"Insanity" is doing the same thing over and over again and expecting different results. – Albert Einstein
Also known as "first-past-the-post," plurality is by far the most common voting system for single-winner races. (Unfortunately.) Your "vote" is the "name of a single candidate," and the most-named candidate wins.
Plurality is simple and easy to count, but its well-known pathologies include the "spoiler effect," the "lesser-of-two-evils dilemma," "vote-splitting," and manipulability of elections via "candidate cloning." Plurality also has the frustrating disadvantage that there is no way for a voter to express any of his opinions about any of the other candidates, and it often is strategically best for a voter to dishonestly vote for somebody who is not his true favorite – the system encourages lying.
You then get a great choice between being a liar and being a fool. (More amusing but equivalent wording: "artificial monopoly for the unpopular Republocrat party, with its ridiculous good-cop/bad-cop scam.")
With plurality voting, you can win without a majority – both Clinton and G.W.Bush won even though the majority voted against them. That means it's possible for most of the voters to get the candidate they like least! (Probability estimate for how often that occurs.) Plurality can in fact easily elect the candidate as "best" whom it would also decide is "worst," and its notions of "best" and "worst" can reverse almost randomly (except that it isn't random, it is predictable, which is even worse because that means these flaws can be manipulated).
Duverger's Law is the observation that these problems of plurality result in self-reinforcing two-party domination. Indeed, in the USA, plurality plus gerrymandering plus other effects has led to one-party domination!
Once that happens, the remaining two parties tend to become similar ("tweedledum and tweedledee") because it is strategically desirable for each to "grab the central ground." That leads to even-further-reduced voter choice (beyond the reduction we already had from having only 2 parties) and long periods of boredom. However, the long periods of boredom are interspersed with exciting periods of randomly directed and sometimes dangerous extremism, whenever one of the parties suffers a vote split with a temporarily rising popular third party, causing the less-popular and more extreme main-party competitor to be elected.
The plurality system is so bad that (computer simulations indicate) replacing it by range voting would improve society by a comparable or greater amount than the entire invention of democracy in the first place.
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