What Causes Gamson’s Law? Experimental Evidence on Coalition Bargaining and Commitment

Abstract

One of the most robust findings in the literature on coalition formation is that government portfolios are allocated proportionally to the seat shares of coalition parties (Gamson’s Law). Surprisingly, standard bargaining models are not able to reproduce this pattern because in these models outcomes are a function of real and not nominal bargaining weights. This begs the question why Gamson’s Law is empirically so robust. In this paper we consider two possible mechanisms and test them in a laboratory experiment. First, we conjecture that the proportionality might be due to bargaining parties respecting the voters’ preferences expressed in the vote shares. Second, we propose a different structure of the bargaining process where parties first choose with whom to form a coalition before deciding how to split the office rents. This model generates the proportionality predicted by Gamson’s Law. The experimental results provide no support for the first mechanism since behaviour is not affected by bargaining weights being purely abstract. In contrast, we find very strong support for the second mechanism both in terms of comparative statics and point predictions. Overall, our analysis strongly suggest that Gamson’s Law might be caused by parties first forming coalitions before bargaining over portfolios.

About the presenter's visit

Dr Aaron Kamm will be visiting the School of Economics on 22.11.19. While here he will be using room 520A Colin Clark Building. If you would like to meet with him or have lunch or dinner with him please contact David Smerdon who will be his host while at The University of Queensland. David Smerdon can be contacted on d.smerdon@uq.edu.au.