Most scholarly literature on the causes of mass violence against civilians can be broken into two camps—one focusing on rational incentives and the other on psychological impulses. This volume is a tantalizing attempt by authors from complementary disciplines to bridge the gap: Daniel Chirot from international studies and sociology and Clark McCauley from psychology.

The book is well organized and starts by identifying four motivations for genocide: efficiency, revenge, fear, and disinfection. It next offers three reasons why most conflict does not escalate to genocidal violence: mutual benefit from rules of restraint, trade and exchange that further increase this benefit, and internalized moral codes. The book then combines these lessons to forge four policy prescriptions: coercive international intervention, transitional and international justice, incentives for intergroup relations, and the promotion of nonexclusionary ideologies.

The greatest strength of Why Not Kill Them All? is its broad historical literacy, drawing examples from the Bible, eleventh-century England, czarist Russia, nineteenth-century United States, and well-known cases from the last century. Chirot also displays deep personal knowledge of the violent, interethnic dynamics in the less bloody but more recent civil war in Ivory Coast.

Perhaps not surprisingly, the authors do not fully succeed at synthesizing the rational and psychological approaches. Though the book is coauthored, the tension between the chapters suggests that the workload was divided. The first theoretical chapter, on motivation, states explicitly that all mass killings “have a logic and rationale behind them” (p. 4). But the following chapter, on psychology, favors Jon Elster's counterargument that “short-lived passions undermine the theory of the rational actor” (p. 61).

This tension is pervasive, including the book's effort to combine two starkly different definitions of the genocidal motivation “revenge.” In one chapter, revenge is a purely emotional response to being humiliated by a perceived inferior challenger (p. 78). But in another, revenge is a coolly rational decision of strong states to establish a tough reputation by retaliating disproportionately against one challenger in order to deter other potential ones (p. 26).

To their credit, the authors offer many hypotheses about the causes of, and preventives against, mass violence. But the evidence is largely anecdotal, and they provide no new empirical tests of its hypotheses, nor methodological guidance on how such tests would be operationalized. Thus, the volume reads somewhat like a textbook, offering deductively logical arguments illustrated by historical events. The authors also provide evidence that contradicts their hypotheses, but fail to refute it convincingly or amend their theory accordingly. For example, they cite a 1974 empirical finding that “the higher the level of exchange between groups, whether in trade goods or intermarriage, the higher the incidence of war”—but then insist that anecdotal evidence to the contrary is “persuasive” (p. 105).

The book is rich in history but lighter on political science. The chapter explaining why conflict usually does not escalate to genocide is essentially a restatement of liberal and constructivist theories of cooperation in international relations—that is, economic interdependence, neoliberal institutions, and norms—but without much explicit reference to these theories. Though the book discusses why weak states are vulnerable to internal ethnic violence, it fails to mention the leading proponents of this idea (Barry Posen, Chaim Kaufmann, and Barbara Walter) or their proposed remedies.

One trade-off for any book so broad in scope is the inability to delve deeply into individual cases, which leads to the misinterpretation of some conflicts with which I am familiar. The authors claim that Slobodan Milosevic “was a master at making Serbs fearful about what would happen to them if they failed to maintain an ethnically pure Greater Serbia” (p. 59). But while Milosevic surely fanned Serbian nationalism and supported paramilitaries in Bosnia and Croatia, he never pursued an ethnically pure Serbia, greater or otherwise. Likewise, the book claims that NATO's 1999 military intervention in Kosovo “did stop the mass murder and ethnic cleansing” (p. 174). But in reality, NATO intervention increased the death rate in Kosovo approximately thirtyfold, from about a hundred per month to more than three thousand, by provoking Serb forces to switch from counterinsurgency to ethnic cleansing. Concerning Rwanda, the authors mistakenly accuse Hutu of dehumanizing Tutsi with the epithet “cockroaches” (p. 80), when it was actually Tutsi rebels themselves in the 1960s who adopted this moniker to signal their relentlessness.

The authors' policy prescriptions to reduce genocide are reasonable but underdeveloped. Their main advice is to promote “the Enlightenment notion that the individual is important and has autonomy apart from any family, community, tribe, or nation” so that people will “reject collective punishment” (pp. 140, 206). But they never square this advice with their own evidence that the United States, champion of enlightenment values, has repeatedly targeted civilians—both within its borders against Native Americans and overseas, notably in Hiroshima and Nagasaki.

The authors advocate federalism as another preventive: “When local groups effectively control more resources, and the way in which these are distributed is both transparent and reasonably fair, political struggle for control of the central government then becomes less critical” (p. 198). Although this may reduce revolutionary conflict, the book fails to acknowledge that devolution of power also gives the constituent entities more incentive and ability to exit, raising risks of secessionist conflict, as noted by Svante Cornell, David Lake, Donald Rothchild, and others.

In sum, while scholars of mass violence may be frustrated by this volume's lack of rigorous empiricism, the larger universe of academics and students may be equally intrigued by its historical breadth and deductively plausible hypotheses.