This article examines how political institutional structures affect political instability. It classifies polities as autocracies or democracies based on three institutional dimensions: election of the executive, constraints on executive decision-making authority, and extent of political participation. It hypothesizes that strongly autocratic and democratic regimes will exhibit the greatest stability resulting from self-enforcing equilibria, whereby the maintenance of a polity's institutional structure is in the interest of political elites, whether through autocratic or democratic control. Institutionally inconsistent regimes (those exhibiting a mix of institutional characteristics of both democracy and autocracy) lack these self-enforcing characteristics and are expected to be shorter-lived. Using a log-logistic duration model, polity survival time ratios are estimated. Institutionally consistent polities are significantly more stable than institutionally inconsistent polities. The least stable political systems are dictatorships with high levels of political participation. The most unstable configuration for polities with an elected executive is one where the executive is highly constrained, but the electorate is very small.