Unpacking logM: Toward a more general theory of party system density

The standard model of political party system density combines two traditions to explain why some countries have more political parties than others, one tradition that emphasizes social cleavages and another that emphasizes electoral institutions, especially district magnitude. Despite its considerable success, there are several reasons to be less than fully satisfied with the standard model. We examine two of these problems associated with the scope of strategic voting and the functional form of the specification used to test the model. In doing so, we contrast the standard interpretation with an organizational ecology model that accounts for what the standard model did so well, but also accounts for important anomalies it ignores. We reexamine some of the key analyses that have been used to test the standard model to assess the severity of its limitations and the utility of the rival organizational ecology account.