Duvager’s law – more guidelines than actual rules?

Dylan Difford, guest contributor. Opinions and research are solely the author's and do not necessarily reflect the opinions of the ERS.

Posted on the 9th March 2022

Wherever there are two dominant political parties, there is a steady stream of new parties hoping to break the system. Britain has had several over the years and America’s latest is the Forward Party of one-time Democratic presidential primary candidate Andrew Yang. And yet nearly all of these newcomers end up the way of The Independent Group for Change (formerly Change UK), failing to even dent the duopoly. The reason for these failures is often attributed to an early 1950s theory called Duverger’s Law.

What does Duverger’s law state?

Duverger’s Law is one of the oldest such ‘laws’ in political science and is named after French political theorist Maurice Duverger. It states that countries that use First Past the Post (FPTP) tend towards a two-party system. This supposedly happens through both a mechanical effect, whereby it is difficult for third parties to win seats, and a psychological effect, whereby voters ignore third parties for fear of wasting their vote.

But Duverger’s Law is contentious. A simple glance at the countries that use First Past the Post reveals many with more than two sizeable parties. So how valid is the rule and, if it isn’t, why isn’t it?

Average Number of Significant Parties, 2000-19

Seats Votes
Canada 4.1 5.0
India 8.8 9.3
United Kingdom 3.0 4.5
United States 2.0 2.0

Sizeable party defined as having more than 2% of votes or seats. Average of general election results 2000-19.

The impact of First Past the Post on winning seats (Mechanical Effect)

Despite questions over its total validity, there is undoubtedly some truth to Duverger’s observation – countries that use First Past the Post do tend to have fewer parties than those that use proportional representation, even if they don’t always have true two-party systems. We can see this by looking at the Effective Number of Parties scores of various countries. This is an index that attempts to account for the relative size of parties within a party system, with ENPP representing parties in parliament and ENEP representing parties contesting elections.

The United States, United Kingdom, Canada and India use First Past the Post, Australia and France use other majoritarian systems. 

Perhaps unsurprisingly, First Past the Post users Britain, Canada and the United States are among the lowest scorers. This is because the mechanical effect highlighted by Duverger does largely work. To win seats under First Past the Post, you need to be the most popular party somewhere and third parties rarely are – their support is not just at a lower level but also tends to be more evenly spread than that of major parties. This is why the third parties that are successful under First Past the Post are almost always those with a strong base where the party system is somewhat detached from the rest of the country – such as the SNP or Bloc Québécois. In fact, First Past the Post can often overrepresent such parties if they achieve dominance in their respective region.

The strength of this constraining effect is particularly clear when you see First Past the Post try to deal with multi-party election results. In the German election last year, the CDU/CSU and SPD took 88% of the First Past the Post seats on only 55% of the vote. Even with an historically low two-party vote, Germany’s several medium-sized parties were unable to take more than a handful of First Past the Post seats each because the way their vote is structured isn’t conducive to being the plurality winner in many places.

The psychological impact of First Past the Post on voters

But while the mechanical effect can shape the parliamentary party system, the ability of the psychological effect to influence the electoral party system is more questionable. The fact that it is difficult for third parties to win seats should theoretically discourage voters from choosing them. But even in First Past the Post countries where nearly all seats are taken by the two major parties, it is common to see a sizeable chunk of voters opting for third parties.

Over the last 50 years, an average of 26% of British voters, though once as high as 35%, have voted for parties other than main two at general elections. Even during the ‘high-point’ of two-partyism in the 1950s, the Liberals were still taking 1 in 6 votes in the constituencies where they stood. Canada has seen even more divergence, with an average two-party vote share of just 70% and a low point of 58%.

The problem is that the psychological effect is, to some extent, idealised voter behaviour. Even though ERS research suggests that a quarter of British voters are prepared to vote tactically for a less liked party, this still leaves a majority of voters who are either unaware or simply don’t care that a vote for a third party under First Past the Post is often a wasted vote. As only some voters feel pressured by First Past the Post to vote for a less favoured party, support for third parties persists.

Why do Political Parties Exist?

One of the key problems with Duverger’s Law is that it attributes to voting systems a power which they don’t necessarily have. While a voting system can be a potential constraining force on the party system, it is not the key determinant in why the parties exist in the first place. Parties tend to reflect the key divides in a society called ‘cleavages’, such as class, religious/secular or urban/rural.

If the divisions are weak, the voting system can perhaps influence party behaviour enough to force two weakly divided parties together, as with the Canadian Alliance and Progressive Conservatives or the Liberals and the SDP. But if the cleavages run deep and the divisions cannot be reconciled, even the most constraining voting system cannot override the creation of multiple parties. This is the case in Scotland, where the Conservatives, Labour and the SNP are all divided by cross-cutting cleavages and so a three-party system exists regardless of the constraints imposed by First Past the Post.

In fact, it has often been party system change that has driven electoral system change rather than the other way round. The failure of First Past the Post to adequately represent the growing multi-partyism of New Zealand voters was one of the key reasons that drove the switch to PR in the 1990s – the party system hasn’t particularly changed since. Similarly, it was the development of multi-partyism that spurred the introduction of PR in most European countries in the early 20th century. Newer parties have only really arisen since the 1960s as new cleavages have emerged.

Does Duvager’s law still hold in the USA?

Of course, this does throw up some interesting questions about the US and the prospects for new parties like Andrew Yang and his ‘Yang Gang’. By any stretch, the US is a very large, complicated country with 50 individual states and many cleavages. Even with First Past the Post, we would expect a country like the US to have some degree of multi-partyism, or at least different two-partyism in some states (similar to India). Yet it is one of only a few sizeable countries to have a true two-party system at both the electoral and parliamentary level.

This is because third parties in America struggle with far more restrictions than simply First Past the Post. The main two parties are heavily institutionalised to the extent of nearly being a de facto part of the constitution. Internal party elections are designed and conducted by state agencies, voters officially register their party support with the government and restrictive ballot access rules decide who can stand for election. This is compounded by the structure of Congress and media debate. The two-party system is such a deeply ingrained part of the American political system that its causes and stability go far beyond the constraining effects of any voting system.

Duverger’s Pattern?

While it may not be the case that First Past the Post inherently leads to a pure two-party system, it is clear that it does have some potentially constraining influence on the evolution of parliamentary party systems, even if its ability to influence which parties the public pick is a lot more questionable.

Duverger’s theory might still influence the thinking and planning of many politicians and some voters, but it is far from a watertight law. The voting system is only one factor that influences the overall party system and a country with multiple cross-cutting cleavages will nearly always end up with some degree of multi-partyism, even if saddled with a voting system that means some of those parties are inadequately represented.

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