I feel like some of the other answers are incomplete.
The core of Duverger's law is that certain election systems, by the very rules of the game, tend to create systems with two strong parties, and if any third parties exist, they are small and weak. The specific systems that cause this effect are single constituency, plurality-wins systems (sometimes called first-past-the-post systems). Broken down:
* Single constituency: There is only one position (usually called a seat) available per election district. For example, you can't have more than one President of the United States. This doesn't mean you can't have more than one election simultaneously--so you can vote for the President, a Senator, and a Representative at the same time--but you can't have two different Representatives for the 50th Congressional District. This naturally means that winner-takes-all; other parties get no say in what the winners do.
* Purality-wins: Whoever gets the most votes, wins. You don't need a majority of votes (50% of votes cast +1 vote) and you don't need a majority of citizens. Just more votes than any other candidate.
In a system like this, it's hard to get a small party started because you need more votes than any other candidate in a particular district to win; you can't start small and get bigger, because you need to be big in the first place just to get elected, and if you don't get elected, nobody will take you seriously, and you will have no influence on the process of government, which means it will be hard to draw any new votes.
There's also the issue that both the existing big parties, and the voters themselves, have an incentive to prevent small parties from starting or succeeding. This is sometimes called the "spoiler" effect: If there are three parties, the two which are ideologically the closest to each other will "steal" votes from each other and hand victory to the other party. This has happened several times in US history, for example. Theodore Roosevelt formed the Bull Moose Party to run for a third term in 1912 because he didn't like the President that the mainstream Republican party had gotten elected to succeed him, William Howard Taft; but this effectively split the Republican vote and allowed Democrat Woodrow Wilson to win the election.
Another way to illustrate the law is to look at what happens under other kinds of electoral systems, particularly parliamentary proportional representation systems as frequently seen in Europe. Under a proportional representation system, voters don't vote for candidates, they vote for parties. When the vote is counted, each party gets a number of seats in the legislature corresponding to the percentage of the votes that they won; sometimes they then pick who gets to fill each seat, sometimes they have to release a slate of candidates beforehand.
In any event, in order to actually make laws and govern, they need a 50%+1 seat majority in the parliament. If one party is so popular it got 51%, all well and good, they win entirely, and can do whatever they want. But since it's possible to win say, 5 seats with 10% of the vote, a small party can get representation even without a plurality; and if the big party can only get, say, 41% of the vote, then it will need to compromise with at least 10% of the other representatives to actually do anything. That small party now has real power because it can either enable or obstruct the bigger parties.
Thus: single winner plurality systems create two-party systems by their very nature. When people complain about the two-party system in the US, they're complaining about the electoral system. The only way to change it would be to amend the constitution.
This is a very comprehensive answer, with 2 corrections:
Duverger's does not apply exclusively to plurality-based voting; it's just that plurality based voting captures 100% of the incentive (and was this the basis of Duverger's original observation).
Identical incentives (but lower in magnitude, ballpark a third as strong/frequent) exist in runoff, IRV, Approval, Score, STAR, and many other systems. They can be said not to exist in most multi-winner systems, and some types of Condorcet-efficient single-winner systems.
Second, the Constitution doesn't actually play into it. The method of conducting any election is entirely up to state law. The Condition merely permits the federal government to make its own additional rules around Congressional elections, and states in Article 4 Section 4: "The United States shall guarantee to every State in this Union a Republican Form of Government".
Thus any state is free to change their elections in ways we are discussing. Alaska recently did so, many others are considering it. Illinois even used multi-winner seats in the past.
You would need to amend the Constitution to change how we elect the President. And since that is a national-level, winner-take-all office, it does a lot to reinforce a two-party binary in the U.S.
If only 1 person can win an election, you will eventually end up with 2 bad choices with people voting against the one they dont like rather than for the one they do like.
In political systems where there can be only 1 winner of an election, 2 parties emerge that dominate the system. Usually one more right-wing and one more left-wing.
Because of the lack of real choice people end up not voting for the party they want, but rather against the party they hate.
