The description of the long-run historical development of parliamentarism has presented an empirical and methodological challenge because it is only loosely related to constitutional writings. This article offers a solution. Using a wide variety of historiography, I collect data on government terminations in eleven West European states from the establishment of national parliaments until today. To describe the evolution of parliamentarism, I apply a Bayesian learning model that estimates institutional development as the change in current expectations about interactions grounded in past experience. The result is the first long-run continuous description of parliamentarism at the country level, which suggests that parliamentarism in many cases was established later than hitherto believed. In general, it is an institution of the Postwar period. The finding that unelected heads of state in several countries influenced government terminations well into the twentieth century also has implications for ideas about democratization.
