A new approach measuring political diversity through ideological distinctiveness* Ignacio Ortuño-Ortín † Yuval Weber° August 7, 2008 Abstract: We propose a new index of political diversity that takes into account not only the number and size of political parties, but also their ideological distinctiveness. By using election manifestos from 49 major democratic countries as proxy markers for ideological distinctiveness of political parties, we demonstrate that our index and the widely used "effective number of parties" (ENP) yield different conclusions. In particular, our index suggests a weaker link between the type of electoral system and the degree of political diversity than that derived by ENP. In addition, the structure of our index allows us to examine a relationship between political diversity of the electorate and the elected parties in countries with pluralist voting systems. ------------------------------------------------------------------------------------------------------------------------------------------------ *We are grateful to Shlomo Weber for valuable comments and suggestions. † Department of Economics, Universidad Carlos III de Madrid. Email: iortuno@eco.uc3m.es ° Department of Government, University of Texas, Austin. Email: yweber@mail.utexas.edu.