We present a theory of comparative political institutions based on the concept of consensus democracy and social choice theory. Unlike Lijphart, we argue that “consensus democracy” is not a special form of democracy characterized by mutual vetoes, but rather the simplest form of democracy, which we refer to as PR-majority rule. We construct a typology of political institutions based on differences with this simple model. Contra Tsebelis’s veto players approach, our theory predicts that PR-majority rule should be the most flexible form of democracy. We test this with data on overall patterns of government spending and on welfare state reform.