Abstract
Efforts to test Duverger's law in the new democracies of postcommunist Europe have had mixed results. Research argues that mixed systems have an effect on the number of effective partics that is distinct from that of single-mandate district and proportional representation systems. Less attention has been given to the effect of other institutions on the party system, particularly strong presidents. Analyzing election results in postcommunist Europe, the authors find support for Duverger's law after controlling for the strength of the executive. They argue that strong presidents substantially reduce the incentive for parties to seize control of the legislative agenda. Hence, the restraint that electoral systems exercise on the proliferation of parties and independent candidates is weakened. The authors find that a further consequence of strong presidents is that the incentive for majority control of committees and the legislative agenda is weakened.