Abstract
Duverger’s law asserts that the proportional representation election system tends to multipartism and that plurality rule elections structured within single-member districts tend to favour a two-party system. The efforts to verify the validity of Duverger’s law in post-communist systems have produced no unanimous results yet. The amended Lithuanian Law on Elections before the 2000 elections to the Seimas provided a unique opportunity to verify whether electoral laws are characteristic of the strategic behaviour of the political parties and the electorate of this region. The paper starts with the analysis of the effect of the amendments to the Lithuanian Law on Elections on the country’s party system. Having discovered that the decrease in effective political parties in Lithuania in 2000 did not reduce the number of effective parties in the Seimas, we focused on this anomaly. Finally we are searching for an answer to the question why the party systems of Lithuania and Russia – the states that have mixed-system elections – considerably differ. This is determined by the presidential regime peculiarities in Russia. The key argument of the paper is that the nature of the relation between the legislative and the executive is the factor determining the effect of the election laws on the strategic behaviour of the electorate and political parties. When the presidential power is considerable yet restricted, political parties have more possibilities to control legislation according to the above Duverger’s law, hence a strong party system forms. When the presidential power is almost unrestricted, the probability of the formation of a strong party system is small. In such a system it is impossible to prevent the increase in the number of weak parties with the help of election laws.