Strategic opposition and government cohesion in westminster democracies

Cohesive government-versus-opposition voting is a robust empirical regularity in Westminster democracies. Using new data from the modern Scottish Parliament, we show that this pattern cannot be explained by similarity of preferences within or between the government and opposition ranks. We look at differences in the way that parties operate in Westminster and Holyrood, and use roll call records to show that the observed behavior is unlikely to be determined by preferences on any underlying issue dimension. Using a simple variant of the agenda-setting model in which members of parliament can commit to their voting strategies we show that the procedural rules for reaching collective decisions in Westminster systems can explain this phenomenon: in the equilibrium, on some bills, members of the opposition vote against the government irrespective of the proposal. Such strategic opposition can reinforce government cohesiveness and have a moderating effect on policy outcomes. We introduce new data from the House of Lords, the Welsh Assembly, and the Northern Ireland Assembly to distinguish our claims from competing accounts of the data.