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The standard model of political party density emphasizing the interaction of social cleavages and district magnitude (M) is incomplete in accounting for number of parties in cases of high values of M in an arbitrary way. We explore an alternative model for such cases emphasizing the slack in the issue agenda available to parties with which to construct viable identities or niches they can employ to mobilize cognitively-limited voters. The model is tested with time series data and event history analysis on the sizes of the public policy agenda and the political

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. P olitical scientists have traditionally explained the density of political party systems-number of parties-in two ways. The first, associated with political sociologists such as Grumm (1958), Lipset and Rokkan (1967), and Rose and Urwin (1970), emphasized social cleavages, arguing that parties represent preexisting regional, religious, or ethnic divisions. The second, based on the work of Lijphart (1990), Riker (1982, Taagepera and Shugart (1993), and Cox (1997), emphasized instead electoral institutions. Based on Duverger's (1954) observation that simple-majority, single-ballot institutions favor the development of a two-party system, these scholars explored how a range of electoral rules severely condition how many parties can meaningfully compete for legislative seats and executive offices. These approaches now have been combined into a single integrated or interTaagepera and Grofman 1985). A strong research program has developed around this interactive model exploring individual-and systemlevel properties of and interactions between social systems and electoral institutions (Despite this real success, there are several reasons to be less than fully satisfied with this standard model of party system density. After describing this model, we examine two key problems, contrasting the standard interpretation with an alternative 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

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