From political parties to environmental organisations, citizens join a host of associations in order to influence policymaking and political agendas. Yet why in some western democracies do citizens join political organisations much more than in others? Drawing on a large number of cross-national surveys and data sets, Morales shows that huge cross-national variations in political membership are not so much related to social or attitudinal differences between these countries' citizens, but are explained to a great extent by the structure of the political system of each nation.