via Unfutz

According to a principle called Duverger’s Law, a first-past-the-post election system (candidate with most votes wins) like we have naturally leads to a two-party system.

Apparently this doesn’t rule out third parties, but a successful third-party will ultimately just replace one of the other parties, as happened when the modern Republican Party replaced the Whigs in the 1850’s. (By the way, I’m just paraphrasing the article here. I slept through most of Poli-Sci in college.)