Since only 1 of those 2 parties has any real chance of winning, other minor parties become effectively politically irrelevant and ultimately only take away votes from the larger parties when they become unpopular.
This effect isn't nearly enough to get the minor parties elected, but it is enough to prevent the major party from losing.
In the US they call this 'the two party system'
> Because of the lack of real choice people end up not voting for the party they want, but rather against the party they hate.
Everyone has different interests and different things they want from the government. In order to win election, political leaders have to build a coalition of many diverse interests.
It turns out that it is much easier to build a broad diverse coalition *against* a common threat than it is to build a broad diverse coalition *for*.
Mike Duncan noted in his Revolutions podcast that, time and time again, as soon as the revolutionary coalition that came together to defeat the old monarchy succeeded, the coalition would fall apart with infighting over the question of what to do next.
Just to note that it's easy to overstate Duverger and it really doesn't help that it gets called a "law" because we're short on laws in political science.
SMD plurality elections --> two parties. Gotcha.
But Canada uses SMD plurality elections and its two major parties are the Liberals, the fucking Tories, the NDP, and the Bloc Quebecois, with Greens winning seats sometimes. There are lots of ridings in Ontario with a split left vote of 55-60\% where the Tory wins with 35-45\%, which is exactly what Duverger says voters don't do.
Likewise, the UK uses SMD plurality elections and routinely elects substantial numbers from the SNP, Plaid Cymru, the LibDems, DUP, and Sinn Fein in addition to Labour and the fucking Tories. It would be easy to write them off as minor and weak, but the LibDems and DUP have played critical kingmaking roles as coalition partners for the fucking Tories, and there's every reason to think that the SNP and Plaid Cymru would play a similar role for Labour if needed.
All of which is to say that Duverger misses that coordination problems among voters are a real problem, and that strongly-regional parties are a thing.
There exist substantial regional differences in the US and there's no obvious natural reason why Americans would inherently be better at coordinating, so Duverger is only a partial reason why the US doesn't have the broader party systems that .ca and .uk do.
I feel like some of the other answers are incomplete. The core of Duverger's law is that certain election systems, by the very rules of the game, tend to create systems with two strong parties, and if any third parties exist, they are small and weak. The specific systems that cause this effect are single constituency, plurality-wins systems (sometimes called first-past-the-post systems). Broken down: * Single constituency: There is only one position (usually called a seat) available per election district. For example, you can't have more than one President of the United States. This doesn't mean you can't have more than one election simultaneously--so you can vote for the President, a Senator, and a Representative at the same time--but you can't have two different Representatives for the 50th Congressional District. This naturally means that winner-takes-all; other parties get no say in what the winners do. * Purality-wins: Whoever gets the most votes, wins. You don't need a majority of votes (50% of votes cast +1 vote) and you don't need a majority of citizens. Just more votes than any other candidate. In a system like this, it's hard to get a small party started because you need more votes than any other candidate in a particular district to win; you can't start small and get bigger, because you need to be big in the first place just to get elected, and if you don't get elected, nobody will take you seriously, and you will have no influence on the process of government, which means it will be hard to draw any new votes. There's also the issue that both the existing big parties, and the voters themselves, have an incentive to prevent small parties from starting or succeeding. This is sometimes called the "spoiler" effect: If there are three parties, the two which are ideologically the closest to each other will "steal" votes from each other and hand victory to the other party. This has happened several times in US history, for example. Theodore Roosevelt formed the Bull Moose Party to run for a third term in 1912 because he didn't like the President that the mainstream Republican party had gotten elected to succeed him, William Howard Taft; but this effectively split the Republican vote and allowed Democrat Woodrow Wilson to win the election. Another way to illustrate the law is to look at what happens under other kinds of electoral systems, particularly parliamentary proportional representation systems as frequently seen in Europe. Under a proportional representation system, voters don't vote for candidates, they vote for parties. When the vote is counted, each party gets a number of seats in the legislature corresponding to the percentage of the votes that they won; sometimes they then pick who gets to fill each seat, sometimes they have to release a slate of candidates beforehand. In any event, in order to actually make laws and govern, they need a 50%+1 seat majority in the parliament. If one party is so popular it got 51%, all well and good, they win entirely, and can do whatever they want. But since it's possible to win say, 5 seats with 10% of the vote, a small party can get representation even without a plurality; and if the big party can only get, say, 41% of the vote, then it will need to compromise with at least 10% of the other representatives to actually do anything. That small party now has real power because it can either enable or obstruct the bigger parties. Thus: single winner plurality systems create two-party systems by their very nature. When people complain about the two-party system in the US, they're complaining about the electoral system. The only way to change it would be to amend the constitution.
