Duverger's Law

Lynette Lewis

PL SC 150 – Hawkins

Blog 7 – Duverger’s Law

                When considering the electoral system of a state, one correlation between electoral rules and effective number of parties is in Duverger’s law.  This states that the effective number of parties in any electoral district is a function of the electoral rules and Single-member-district plurality rules produce two effective parties, while proportional representation leads to more.  We can test this law by looking at the example of the United Kingdom to see that this rule does not always hold true.

                In the case of the UK we see an example of a single-member-district plurality system (Parline), which I will refer to as SMDP for the remainder of this paper.  An SMDP system is an electoral system in which “the state is divided into a set of districts, usually having roughly equal populations.  One representative is elected from each district to be a member of the legislative body of the state, and whoever gets a plurality of the votes wins the seat”(Shively, 231).

 The House of Commons is selected by voting on representatives from their 650 single-member constituencies (Parline).  Generally, a first-past-the-post system is found “in places that are, or once were, British colonies” (ERS), but two years ago we saw a shift in the House of Commons.  After a vote in February 2010 the House of Commons now uses an Alternative Votes System (Praline).  This system has “voters rank candidates in order of preference. A candidate obtaining more than 50 percent of the first choice votes is declared elected. If no candidate secures more than 50 percent of the votes, the candidate with the fewest number of votes is eliminated and voters' second choices are allocated to the remaining candidates. This process continues until a winner emerges” (Praline).  While this system is less obvious than some to be an SMDP, it is a vote based on districts to represent the people and the vote is similar to a plurality.

                If this law holds true then because the UK has an SMDP system we should see two effective parties.  In the 2011 election we find the Labour Party at the top with approximately 36% of the vote and Conservative and Liberal Democrats receiving 52% of the remaining vote and the other 12% being split between a large number of parties (Wikipedia¹).  Based off the information on the past election we can calculate the effective parties in the UK by finding the sum of the percentage of seats in the house for each party squared, and take the inverse.  By doing this we find the UK to have 3.7 effective parties.  We also find that there is no voter threshold as parties with as little as .1% may receive some representation, like Independent-Sylvia Hermon who earned .1% of the vote and received .2% of the seats (Wikipedia¹). 

We may think that this exception to Duverger’s law is due to the electoral system change in 2010, however the UK has appeared to have 3 effective parties for a number of years.  We can do the same calculation and find that in the 2005 election there were 3.6 effective parties, based on election results from that year (Wikipedia²).

Based on this information we can infer that while Duverger’s Law may hold true most of the time, there is an exception in the example of the United Kingdom, where despite being mainly an SMDP electoral system there are more than two effective parties.

Works Cited

"IPU PARLINE Database: UNITED KINGDOM (House of Commons)." IPU PARLINE Database: UNITED KINGDOM (House of Commons). Inter-Parliamentary Union, 2009. Web. 2 Nov. 2012. <http://www.ipu.org/parline-e/reports/CtrlParlementaire/2336_F.htm>.

Shively, W. Phillips. "Elections." Power and Choice: An Introduction to Political Science. 13th ed. New York [etc.: McGraw-Hill, 1993. 230-31. Print.

“Voting Systems Made Simple: First-Past-the-Post.” Electoral Reform Society. n.d. Web. 02 Nov. 2012. <http://www.electoral-reform.org.uk/first-past-the-post>.

¹Wikipedia Contributors. “United Kingdom General Election, 2005.” Wikipedia, The Free Encyclopedia. 28 Oct 2012. Web. 2 Nov. 2012

²Wikipedia Contributors. “United Kingdom General Election, 2010.” Wikipedia, The Free Encyclopedia. 28 Oct 2012. Web. 2 Nov. 2012