Opposites Do Not Attract: The Impact of Domestic Institutions, Power, and Prior Commitments on Alignment Choices

States that choose to involve themselves in an ongoing dispute do so by choosing to align with or against one of the original disputants. What factors lead states to prefer to help one side over the other? We consider the effect of the disputants' power, political and economic institutional similarities between each disputant and the aligning state, and formal alliance commitments between each disputant and the aligning state on these alignment choices. We evaluate these expectations empirically by examining the alignment choices of states that joined with one side or another in a Militarized Interstate Dispute during the period of 1816 to 1986. The results indicate that regardless of regime type, institutional similarities matter to the aligning state's decision. We also find that power concerns matter only to autocracies; democracies do not seem to base their alignment choices on the power of the sides in the dispute. Finally, the evidence indicates that the alignment choices of democracies cannot be anticipated by their prior alliance commitments, although the alignment choices of autocracies can. These results suggest interesting implications for research on the democratic peace, the determinants of threat in the international system, and the impact of selection effects. The consistent empirical evidence that institutional similarity affects alignment decisions also increases our confidence that future investigations of institutional similarity generally, rather than an exclusive focus on joint democracy, will prove fruitful.