Political system similarity and the choice of allies - Do Democracies flock together, or do opposites attract?

Does the nature of a nation's political institutions influence the types of countries with which it allies? Some previous research has suggested that democracies tend to ally with other democracies. This study reexamines alliance patterns by assessing the broader linkage between regime type and alliance partnership. The authors present a refinement of previous research designs, using new data from Polity III and the updated correlates of war (COW) alliance data sets to analyze all alliances from 1815 to 1992. The bipolar alliance structures of the cold war (NATO and the Warsaw Pact) appear to be aberrations in their strong ideological content In general, there is very little correlation between alliance dyads and regime type. Surprisingly, democracies are less likely to ally with one another than highly autocratic regimes. Regimes of most types seem to prefer to ally with partners of dissimilar type. The authors conclude that this is due to so-called gains from trade within alliance dyads.