An impressive body of work is emerging that aspires to explain what appears to defy explanation: genocide. Although most of these studies focus on individual cases, scholars are increasingly developing theoretical frameworks and applying a comparative method to explain not individual genocides but the very phenomenon. Manus Midlarsky's highly ambitious, original, and impressive book aspires to bring us closer to understanding genocide in the contemporary age. Avoiding monocausal explanations that locate homicidal tendencies either in cultural or strategic variables, Midlarsky draws from international relations theory, comparative politics, and psychology to explain what makes genocide imaginable and executable. Written with controlled passion by a scholar deeply committed to using social science theory and methods to understand this highly emotional topic, The Killing Trap is an invaluable contribution to genocide studies.

The book is divided into six parts. Part I briefly surveys the literature on genocide; rightly claims that theory has a place for explaining events that appear to be produced by the idiosyncratic conjunction of historical forces; and defends the author's use of a highly restrictive definition of genocide—“the state-sponsored systematic mass murder of innocent” people defined by a “particular ethnoreligious identity, with the purpose of eradicating that group from a given territory” (p. 10), which limits his case selection to the Holocaust, Rwanda, and Armenia.

Part II presents the theory. Various background factors prepare the ground. There must be a social differentiation that creates a minority group that both is treated as distinct from the dominant culture and, over time, can be constructed as a possible threat to its identity and survival. Also present is a prior history of mass killing against minority groups. These episodes were not “dry runs” but, rather, contributed to making state killing an acceptable policy. Last, but most important, is “loss,” which Midlarsky defines fairly broadly and includes both past and future losses. War, either domestic or international, can cause the government to treat the minority group as a threat to the state; whether it sees mass killing as a solution depends on its adoption of an “imprudent” realpolitik, defined as the use of brute force to accomplish the state's goals.

Midlarsky also wants to understand why the state shifts its policy from mass killing to genocide—that is, when it is no longer content with killing a subset of that population and now aspires to exterminate all who are classified as having that identity. To understand this critical shift, he draws from prospect theory; it helps explain why state leaders might react to future losses by taking extraordinary risks, even to the point of concentrating resources away from the war to the genocide. Yet such a policy, to be successful, requires a legion of associates. Why do “ordinary” people follow? Borrowing from experimental psychology and the concept of “altruistic punishment,” Midlarsky argues that a critical role is played by an actor, motivated not by personal gain but rather by a commitment to the group, who is willing to punish those who do not contribute to the collective “good.” His framework, then, aspires to explain the background and triggering causes of genocide, why mass killing becomes genocide, and why there are varying rates of killing over time and space.

Relying on already published work, Part III applies the theory to the critical cases of the Holocaust, the Tutsis, and the Armenians. Because of his own interests and the relative abundance of literature on the Holocaust, it commands most of the author's attention; the value of his narrative is immediately apparent as he interrogates critical issues of the Holocaust, including when and why mass killing became genocide and why in some parts of Europe there were enthusiastic killers, in other places reluctant accomplices, and in others dissidents. Part IV provides a series of fascinating but at times disjointed considerations of the magnitude and the kinds of killing, and the analysis rightly incorporates the extent and possibility of rebellion by the victims. Part V examines cases of mass killing that, in his view, fell short of genocide, such as Cambodia, and those instances that seemed primed for genocide but where it did not occur. He concludes the book by comparing the different cases, considering consequences, and speculating about the possibility of prevention and response, given his analysis.

Any book that aspires to unlock the dynamics of genocide should be judged not only on whether it accomplishes this (impossibly ambitious) goal but also on whether its shortcomings suggest areas for future research. Four areas stand out. One is the impact of past episodes of mass killing. Midlarsky points to their presence but provides little systematic evidence of their importance. Do past killings contribute to a belief that mass killing is an appropriate state tool? Does it contribute to a “culture of violence”? How, exactly, does the past become embedded in the present? We also need more sophisticated, dynamic, models regarding the rates of participation and the production of collective violence. Although Midlarsky points to a variety of factors, his reliance on “altruistic punishment” potentially misses the mixture of motives of these enforcers and of those who will follow. The model also has a static quality when, in fact, there is a dynamic element to collective violence. For instance, in The Order of Genocide (2006), Scott Straus argues that a dynamic interaction between the civil war and local elite competition shaped the rates of participation in the Rwandan genocide. Furthermore, while Midlarsky rightly considers situations where genocide did not occur even though all the conditions were present, these cases of “dogs that do not bark” need greater treatment. Finally, we need not only more systematic comparisons across cases but also a comparison of existing explanations of these instances. The author, at times, shows how his argument overlaps with or contrasts with some explanations, but there are surprisingly few accounts that systematically survey the range of existing accounts.

Although I do not hold out the possibility for a theory of genocide, and, thankfully, we continue to have more variables than instances, contributions like The Killing Trap bring us closer to an understanding of what seems incomprehensible.