups, or cultures that enable these to survive. Focusing on groups, Johnson and Reeve begin by pointing out that evolution could just as well favor “nasty” consequences as “nice” ones. The fittest, not necessarily the most agreeable, survive. One factor that helps groups survive is a capacity to defend themselves against rivals, and religion enables this in many ways. Most centrally, it helps overcome the “collective action” problem of war, in which individuals are naturally wary of participating in something that is likely to injure them personally. Religion inspires confidence in the justice of the cause, instils comradeship and cohesion, offers supernatural rewards in the event of death, and dehumanizes enemies. Despite the official messages of peace, it is not surprising that the historical record shows religion to be so often associated with war.

C. Daniel Batson's chapter exemplifies the approach of experimental psychology. He asks “whether religion functions in people's lives as a depressant or stimulant of tolerance and compassion” (88). Batson's review of the psychological evidence starts in the 1940s when the first systematic studies showed that “the answer is very clear: In spite of what religions preach about universal brotherhood, the more religious an individual is, the more intolerant he or she is likely to be” (89). However, this initial answer was soon refined through the introduction of a distinction between different ways of holding religious beliefs. “Extrinsic” orientations use religion as instruments for other purposes, while “intrinsic” orientations take religion to be desirable in itself. The work of Gordon Allport seemed to show that while extrinsic attitudes correlated with high levels of intolerance, intrinsic orientations were more tolerant. Again, though, doubts surfaced, with evidence that the apparent tolerance of the intrinsic outlook largely resulted from subjects wanting to say the right thing to researchers. Once the “self-presentation” problem had been corrected for, the intrinsic outlook turned out to be little more tolerant than the extrinsic. More recently, a third orientation has been identified, the “Quest” approach, in which religion is seen less as a body of doctrine and more as an individual journey. According to Batson, the latest studies show that, in contrast with both extrinsic and intrinsic orientations, the Quest approach is a generally tolerant one.

From a philosophical perspective, Walter Sinnott-Armstrong argues that “certain kinds of religious beliefs have a tendency to undermine good compromises” (222). The religions that concern him are what he calls “absolutist” doctrines, featuring “a God who is all-good and all-knowing, who revealed his will in a sacred text, and who sends some people to Hell and others to Heaven because of how they act” (226). These absolutist religions include most fundamentalist forms of Christianity, Islam, and Judaism. The compromises threatened by religions of this type are (following terminology coined by Avishai Margalit) “thick” compromises, involving “some sacrifice in [one's] central values” in the name of peace or friendship, together with recognition of the legitimacy of others' point of view (222–223). Absolutist religions typically refuse to trade-off any central values, even for the sake of peace or friendship, and deny any legitimacy to competing perspectives. On the assumption that at least some thick compromises are independently good or justified, absolutist religions can rightly be identified as a major obstacle to any sensible and decent politics.

Toward the end of the book, John Perry and Nigel Biggar offer “A Critical Commentary” on some of the contributions. Writing from a perspective distinctly sympathetic to religion, they are indeed critical of those they discuss, and they are sometimes a little testy. According to Perry and Biggar, Batson's work is “trivial” because it wrongly assumes that tolerance is always desirable, when in fact the desirability of tolerance depends on the value of what is being tolerated (257). Similarly, war is not necessarily bad (contrary to Johnson and Reeve), and (against Sinnott-Armstrong) compromise is not necessarily good.

Perry and Biggar are right that much discussion of tolerance assumes the value of its objects, but the writers they criticize can reply that there is good reason for such an assumption in the cases they address. None of these writers is arguing for tolerance of murder, theft or terrorism; typically, the discussions are about attitudes toward people from other races and religions. Surprisingly, Perry and Biggar propose tolerance of homosexuality as an example of what concerns them. Apparently we shouldn't assume that homosexuality represents a legitimate form of life, but should rather be discussing “whether homosexual practice is morally and socially harmful” (257). With friends like Perry and Biggar, religion doesn't need enemies. Of course, there are other cases where there is, as Perry and Biggar observe, “ample room for reasonable disagreement” (256). But this, too, is a context in which the value of tolerance can safely be accepted, since by definition these are cases where no amount of discussion will determine that one view is superior to another.

What should certainly not be assumed is that this collection is thoroughly hostile to religion. As the editors point out, it contains many different voices, some quite welcoming to the role of religion in society and politics. Overall, this is a superb book, full of high-quality contributions that provide both an introduction for newcomers and a state-of-the-art handbook for specialists in these important debates.