Amid recent blood-soaked conflicts in Ukraine, Ethiopia, and Sudan, it might be surprising to learn that both interstate and civil wars have declined since the mid-1990s. Particularly, ethnonationalist wars pitting governments against ethnically organized rebels have become less frequent. Why? Early generations of civil war scholarship emphasized political discrimination and grievances as root causes of conflict. A subsequent wave of quantitative conflict research, by contrast, tended to focus on material variables like natural resources, foreign interventions, or states’ counterinsurgency capabilities. In their authoritative new book, Lars-Erik Cederman, Simon Hug, and Julian Wucherpfennig recenter grievances in the study of ethnic conflict. Their main argument is that interethnic peace stems from inclusive governance: when states share power broadly among ethnic subgroups, both present grievances and fears of future discrimination that could foment rebellion are reduced. Power-sharing arrangements in places like Northern Ireland, Bosnia and Herzegovina, and South Africa have taken the winds out of the sails of ethnic elites who might otherwise have returned their societies to violence.

The idea that power sharing yields peace is intuitive, yet few hypotheses have attracted more vociferous debate among conflict scholars. Skeptics argue that ethnic power sharing is at best a superficial remedy to conflict and at worst an arrangement that dangerously crystallizes social divisions—as purportedly illustrated by the failure of ethnic inclusion in places like Rwanda, Afghanistan, and South Sudan. But it would be misleading to draw sweeping conclusions from such extreme examples. After all, just because hospitalized people are more ill on average than the nonhospitalized does not mean hospitals do not improve health. To understand the effects of power sharing one must study both peaceable and war-torn societies—Switzerland as well as Syria.

Enter Cederman, Hug, and Wucherpfennig’s volume, an empirical tour de force that aims to dispel doubts about the pacifying effects of power sharing.

Synthesizing over a decade of the authors’ past research on civil conflict, the book advances several arguments. Ethnic groups that enjoy access to central government power are less likely to rebel than excluded ethnic groups. Territorial power sharing (e.g., decentralization) also reduces conflict, particularly when paired with governmental power sharing. The practice of power sharing is crucial; formal institutions matter only insofar as they shape the actual distribution of power. Finally, power sharing is strategically adopted by governments to co-opt threatening challengers. In other words, power sharing occurs in “hard cases.” Naïve comparisons of cases with and without power sharing thus underestimate the effects of inclusion.

After developing these theoretical propositions, the authors present their evidence across eight chapters that quantitatively analyze the relationship between power sharing and ethnic conflict. The well-known Ethnic Power Relations (EPR) project serves as the book’s workhorse dataset, providing country- and group-level measures of ethnic groups’ de facto access to state power since World War II. Chapter 4 establishes the book’s empirical baseline with a straightforward model of intrastate conflict between a government and ethnic groups that choose to rebel or not, conditional on their access to political power and other variables that shape their expected payoffs. These models reveal that while government power-sharing practices are associated with a decline in conflict probability, the evidence for territorial power sharing is mixed. Formal power-sharing institutions (as expressed in constitutions or peace agreements) also appear to have no effect on conflict. The interplay between formal institutions and practices is taken up in further detail in chapter 5. Employing mediation analysis, the authors show that the “work” of governmental power sharing occurs mainly through behavioral practice, raising questions about the validity of research with an exclusive focus on formal institutions.

Perhaps the thorniest problem in past studies of power sharing is the endogenous relationship between institutions and conflict. If governments share power with ethnic groups with an eye toward future rebellion, then the causal arrow may run from conflict to power sharing rather than the reverse. Cederman, Hug, and Wucherpfennig tackle this endogeneity problem head-on in chapters 6 through 9. First, the authors leverage differences between the colonial state-building strategies of France and the United Kingdom that shaped ethnic groups’ early access to state power in Africa and Asia. Whereas French tenets of centralization and assimilation undermined the influence of peripheral colonial subjects, the British reliance on existing customary institutions empowered the elites of peripheral groups (for example, the Hausa-Fulani in Nigeria). The authors thus use the interaction between colonizer identity and groups’ distance from the coast to instrument for power access and recover estimates of the (positive) effect of both governmental and territorial power sharing on peace. Second, the authors employ strategic interaction analyses that directly model the initial selection of power-sharing institutions by governments depending on the threat posed by challengers, the credibility of government commitments, and a degree of random error. Essentially, the models recover counterfactual estimates of how groups might have behaved under alternative choices by the government. Overall, these chapters—which are the most technical of the book—provide novel and convincing evidence for the inclusion-peace thesis.

Having established a firm link between power sharing and peace, chapter 10 zooms out to explore the diffusion of power sharing since 1945. Power sharing has become more prevalent globally, a trend driven by the spread of norms concerning minority group rights and the political influence of regional actors like the European Union (in Eastern Europe) and South Africa (in Sub-Saharan Africa). Chapter 11 examines the importance of accommodative institutions relative to other causes of peace like democratization and peacekeeping. Strikingly, the authors demonstrate that rising ethnic inclusion is arguably the critical variable explaining the decline of global conflict since the mid-1990s.

There is little doubt that Cederman, Hug, and Wucherpfennig have written a landmark study in the power-sharing and conflict literature. The sheer volume of empirical evidence in the book is astonishing and sets the standard for cross-national research on civil war. Scholars, students, and policy makers interested in ethnic politics, war, and conflict resolution will all benefit from reading it—especially those inclined to skepticism of power sharing (as this reviewer was). I would also recommend the book to anybody working with observational evidence subject to selection issues, as the authors provide an exemplary model for rigorous and transparent analysis of cross-national data.

Will this book be the last word on power sharing? Surely not. Power sharing within military institutions receives little attention in the book, despite the frequent use of these arrangements in postconflict countries. I suspect this form of power sharing may yield less impressive peace dividends, due to the special commitment problems that arise when rival armed forces (who, until recently, were shooting and bombing each other) are asked to forfeit their autonomy and merge under a unified command. It is also unclear to me whether governments co-opt challengers with military power sharing in the way proposed by Cederman, Hug, and Wucherpfennig, or whether governments prefer to exclude rivals from sensitive positions in the security apparatus (perhaps while compensating with political or territorial power sharing). The complex interaction between these varieties of power sharing deserves further study.

Another question the authors touch on only briefly is the relationship between power sharing and the recent global surge of populism and democratic backsliding. Will the diffusion of nativist attitudes and hostility toward multiculturalism hollow out accommodative institutions that currently keep the peace in many divided societies? Perhaps the challenge for proponents of power sharing today is not merely to sustain tolerant attitudes, but also to bolster institutions (e.g., courts, parties, etc.) that can constrain executive power and keep promises of inclusion credible. Investigating these questions is urgent for students and practitioners of power sharing.