Voter turnouts differ significantly across societies, and in some polities voting is compulsory. To explain differences in voter turnout it is necessary first to explain the variation in compulsion policies that exists across societies. A threshold model of collective action is used to provide an explanation for compulsion policies, which are treated as endogenous. Governments and parties are viewed as “demanding” turnout as a means to enhance political support. This theory suggests that voter turnout is high, and possibly government mandated, in societies that have high levels of political polarization and that use proportional representation. The predictions of the theory are tested using cross-country data.