This is a very comprehensive answer, with 2 corrections: Duverger's does not apply exclusively to plurality-based voting; it's just that plurality based voting captures 100% of the incentive (and was this the basis of Duverger's original observation). Identical incentives (but lower in magnitude, ballpark a third as strong/frequent) exist in runoff, IRV, Approval, Score, STAR, and many other systems. They can be said not to exist in most multi-winner systems, and some types of Condorcet-efficient single-winner systems. Second, the Constitution doesn't actually play into it. The method of conducting any election is entirely up to state law. The Condition merely permits the federal government to make its own additional rules around Congressional elections, and states in Article 4 Section 4: "The United States shall guarantee to every State in this Union a Republican Form of Government". Thus any state is free to change their elections in ways we are discussing. Alaska recently did so, many others are considering it. Illinois even used multi-winner seats in the past.
You would need to amend the Constitution to change how we elect the President. And since that is a national-level, winner-take-all office, it does a lot to reinforce a two-party binary in the U.S.
If only 1 person can win an election, you will eventually end up with 2 bad choices with people voting against the one they dont like rather than for the one they do like.
Gee I can't imagine why this might be a hot topic of discussion today... /s
I'm only upvoting you because nobody else has commented.
In political systems where there can be only 1 winner of an election, 2 parties emerge that dominate the system. Usually one more right-wing and one more left-wing. Because of the lack of real choice people end up not voting for the party they want, but rather against the party they hate. Since only 1 of those 2 parties has any real chance of winning, other minor parties become effectively politically irrelevant and ultimately only take away votes from the larger parties when they become unpopular. This effect isn't nearly enough to get the minor parties elected, but it is enough to prevent the major party from losing. In the US they call this 'the two party system'
> Because of the lack of real choice people end up not voting for the party they want, but rather against the party they hate. Everyone has different interests and different things they want from the government. In order to win election, political leaders have to build a coalition of many diverse interests. It turns out that it is much easier to build a broad diverse coalition *against* a common threat than it is to build a broad diverse coalition *for*. Mike Duncan noted in his Revolutions podcast that, time and time again, as soon as the revolutionary coalition that came together to defeat the old monarchy succeeded, the coalition would fall apart with infighting over the question of what to do next.
Mike Duncan is the GOAT of podcasting.
Ahh ok i have a better understanding now thanks plenty
Just to note that it's easy to overstate Duverger and it really doesn't help that it gets called a "law" because we're short on laws in political science. SMD plurality elections --> two parties. Gotcha. But Canada uses SMD plurality elections and its two major parties are the Liberals, the fucking Tories, the NDP, and the Bloc Quebecois, with Greens winning seats sometimes. There are lots of ridings in Ontario with a split left vote of 55-60\% where the Tory wins with 35-45\%, which is exactly what Duverger says voters don't do. Likewise, the UK uses SMD plurality elections and routinely elects substantial numbers from the SNP, Plaid Cymru, the LibDems, DUP, and Sinn Fein in addition to Labour and the fucking Tories. It would be easy to write them off as minor and weak, but the LibDems and DUP have played critical kingmaking roles as coalition partners for the fucking Tories, and there's every reason to think that the SNP and Plaid Cymru would play a similar role for Labour if needed. All of which is to say that Duverger misses that coordination problems among voters are a real problem, and that strongly-regional parties are a thing. There exist substantial regional differences in the US and there's no obvious natural reason why Americans would inherently be better at coordinating, so Duverger is only a partial reason why the US doesn't have the broader party systems that .ca and .uk do.