Duverger’s law is a theoretical political rule devised by Maurice Duverger. The “rule” implies that in electoral systems where members are chosen via a plurality voting system (for example, first-past-the-post like in Britain), two-party politics dominates. Conversely, this implies that elections held under proportional voting systems will lead to multi-party politics.
The theory behind this is that as only candidates from the top two parties have any significant chance in winning seats, third parties will struggle to gain any substantial support.
In recent elections, it appeared that the rule no longer held in Britain. The Liberal Democrats enjoyed success, leading to the phrase of a two-and-a-half party system being used, but following the 2015 election the party was reduced to eight seats. UKIP surged in terms of votes, but only won one MP, and in Scotland, the SNP won 56 seats, showing regional differences across the United Kingdom.
But the 2017 election seems to provide evidence in favour of the “rule”. Both Labour and the Conservatives gained votes, the SNP gave way to the two main parties, and UKIP were decimated in popular vote terms while the Lib Dems floundered at less than 8% of the vote. The 2017 election makes a pretty convincing case that while there have been recent upsets, things are returning to what Duverger’s law would predict.
Writing for LSE Blogs, Patrick Dunleavy however, has pointed out that while the rule clearly holds in America, where the Republicans and Democrats are dominant, in other FPTP countries such as India, there is limited evidence in favour of the rule.
Furthermore, a model devised by Forand and Maheshri found that “the number of parties under plurality rule should be less variable than under proportional representation”, suggesting some evidence in favour of Duverger’s law.
And in Canada, which uses first-past-the-post, the NDP are a persistent third force, having even been the second largest party before Justin Trudeau’s Liberals came to power in 2015, showing that the rule does not hold everywhere.
Overall, the 2017 general election results suggest that Duverger’s Law holds, however, it is worth noting that the collapse of UKIP which helped facilitate this return to two-party politics did not happen because first-past-the-post; it happened because the two main parties embraced Brexit, thus leading to a decline in UKIP support.
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