"In first-past-the-post, the electorate is divided into a number of districts, each of which selects one person to fill one seat by a plurality of the vote. First-past-the-post is not conducive to a proliferation of parties, and naturally gravitates toward a two-party system, in which only two parties have a real chance of electing their candidates to office. This gravitation is known as Duverger's law. Proportional representation, on the other hand, does not have this tendency, and allows multiple major parties to arise.
A two-party system requires voters to align themselves in large blocs, sometimes so large that they cannot agree on any overarching principles. Along this line of thought, some theories argue that this allows centrists to gain control. On the other hand, if there are multiple major parties, each with less than a majority of the vote, the parties are strongly motivated to work together to form working governments. This also promotes centrism, as well as promoting coalition-building skills while discouraging polarization." From wikipedia Multi-Party System article.