The Psychological Impact of Electoral Laws: Measuring Duverger's Elusive Factor

Duverger's law regarding the impact of electoral systems on party competition depends upon two effects: the mechanical and the psychological. The former is well defined and well documented, whereas the latter has more often been a matter for theoretical speculation. In this article we provide an operational definition of the psychological effect of electoral systems and measure its impact across twenty democratic systems over more than a century. Our findings suggest that it does exist, that it works as Duverger predicted and that its impact is about the same magnitude as the mechanical effect.