The Laakso—Taagepera index of the effective number of parties, while satisfying most of the requirements of this aggregate quantity, tends to produce unrealistically high scores for party constellations in which the shares of the largest parties exceed 50 percent, and can produce misleading results in several other situations. After reviewing the structural properties of the Laakso—Taagepera index and supplements or alternatives to it proposed in the literature, this article proposes a new index that eliminates several problems inherent in the mathematical form of indices using the Herfindahl—Hirschman measure of concentration as computational core. The proposed index is tested on a set of party constellations from recent elections in new democracies and non-democratic electoral regimes. The results confirm that the new index works better than earlier proposed alternatives in measuring the effective number of components in highly fragmented and highly concentrated party systems.

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This article uses a representative sample of elections held in 80 countries for a statistical analysis of societal and institutional factors of party system nationalization. The analysis demonstrates that high degrees of political decentralization, such as in federations and devolved states, exert a strong negative effect on party system nationalization. However, in the long-term federalism can accommodate highly nationalized party systems. Another factor that strongly contributes to party system nationalization is a high quality of democracy. The degrees to which different varieties of electoral rules support party system nationalization are associated with a lack of incentives for cultivating the personal vote. Countries with large populations and societies divided along linguistic and/or religious lines tend to have party systems that are less nationalized, but these factors are not as important as institutional determinants. The impact of some of the factors is mitigated by party system fragmentation.

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