Small Parties Get a Boost—A New Model Explains Gamson’s Law

🔎 The Puzzle and the Move

Addresses the long-standing portfolio allocation paradox (Warwick and Druckman 2006) by building on intraparty politics research (Giannetti and Benoit 2009; Laver 1999; Strøm 2003). Introduces a new government-formation model grounded in two central assumptions:

• No actor holds a structural bargaining advantage during government formation negotiations.
• Every actor capable of depriving a coalition of a majority (or another critical threshold) must be included in negotiations—not only formal parties.

⚙️ What the Model Predicts

Contrasts with standard bargaining models that contradict Gamson’s Law. The proposed model implies that equilibrium portfolio allocations will generally follow Gamson’s Law (i.e., ministries proportional to party weights) but display a systematic small-party bias—precisely the pattern long documented in empirical studies.

📊 Evidence and How the Model Was Tested

• Empirical tests compare the new model’s fit to the literature’s standard specification (Browne and Franklin 1973).
• The new model outperforms the Browne and Franklin specification in accounting for observed portfolio allocations.
• A novel prediction—that candidate-centered electoral rules increase the likelihood of Gamsonian portfolio allocations—is supported by the empirical evidence.

📌 Why It Matters

This approach reconciles theoretical bargaining models with empirical regularities by explicitly modeling intraparty actors and inclusion rules in negotiations. The findings clarify why coalitions tend to be Gamsonian in allocation while systematically favoring smaller parties, and they identify how electoral institutions (candidate-centered rules) shape portfolio outcomes.