Americans often bemoan the lack of options on the ballot. Why are there only two legitimate options? Can’t there be a third alternative to voting for a Democrat or a Republican?
Nope. It’s against the law — Duverger’s Law.
Maurice Duverger died last week at the age of 97. The scholar, politician, and columnist will be remembered for many things. Among political scientists he is known for “Duverger’s Law,” his simple — yet powerful — explanation for why some political systems are destined to have two parties while others have many more.
The U.S.A. is a two party system. Occasionally, we’ve had a third party pop up and grab more votes, but these parties never last. We’re a land of two political parties and, as Duverger figured out, it always will be this way.
Duverger figured out that a two party system is the outcome of how we count votes. Nearly all of our elections are “first past the post” or “plurality” contests. A winner is the person with a plurality of the vote — just one more vote than any other candidate.
In this type of electoral system, a party may win even if it isn’t in the majority.  When this happens, there is a strong incentive for people to vote against a third party.
Even if in one election a third party candidate garners votes, voters will be more reluctant to let this happen in the next election. Parties also want to avoid losing. They will change their positions or form coalitions. In the example of the left-leaning district, the Democrats could recruit a Green-Party-style candidate to run as a Democrat, emphasize issues that Green voters prioritize, or even broker a deal with the Green Party not to run a challenge to them.
Another way to think of the power of voting rules on the party system is to consider what would happen if votes were cast proportionally. Most democracies have parliaments in which parties receive seats based on their votes in the election. A party that receives 25 percent of the vote earns 25 percent of the seats.
In a proportional system, there is no incentive to keep with just two parties. If a party can get five percent of the vote, then it will still have a voice in government. A vote for your top choice is never wasted. Parties can win by finding groups of voters who support them, even if this group doesn’t have a chance of ever.
In the U.S., we would be unlikely to have just two parties if Congress and state legislatures were divided up proportionally. The Republican Party would probably be divided with a Christian right party, a libertarian, an anti-immigrant party, and a pro-business party. Democrats would be split between a labor party, environmental party, socialists, and perhaps a party for African-Americans or Latinos.
There are some proportional systems that have two parties (or even just one); proportional representation doesn’t guarantee multiple parties. But in a plurality system like ours, the voting rules mean that there is a de facto cap on the number of parties. When there’s a district in which there is just one winner chosen by a plurality of votes, then there will be no more than two parties.
People may not like that outcome, but it’s the law.
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