Duverger's Law

Is Duverger’s law correct when it states that the effective number of parties in any electoral district is a function of the electoral rules? The rule states that Single Member District Plurality (SMDP) rules produce two effective parties while a Proportional Representation (PR) rules allow for more parties to flourish and be effective. I decided to use Costa Rica as my study case in observing the electoral rules and seeing if this law holds true. Costa Rica’s electoral system is a Proportional Representation with a closed party list. 

Costa Rica’s Legislative Assembly is comprised of 57 seats that are directly elected from 7 multi-member constituencies, one for each province, that contains a district magnitude anywhere from 21 seats for the largest and most populated province down to 4 seats for the least populated province. Under this system the “parties which have attained the required quotient are entitled for seat distribution. Seats remaining unfilled on the basis of the quotient system are distributed among parties in the order of their residual votes.” That quotient is formed by “dividing the total number of valid votes cast in a particular province by the number of seats to be filled in the same province” (IPU Parline). The electoral system is two-round with a 40 percent threshold.

The fact that Costa Rica has a closed party lists means that those voting can only vote for the party they prefer as a whole. They cannot choose the officials they would like to be elected and receive a seat. This closed party system is likely a reason why there can be many effective parties as voters do not want to put only two parties in congress without knowing exactly who from those partied will be represented. By distributing the seats across more parties it helps ensure that more voices and different opinions will be heard. 

When using the Neff formula by plugging in the number found in the chart below it shows that there are roughly 4 effective parties (1/(.421)²+ (.193)²+(.158)²+(.07)²+(.105)²+(.018)²+(.018)²+(.018)²= 3.9009). It is visible in the chart that after the National Liberation Party that the number of seats distributed is scattered somewhat evenly between the next few parties. The added element of number of seats based off of population helps distort the order of the parties slightly as the Access without Exclusion party garnered more votes but eventually ended up with less seats than the Social Christian Unity Party.  The chart shows though that 5 parties were able to receive 8% or more of the total number of seats. That distribution helps confirm Duverger’s law and prove that Proportional Representation systems typically are able to produce more effective parties than a Single Member Plurality District does. 

Parties	Votes	Seats	Proportion
National Liberation Party (Partido Liberación Nacional)	708,043	24	0.421
Citizens' Action Party (Partido Acción Ciudadana)	334,636	11	0.193
Libertarian Movement Party (Partido Movimiento Libertario)	275,518	9	0.158
Access without Exclusion (Accesibilidad sin Exclusión)	171,858	4	0.07
Social Christian Unity Party (Partido de Unidad Socialcristiana)	155,047	6	0.105
Costa Rican Renovation Party (Partido Renovación Costariccense)	73,150	1	0.018
Broad Front (Frente Amplio)	68.987	1	0.018
National Restoration (Restauración Nacional)	29.530	1	0.018
Total (turnout 69.08%)	1,816,769	57

Sources

IPU PARLINE Database: COSTA RICA (Asamblea Legislativa), Electoral System." IPU PARLINE Database: COSTA RICA (Asamblea Legislativa), Electoral System. N.p., n.d. Web. 01 Nov. 2012. <http://www.ipu.org/parline-e/reports/2073_B.htm>.

Costa Rican General Election, 2010." Wikipedia. Wikimedia Foundation, 22 Oct. 2012. Web. 01 Nov. 2012. <http://en.wikipedia.org/wiki/Costa_Rican_general_election,_2010>.