Matt Golder

In ‘Understanding Interaction Models: Improving Empirical Analyses’, we report the results of several replications that we conducted of analyses examining electoral institutions and party systems. One of these replications was of Carles Boix’s article ‘Setting the Rules of the Game: The Choice of Electoral Systems in Advanced Democracies’ which appeared in the British Journal of Political Science in 1999. In our article, we said that:

‘In an award-winning article in the American Political Science Review, Boix (1999) examines the factors that determine electoral system choice in advanced democracies. He makes two main conclusions. First, ethnic or religious fragmentation encourages the adoption of proportional representation in small and medium-sized countries (621). He draws this conclusion based on a model that includes an interaction term between ethnoreligious fragmentation and country size. However, he does not include either of the constitutive terms. When these terms are included, there is no longer any evidence that ethno-religious fragmentation ever affects the adoption of proportional representation. The second conclusion is that countries are more likely to shift to proportional representation when the proportion of Socialist votes and the effective number of non-socialist parties are both large. This conclusion comes from a model in which there is an interaction term between the strength of socialist parties and the number of non-socialist parties but no constitutive terms. In this case, the coefficient on the interaction term from the primary model remains significant (albeit only at the 90% level now) once the constitutive terms are included, but its magnitude increases by 340%. Thus, the original analysis considerably underestimates the interactive effect of these two variables. Moreover, the failure to include constitutive terms means that the predicted electoral thresholds reported by Boix are off by up to 80%.’

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