Electoral systems of proportional representation (PR) feature prominently as explanations of high levels of party fragmentation. These accounts engage a demand-side point of view: diversity in electoral preferences of the electorate entails party diversity in parliament. From a supply-side point of view, of members of parliament (MPs) as those who provide the electorate with a variety of policy offers, we add a qualification to these accounts. We identify succumbing to the leadership’s wishes or splitting from the party as two routes for MP behavior under PR. We derive these routes from a trade-off faced by party leaders: Disciplining MPs via reducing their chances for re-election within the party entails a relative increase in profitability from the point of view of these MPs of splitting from the party and running for another party or on their own in the next election. Since this tradeoff is particularly pronounced in low-threshold PR systems, we argue that it is these systems rather than PR in general that witness high levels of party fragmentation. We provide evidence for our claims with the help of a series of novel datasets on candidates, MPs, and MP behavior in roll call votes in the German Imperial and Weimar eras. Our findings also go beyond enduring findings on determinants of the “upper bound” of the number of competitive parties in electoral systems.
