This study examines the 1919 introduction of proportional representation (PR) in Norway. Using an extraordinary dataset that captures party vote shares before and after reform, it analyzes fragmentation incentives.

Context & Data: Tracking parties across single-member districts pre-reform to multi-member districts post-PR introduces unique challenges due to district boundaries persisting through the transition.

Key Findings: The authors find two main effects. First, PR itself caused party system fragmentation, driven partly by strategic party entry. Second, crucially for voters, many shifted parties (especially between Liberals/Conservatives) as incentives to strategically coordinate against Labor disappeared.

Implications: This research demonstrates how electoral rules can fundamentally alter both elite behavior and voting patterns in a specific historical context.