Barry Hughes: Game theory explains why there are only two major political parties in the US

Written by Barry Hughes Wednesday, 12 September 2012 21:05
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With the US elections coming up soon, have you ever wondered why there are only two major parties?


Is it just a coincidence or is there something more fundamental going on? It fact it is nearly inevitable that the kind of voting system used in the United States will lead to a two party system.


This will happen in any election which has the following characteristics:

  1. Voters have a single vote
  2. They vote for a single candidate in their district
  3. The district has one legislative seat available in the election
  4. The winner is determined solely by who has the most votes

Tactical voting

What happens in this type of election when a third party enters is that it makes voters start to think tactically.


Imagine that there are two parties, one right-wing and one left-wing and that in the election the right-wing party is predicted to get 55% of the vote to the 45% for the left-wing party. The right-wing party wins (please don’t be offended if you would want the left-wing party to win, they will in a moment!)

Now a third party decides to join the race, it is a more extreme right-wing party and 20% of the voters agree with the policies of this new party. If everyone now voted for the party whose policies they believe in then the left-wing party still gets 45% of the vote, the moderate right-wing party gets 35% of the vote (55% less the 20% that move to the more extreme party) and the extreme right-wing party gets 20% of the vote.


This means that the left-wing party wins as the right-wing vote is split.


In practice voters realise that voting for the extreme party, although in line with their beliefs, would let in the left-wing party that they really don’t want to win. To avoid this they stick with the moderate right-wing party that they believe can win the election and is closest to their beliefs of the two main parties.


Unless a third party can convince enough voters that they really have a chance to win overall then voters won’t switch and the two parties will continue to dominate.


This idea was first put forward my Maurice Duverger and is now known as Duverger’s law.


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Last modified on Wednesday, 12 September 2012 22:22

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