All other things being equal, party system change implies change of party’s electoral strength, and therefore changing numbers of electoral parties –party strength volatility- and the increase of electoral volatility as a consequence. Party system (in)stability on the contrary would imply (un)stable electoral support and (un)stable party representation. This being said, the place of electoral volatility in the discussion of the proportional representation of parties is not very clear. In this paper we study the effects of the electoral institution on party system dynamics, including electoral volatility. Since the 1960s studies of proportionality have focused on two elements: the electoral formula and district magnitude (or the total number of available seats). This paper looks at the ways in which the mechanical side of the electoral institution have an effect on the psychology of the voter (vote swing, strategic voting etc.) and offers guidelines on study those effects in the context of a single case study.
