Editing Duverger's law (section)

== Strength of effect ==
William Clark and Matt Golder (2006) find the effect largely holds up, noting that different methods of analyzing the data might lead to different conclusions. They emphasize other variables like the nuances of different electoral institutions and the importance that Duverger also placed on sociological factors.<ref name=":4">{{Cite journal |last1=Clark |first1=William Roberts |last2=Golder |first2=Matt |date=August 2006 |title=Rehabilitating Duverger's Theory: Testing the Mechanical and Strategic Modifying Effects of Electoral Laws |url=http://journals.sagepub.com/doi/10.1177/0010414005278420 |journal=Comparative Political Studies |language=en |volume=39 |issue=6 |pages=679–708 |doi=10.1177/0010414005278420 |issn=0010-4140 |s2cid=154525800 |quote=The central hypothesis Duverger’s theory generates is that social heterogeneity should increase the number of parties only once the electoral system is sufficiently permissive.}}</ref> [[Thomas Palfrey|Thomas R. Palfrey]] argued Duverger's law can be proven mathematically at the limit when the number of voters approaches infinity for one single-winner district and where the probability distribution of votes is known (perfect information).<ref>Palfrey, T. (1989) ‘A mathematical proof of Duverger’s law’, in P. Ordeshook (ed.) [https://archive.org/details/modelsofstrategi0000unse/page/68/mode/2up?view=theater Models of Strategic Choice in Politics], Ann Arbor: University of Michigan Press, 69–91.</ref>

Duverger did not regard this principle as absolute, suggesting instead that plurality would act to delay the emergence of new political forces and would accelerate the elimination of weakening ones, whereas proportional representation would have the opposite effect.<ref name="Duverger" />

The U.S. system has two major parties that have won, on average, 98% of all state and federal seats.<ref name="kaan :3" /> There have only been a few rare elections where a minor party was competitive with the major parties, occasionally replacing one of the major parties in the 19th century.<ref name=":0" /><ref name="riker" />

In Matt Golder's 2016 review of the empirical evidence to-date, he concluded that despite some contradicting cases, the law remains a valid generalization.<ref>Golder, M. (2016). "Far from equilibrium: The state of the field on electoral system effects on party systems." In E. Herron, R. Pekkanen, & M. Shugart (Eds.), ''The Oxford Handbook of Electoral Systems''. Oxford University Press.</ref>

[[Steven Reed (political scientist)|Steven R. Reed]] argued in 2001 that Duverger's law could be observed in Italy, with 80% of electoral districts gradually but significantly shifting towards two major parties.<ref name=":2">{{cite journal |last1=Reed |first1=Steven R. |date=April 2001 |title=Duverger's Law is Working in Italy |journal=Comparative Political Studies |volume=34 |issue=3 |pages=312–327 |doi=10.1177/0010414001034003004 |s2cid=154808991}}</ref> He finds a similar effect in Japan through a slow trial-and-error process that shifted the number of major parties towards the expected outcome.<ref name=":1">{{cite journal |last1=Reed |first1=Steven R. |year=1990 |title=Structure and Behaviour: Extending Duverger's Law to the Japanese Case |journal=British Journal of Political Science |volume=20 |issue=3 |pages=335–356 |doi=10.1017/S0007123400005871 |jstor=193914 |s2cid=154377379}}</ref>

Eric Dickson and Kenneth Scheve argued in 2007 that Duverger's law is strongest when a society is homogenous or closely divided, but is weakened when multiple intermediate identities exist.<ref>{{cite journal |last1=Dickson |first1=Eric S. |last2=Scheve |first2=Kenneth |author2-link=Kenneth Scheve |date=20 February 2007 |title=Social Identity, Electoral Institutions and the Number of Candidates |url=http://www.nyu.edu/gsas/dept/politics/faculty/dickson/dickson_candidates.pdf |url-status=dead |journal=British Journal of Political Science |volume=40 |issue=2 |pages=349–375 |citeseerx=10.1.1.75.155 |doi=10.1017/s0007123409990354 |jstor=40649446 |s2cid=7107526 |archive-url=https://web.archive.org/web/20180721203318/http://www.nyu.edu/gsas/dept/politics/faculty/dickson/dickson_candidates.pdf |archive-date=2018-07-21 |access-date=2017-10-26}}</ref> As evidence of this, Duhamel cites the case of India, where over 25 percent of voters vote for parties outside the two main alliances.<ref>{{cite book |last1=Duhamel |first1=Olivier |url=https://books.google.com/books?id=ZWSzDwAAQBAJ&pg=PT297 |title=Droit constitutionnel et institutions politiques |last2=Tusseau |first2=Guillaume |date=2019 |publisher=[[Éditions du Seuil]] |isbn=9782021441932 |edition=5e |location=Paris |page=297 |language=fr |trans-title=Constitutional law and political institutions |oclc=1127387529 |quote=[L]a loi selon laquelle le scrutin majoritaire à un tour tend à produire le bipartisme ne vaut que dans une société relativement homogène et un État assez centralisé.  Dans le cas contraire, le système de parti national se voit concurrencé par des sous-systèmes régionaux. |author-link1=Olivier Duhamel |author-link2=Guillaume Tusseau}}</ref>

Two-party politics may also emerge in systems that use a form of proportional representation, with Duverger and others arguing that Duverger's Law mostly represents a limiting factor (like a brake) on the number of major parties in other systems more than a prediction of equilibrium for governments with more proportional representation.<ref>{{cite book |last1=Cox |first1=Gary W. |url=https://archive.org/details/makingvotescount0000coxg/page/273/mode/1up?view=theater |title=Making votes count: strategic coordination in the world's electoral systems |date=1997 |publisher=[[Cambridge University Press]] |isbn=9780521585163 |location=Cambridge |pages=273–274 |oclc=474972505 |author-link1=Gary W. Cox}}</ref><ref name=":4" />