Ethnic Defection in Civil War Stathis N. Kalyvas Yale University The study of ethnicity is dominated by constructivist approaches, yet empirical studies of civil war have been oblivious to their insights. In this article, the author examines the relationship between ethnic identity and civil war and points to several empirical instances of fluidity in the behavioral expression of ethnic identities within civil war. The author identifies two processes that are consistent with constructivist theorizing: identity shift and ethnic defection. The author provides several empirical illustrations along with a micro-level test of the determinants of ethnic defection. At the micro level, ethnic defection is best predicted by the extent of territorial control exercised by the incumbent political actor and the level of prior insurgent violence. The author also hypothesizes that at the macro level, ethnic defection is a function of the resources available to incumbent actors and conclude by stressing the need to take seriously the endogenous dynamics of civil wars. Keywords: civil war; ethnicity; identity; insurgency; counterinsurgency R ecent research on civil wars 1 has generally been oblivious to construc- tivist insights that stress the social construction of ethnicity and its poten- tial fluidity and malleability. 2 Although constructivist research has focused on processes of constitution and reconfiguration of ethnic groups and identities (Wimmer, 2007), empirical researchers of civil war, particularly in the field of international relations, have tended to treat ethnic groups as unitary actors and ethnic identities as given ex ante, automatically salient, fixed during the conflict, and predictive of individual political behavior (e.g., Posen, 1993; Walter, 2005). In their simplest formulation, these assumptions boil down to the claim that during civil wars, individuals will tend to act in support of orga- nizations claiming to represent their ethnic identity—so much so that indi- viduals and organizations can be conflated into a single actor, the “ethnic group” (Biddle, 2006; Kaufmann, 1996a, 1996b). Comparative Political Studies Volume 41 Number 8 August 2008 1043-1068 © 2008 Sage Publications 10.1177/0010414008317949 http://cps.sagepub.com hosted at http://online.sagepub.com 1043 Author’s Note: I am grateful to Kanchan Chandra, Alberto Diaz-Cayeros, Michael Hechter, Erik Wibbels, Andreas Wimmer, and three anonymous reviewers for their helpful comments on this article.