Stronger Veto, Less Fragmented Party Systems: Influence of Bicameralism on Party System Concentration

While several studies have explored the impact of bicameralism on cabinet and legislative majority formation, its impact on political parties and party systems has been inexplicably overlooked. To fill this gap, this study argues that rational voters and politicians would pre-empt the necessity of majority formation and vote for and form larger political parties and thus concentrate countries’ party systems. We introduce a new bicameral index and analyse 415 elections in 32 countries. The results reveal that a strong and legitimate upper house is associated with larger seat shares for major political parties and the smaller effective number of parties. These findings provide a novel perspective because most prior research focuses on the lower house arena to explain countries’ party politics. Furthermore, they necessitate a re-examination of bicameralism on majority formation, as earlier studies largely treated political parties and party systems as exogenous variables, thus overestimating or underestimating the influence of bicameralism.