Key electoral institutions and rules influencing proportionality and partisan bias in Spanish politics

The current paper focuses on the Spanish electoral rules governing political competition for the central “Congreso de los Diputados”. It is well-documented that the system as a whole has traditionally favoured one or the other of the two main political parties (PP and PSOE) at the expense of proportionality and the remaining political parties. This paper focuses on some key Spanish electoral rules and investigates how much the observed biases could be altered by introducing some alternative rules taken from the Swedish electoral system, ceteris paribus. Measures of disproportionality are made through the Loosemore–Hanby index and the Gallagher index. The electoral raw data used for our estimations comes from the 2011, 2015 and 2016 last three Spanish general elections. The basic contribution of the paper is an empirical one as it provides a new example that institutions matter for results. Keywords: Institutions, Institutional changes, Spanish electoral rules, Adapted Swedish electoral rules, Proportionality, Partisan bias, Quantitative estimations. JEL: H0, C1.