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Mutual Optimism and War


  • Earlier versions of this article were presented at the University of Rochester, Columbia University, and NYU. We thank Scott Ashworth, Bruce Bueno de Mesquita, Songying Fang, Tanisha Fazal, Erik Gartzke, Shigeo Hirano, Adam Meirowitz, John Patty, Pablo Pinto, Robert Powell, Quinn Ramsay, Anne Sartori, Curt Signorino, Branislav Slantchev, Allan Stam, Randy Stone, and Robert Walker, as well as other seminar participants. We would also like to thank the editor of the AJPS and two anonymous reviewers for helpful comments. Any remaining errors are our own. Kris Ramsay acknowledges financial support from NSF grant SES-0413381.

Mark Fey is associate professor of political science, 109E Harkness Hall, University of Rochester, Rochester, NY 14627 ( Kristopher W. Ramsay is assistant professor of politics, 033 Corwin Hall, Princeton University, Princeton, NJ 08544 (


Working with the definition of mutual optimism as war due to inconsistent beliefs, we formalize the mutual optimism argument to test the theory's logical validity. We find that in the class of strategic situations where mutual optimism is a necessary condition for war—i.e., where war is known to be inefficient, war only occurs if both sides prefer it to a negotiated settlement, and on the eve of conflict war is self-evident—then there is no Bayesian-Nash equilibrium where wars are fought because of mutual optimism. The fundamental reason that mutual optimism cannot lead to war is that if both sides are willing to fight, each side should infer that they have either underestimated the strength of the opponent or overestimated their own strength. In either case, these inferences lead to a peaceful settlement of the dispute. We also show that this result extends to situations in which there is bounded rationality and/or noncommon priors.