The fate of the square root law for correlated voting

TL;DR

This paper investigates how voter correlations affect the optimal weighting in two-tier voting systems, challenging the traditional square root law by modeling voter behavior with spin systems from physics.

Contribution

It introduces a model of correlated voters using spin systems and derives new formulas for optimal weights and democracy deficit in such settings.

Findings

Correlated voters can significantly alter optimal weights.

The square root law may not hold under voter correlation.

New analytical expressions for weights and democracy deficit are provided.

Abstract

We consider two-tier voting system and try to determine optimal weights for a fair representation in such systems. A prominent example of such a voting system is the Council of Ministers of the European Union. Under the assumption of independence of the voters, the square root law gives a fair distribution of power (based on the Penrose-Banzhaf power index) and a fair distribution of weights (based on the concept of the majority deficit), both given in the book by Felsenthal and Machover. In this paper, special emphasis is given to the case of correlated voters. The cooperative behaviour of the voters is modeled by suitable adoptions of spin systems known from statistical physics. Under certain assumptions we are able to compute the optimal weights as well as the average deviation of the council's vote from the public vote which we call the democracy deficit.