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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.
Blais et al. (Tue,) studied this question.