Editing Duverger's law (section)

==Mechanism==
A [[two-party system]] is most common under [[plurality voting system|plurality voting]]. Voters typically cast one vote per race. [[Maurice Duverger]] argued there were two main mechanisms by which plurality voting systems lead to fewer major parties: (i) small parties are disincentivized to form because they have great difficulty winning seats or representation, and (ii) voters are wary of voting for a smaller party whose policies they actually favor because they do not want to "waste" their votes (on a party unlikely to win a plurality) and therefore tend to gravitate to one of two major parties that is more likely to achieve a plurality, win the election, and implement policy.<ref>{{cite journal | title = Maurice Duverger and the Study of Political Parties | journal = French Politics | url = http://www.pratiquesciencessociales.net/exposes/S12.%20Maurice%20Duverger%20and%20the%20Study%20of%20Political%20Parties%20%28Schlesinger%202006%29.pdf | access-date = 2011-12-17 | url-status = dead | archive-url = https://web.archive.org/web/20110724020408/http://www.pratiquesciencessociales.net/exposes/S12.%20Maurice%20Duverger%20and%20the%20Study%20of%20Political%20Parties%20%28Schlesinger%202006%29.pdf | archive-date = 24 July 2011 | doi = 10.1057/palgrave.fp.8200085 | year = 2006 | last1 = Schlesinger | first1 = Joseph A. | last2 = Schlesinger | first2 = Mildred S. | volume = 4 | pages = 58–68 | s2cid = 145281087 }}</ref><ref>{{cite book | url = https://books.google.com/books?id=fecTeDjAZZ8C&pg=PA12 | title = The Japanese Election System: Three Analytical Perspectives | first = Junichiro | last = Wada| isbn = 9780203208595 | date = 2004-01-14 | publisher = Taylor & Francis }}</ref><ref>{{cite book |last=Alston |first=Eric |chapter-url=https://www.cambridge.org/core/books/institutional-and-organizational-analysis/legislature-and-executive/70DD1949D5C833D7F30FE49A3E0C9A9F |chapter=The Legislature and Executive |editor-last1=Alston |editor-first1=Lee J. |editor-last2=Mueller |editor-first2=Bernardo |editor-last3=Nonnenmacher |editor-first3=Tomas |date=2018 |title=Institutional and Organizational Analysis: Concepts and Applications |publisher=Cambridge University Press |isbn=9781316091340 |pages=173–206 |doi=10.1017/9781316091340.006 |access-date=September 25, 2023}}</ref>

For legislatures where each seat represents a geographical area and the candidate with the most votes wins that seat, minor parties spread fairly evenly across many districts win less representation than geographically concentrated ones with the same overall level of public support. An example of this was the [[Liberal Democrats (UK)|Liberal Democrats]] in the [[United Kingdom]], whose proportion of seats in the legislature was, until recently, significantly less than their proportion of the national vote. The [[Green Party of Canada]] is another example; the party received about 5% of the popular vote from 2004 to 2011 but had only won one seat (out of 308) in the [[House of Commons of Canada|House of Commons]] in the same span of time. Another example was seen in the [[1992 United States presidential election|1992 U.S. presidential election]], when [[Ross Perot presidential campaign, 1992|Ross Perot's candidacy]] received zero [[Electoral College (United States)|electoral votes]] despite receiving 19% of the popular vote. [[Gerrymandering]] is sometimes used to try to collect a population of like-minded voters within a geographically cohesive district so that their votes are not "wasted", but it tends to require that minor parties have both a geographic concentration and a redistricting process that seeks to represent them. These disadvantages tend to suppress the ability of a third party to engage in the political process.{{Citation needed|date=October 2024}}

The second challenge to a third party is both statistical and tactical. Duverger presents the example of an election in which 100,000 moderate voters and 80,000 radical voters are to vote for candidates for a single seat or office. If two moderate parties and one radical party ran candidates, and every voter voted, the radical candidate would tend to win unless one of the moderate candidates gathered fewer than 20,000 votes. Appreciating this risk, moderate voters would be inclined to vote for the moderate candidate they deemed likely to gain more votes, with the goal of defeating the radical candidate. To win, then, either the two moderate parties must merge, or one moderate party must fail, as the voters gravitate to the two strongest parties. Duverger called this trend polarization.<ref name="Duverger">
{{cite book
 | first = Maurice
 | last = Duverger
 | author-link = Maurice Duverger
 | chapter = Factors in a Two-Party and Multiparty System
 | title = Party Politics and Pressure Groups
 | location = New York
 | publisher = Thomas Y. Crowell
 | year = 1972
 | pages = 23–32
 | chapter-url = http://janda.org/c24/Readings/Duverger/Duverger.htm
}}</ref>

=== Direction of effect ===
Kenneth Benoit suggested causal influence between electoral and party systems might be bidirectional or in either direction.<ref>{{Cite journal |last=Benoit |first=Kenneth |year=2007 |title=Electoral Laws as Political Consequences: Explaining the Origins and Change of Electoral Institutions |journal=Annual Review of Political Science |volume=10 |issue=1 |pages=363–390 |doi=10.1146/annurev.polisci.10.072805.101608 |doi-access=free}}</ref> Josep Colomer agreed, arguing that changes from a plurality system to a proportional system are typically preceded by the emergence of more than two effective parties, and increases in the [[effective number of parties]] happen not in the short term, but in the mid-to-long term.<ref>
{{cite journal |last=Colomer |first=Josep M. |author-link=Josep Colomer |year=2005 |title=It's Parties that Choose Electoral Systems (or Duverger's Law Upside Down) |url=http://www.politicalstudies.org/pdf/edsfavourites/colomer.pdf |url-status=dead |journal=Political Studies |volume=53 |issue=1 |pages=1–21 |citeseerx=10.1.1.563.7631 |doi=10.1111/j.1467-9248.2005.00514.x |s2cid=12376724 |archive-url=https://web.archive.org/web/20060203040309/http://www.politicalstudies.org/pdf/edsfavourites/colomer.pdf |archive-date=3 February 2006 |access-date=2009-05-31 |hdl=10261/61619}}</ref>

===Swap between parties ===
The political chaos in the United States immediately preceding the [[Origins of the American Civil War|Civil War]] allowed the [[Republican Party (United States)|Republican Party]] to replace the [[Whig Party (United States)|Whig Party]] as the progressive half of the American political landscape.<ref name="riker" /> Loosely united on a platform of country-wide economic reform and federally funded industrialization, the decentralized Whig leadership failed to take a decisive stance on the [[Slavery in the United States|slavery issue]], effectively splitting the party along the [[Mason–Dixon line]]. [[Southern United States|Southern]] rural planters, initially attracted by the prospect of federal infrastructure and schools, aligned with the pro-slavery Democrats, while urban laborers and professionals in the northern states, threatened by the sudden shift in political and economic power and losing faith in the failing Whig candidates, flocked to the increasingly vocal anti-slavery Republican Party.