The fact that democracies seldom fight other democracies has been explained monadically — there is something about democratic institutions that constrains decision-makers — and dyadically — there are normative sentiments shared by democracies that make their conflicts less probable in the first place and less likely to escalate when they do occur. The problem is that empirical analyses rarely support the contention that less authoritarian states are discernibly less likely to initiate wars. Moreover, the presence or absence of normative ties between democracies has proven difficult to measure directly. Even more problematic is the tendency to pursue regime type explanations as if regime type arguments, in either their monadic or dyadic manifestations, are likely to be necessary and sufficient. While no one explicitly argues that they are necessary and sufficient, the systematic assessment of competing explanations (regime type versus alternatives) is still very much in its infancy. Not only is it fair to say that we do not know for sure what it is about regime type that restrains conflict within some dyads; we also do not know how much relative explanatory credit to give to regime type.Arguments and evidence on the democracy-war generalization may be found in Quincy Wright, A Study of War (Chicago 1942/65); Immanuel Kant, Perpetual Peace (Indianapolis, 1957; Dean Babst, ‘Elective Governments — A Force for Peace?’, The Wisconsin Sociologist, 3 (1964), pp. 9-14; Peter Wallensteen, Structure and War: On International Relations, 1920-1968 (Stockholm, 1973); R. J. Rummel, ‘Libertarianism and International Violence’, Journal of Conflict Resolution, 27 (1983), pp. 27-71; Steven Chan, ‘Mirror, Mirror on the Wall...Are the Freer Countries More Pacific?’, Journal of Conflict Resolution, 28 (1984), pp. 617-48; Erich Weede, ‘Democracy and War Involvement’, ibid., pp. 649-64; Michael W. Doyle, ‘Liberalism and World Politics’, American Political Science Review, 80 (1986), pp. 1151-69; Bruce Russett, Grasping the Democratic Peace (Princeton, 1993); Max Singer and Aaron Wildavsky, The Real World Order: Zones of Peace/Zones of Turmoil (Chatham, NJ, 1993); and James Lee Ray, Democracy and International Conflict: An Evaluation of the Democratic Peace Proposition (Columbia, MO, 1995). Alternative approaches to developing greater specification, examining rival hypotheses, and presenting still more arguments can be found in Melvin Small and J. David Singer, ‘The War-Proneness of Democratic Regimes, 1816-1965’, Jerusalem Journal of International Relations, 1 (1976), pp. 50-69; William K. Domke, War and the Changing Global System (New Haven, CT, 1988); Zeev Maoz and Nasrin Abodali, ‘Regime Type and International Conflict’, Journal of Conflict Resolution, 33 (1989), pp. 3-35; Richard L. Merritt and Dina A. Zinnes, ‘Democracies and War’, in Alex Inkeles (ed.), On Measuring Democracy: Its Consequences and Concomitants (New Brunswick, NJ, 1989); George Modelski and Gardener Perry III, ‘Democratization from a Longterm Perspective’, in N. Nakicenovic and A. Grubler (eds.), Diffusion of Technologies and Social Behavior (New York, 1991); Stuart A. Bremer, ‘Dangerous Dyads: Conditions Affecting the Likelihood of Interstate War, 1816-1965’, Journal of Conflict Resolution, 36 (1992), pp. 309-41; Zeev Maoz and Bruce Russett, ‘Alliance Contiguity, Wealth, and Political Stability: Is the Lack of Conflict Among Democracies a Statistical Artifact?’, International Interactions, 17 (1992), pp. 245-68; David Lake, ‘Powerful Pacifists: Democratic States and War’, American Political Science Review, 86 (1992), pp. 24-37; Randall Schweller, ‘Domestic Structure and Preventative War: Are Democracies More Pacific?’, World Politics, 44 (1992), pp. 235-69; Bruce Bueno de Mesquita and David Lalman, War and Reason (New Haven, CT, 1992); T. Clifton Morgan and Valerie Schwebach, ‘Take Two Democracies and Call Me in the Morning: A Prescription for Peace?’, International Interactions, 17 (1992), pp. 305-20; Alex Mintz and Nehemia Geva, ‘ Why Don't Democracies Fight Each Other? An Experimental Assessment of the “Political Incentive” Explanation’, Journal of Conflict Resolution, 37 (1993), pp. 484-503; William J. Dixon, ‘Democracy and the Peaceful Settlement of International Conflict’, American Political Science Review, 88 (1994), pp. 14-32; James D. Fearon, ‘Domestic Political Audiences and the Escalation of International Disputes’, American Political Science Review, 88 (1994), pp. 577-92; Christopher Layne, ‘Kant or Cant: The Myth of the Democratic Peace’, International Security, 19 (1994), pp. 5-49; David E. Spiro, ‘The Insignificance of the Liberal Peace’, ibid., pp. 50-86; John M. Owen, ‘How Liberalism Produces Democratic Peace’, ibid., pp. 87-125; and William R. Thompson, ‘Democracy and Peace: Putting the Cart Before the Horse?’ International Organization, 50 (1996), pp. 141-74. In particular, Bremer, ‘Dangerous Dyads’, and Maoz and Russett, ‘Alliance Contiguity’, have made some headway in examining the relative contribution of dyadic regime factors versus other sources of influence. However, Maoz and Russett's examination is limited to a restricted set of variables and the post-1945 era. Bremer's analysis has a longer time-span but his study is characterized by important operationalization problems and also fails to check the temporal stability of the outcome. More work along these lines is definitely warranted.
