Blog 7: Duverger’s Law

The United Kingdom is home to one of the world’s oldest surviving democracies, and its legislative body, Parliament, has served as a model to democracies everywhere. It maintains a very basic structure, and is an interesting example of Duverger’s Law.

The UK has Single Member District Plurality.  There are 650 electoral districts², and each one sends a single Member of Parliament based on the “First Past the Post” system¹. There is not a list system, nor do they have a threshold rule. The following table shows the calculation of the effective number of political parties, based on the 2010 elections.

Political Group	Total	%	p squared
Conservative Party	306	0.470769	0.221624
Labour Party	258	0.396923	0.157548
Liberal Democrats	57	0.087692	0.00769
Democratic Unionist Party	8	0.012308	0.000151
Scottish National Party	6	0.009231	8.52E-05
Sinn Fein	5	0.007692	5.92E-05
Plaid Cymru	3	0.004615	2.13E-05
Social Democratic and Labour Party	3	0.004615	2.13E-05
Greens	1	0.001538	2.37E-06
Alliance Party of Northern Ireland	1	0.001538	2.37E-06
The Speaker	1	0.001538	2.37E-06
Independents	1	0.001538	2.37E-06
Total	650	1	0.387209
Effective Number of Parties	2.582581

We can see how the SMDP rules of Britain have produced two major parties, but this year, something interesting happened. For the first time since 1974, there was a “hung parliament,” where one party did not have a full majority. This caused some turmoil in the British electoral system, demonstrating that these results are the exception, rather than the rule. In general, Britain’s rules create one party with a majority, and one major opposition party.

1."Elections in the United Kingdom." Wikipedia. Wikimedia Foundation, 27 Oct. 2012. Web. 02 Nov. 2012. <http://en.wikipedia.org/wiki/Elections_in_the_United_Kingdom>.

2."IPU PARLINE Database: UNITED KINGDOM (House of Commons), Last Elections." IPU PARLINE Database: UNITED KINGDOM (House of Commons), Last Elections. N.p., n.d. Web. 02 Nov. 2012. <http://www.ipu.org/parline-e/reports/2335_E.htm>.