¥" T "Te rely on data from India and the United States to show that political and economic centralization 1 / 1 / can influence the number of national parties in single-member simple-plurality electoral systems. V V Historically, in both countries the number of parties in local electoral districts has been near two, but the number of national parties has fluctuated. Periods of a small number of national parties in both countries correspond to periods of centralization. We argue that, as national governments centralize power and make policies that affect local areas, candidates have greater incentives to associate with national organizations, and voters have greater incentives to abandon locally competitive but nationally noncompetitive parties. P rominent explanations for differences in the number of political parties across countries, most notably Duverger's Law and the literature related to it, have focused on the role of electoral systems. Differences in district magnitude, electoral formulas, the number of run-offs, and presidentialism have each been hypothesized as determining the number of parties (Cox ). Ethnic heterogeneity has also been associated with multiple party systems.' In accounting for changes in the number of national parties over time within individual countries, however, explanations based solely on electoral systems or population diversity are strained. These features rarely change much within countries, and certainly not as often as party systems undergo change in some countries, such as Italy, India, Mexico, or even, as we shall show, the United States. We examine party system change in India and the United States. Both countries are federal (with relatively strong state governments), and both have singlemember, simple-plurality electoral systems. Yet, in both countries the number of parties has varied over

We develop a model of two-party spatial elections that departs from the standard model in three respects: parties' information about voters' preferences is limited to polls; parties can be either office-seeking or ideological; and parties are not perfect optimizers, that is, they are modelled as boundedly rational adaptive actors. We employ computer search algorithms to model the adaptive behavior of parties and show that three distinct search algorithms lead to similar results. Our findings suggest that convergence in spatial voting models is robust to variations in the intelligence of parties. We also find that an adaptive party in a complex issue space may not be able to defeat a well-positioned incumbent.

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