http://www.slate.com/articles/news_and_politics/kausfiles_sp...
Choice quote:
"But why should we expect the deadlock to be broken at all? Think of it in ... well, cheap Darwinian terms. Imagine that we have a two party system, and each party is a collection of status-seeking individuals looking for power by winning a greater "market share" of the vote. Imagine that they each have their ideological principles --one is more to the left, one more to the right -- but these principles are quite flexible in the face of imminent or repeated failure at the polls. Over time, as each party crafts its message to maximize its appeal -- and adjusts its message after each election to regain any lost share of the votes -- wouldn't one expect the system to reach a roughly 50-50 equilibrium, in which every election was a cliffhanger?"