Due to their practical importance and major theoretical implications, two-party configurations are central in research on elections and party systems. However, the measurement of two-partyness in cross-national political research is often substituted with the measurement of party system concentration/fragmentation, which leads to undesirable information losses. This study overviews the three measures of two-partyness already offered, albeit rarely used, in political science, and develops a new index that minimizes data demands, which is important in the field where the problem of data incompleteness is endemic. At the same time, the new index is transparent in its mathematical build-up, and it satisfies all theoretical requirements that have to be met in the measurement of two-partyness. The properties of the indices of two-partyness, both old and new, are tested on a hypothetical dataset comprising party constellations with widely variable structural characteristics, and on a real-life dataset that embraces elections held in 75 democratic polities of the world in 2001–2023. The analysis shows the utility of the new index in comparative research, at the same time confirming the validity of the previously developed measures.
Amorim NetoOctavioCoxGary W.. 1997. “Electoral Institutions, Cleavage Structures, and the Number of Parties.” American Journal of Political Science41 (1): 149–74.
2.
BikkerJacob A.HaafKatharina. 2002. “Measures of Competition and Concentration in the Banking Industry: a Review of the Literature.” Economic and Financial Modelling9 (2): 53–98.
3.
BoixCarlesMillerMichaelRosatoSebastian. 2013. “A Complete Data Set of Political Regimes, 1800–2007.” Comparative Political Studies46 (12): 1523–54.
4.
BoscoAnnaVerneySusannah. 2018. Crisis Elections, New Contenders and Government Formation: Breaking the Mould in Southern Europe. Routledge.
5.
ClarkWilliam R.GolderMatt. 2006. “Rehabilitating Duverger’s Theory: Testing the Mechanical and Strategic Modifying Effects of Electoral Laws.” Comparative Political Studies39 (6): 679–78.
6.
ClericiPaula. 2021. “Legislative Territorialization: the Impact of a Decentralized Party System on Individual Legislative Behavior in Argentina.” Publius: The Journal of Federalism51 (1): 104–30.
7.
ColemanStephen.1995. “Dynamics in the Fragmentation of Political Party Systems.” Quality and Quantity29 (2): 141–55.
8.
ComellasJosep M.TorcalMariano. 2023. “Ideological Identity, Issue-based Ideology and Bipolar Affective Polarization in Multiparty Systems: the Cases of Argentina, Chile, Italy, Portugal and Spain.” Electoral Studies83: 102615.
9.
CoppedgeMichaelJohnGerringKnutsenCarl H., et al.2023. V-Dem [Country-Year/Country-Date] Dataset v13. Varieties of Democracy (V-Dem) Project. doi:10.23696/vdemds23.
10.
CoxGary W.1997. Making Votes Count: Strategic Coordination in the World's Electoral Systems. Cambridge University Press.
11.
DaalderHans. 2002. “Parties: Denied, Dismissed, or Redundant? A Critique.” In Political Parties: Old Concepts and New Challenges, edited by RichardGuntherJuanLinzMonteroJose R., 39–57. Oxford University Press.
12.
DiwakarRekha. 2007. “Duverger’s Law and the Size of the Indian Party System.” Party Politics13 (5): 539–61.
13.
DownsAnthony. 1957. An Economic Theory of Democracy. Harper & Row.
14.
DunleavyPatrickBoucekFrançoise. 2003. “Constructing the Number of Parties.” Party Politics9 (3): 291–315.
15.
DunleavyPatrickDiwakarRekha. 2013. “Analysing Multiparty Competition in Plurality Rule Elections.” Party Politics19 (6): 855–86.
16.
DuvergerMaurice. 1954. Political Parties, their Organization and Activity in the Modern State. Methuen.
GainesBrian J.TaageperaRein. 2013. “How to Operationalize Two-Partyness.” Journal of Elections, Public Opinion, and Parties23 (4): 387–404.
19.
GeddesBarbaraWrightJosephEricaFrantz. 2014. “Autocratic Breakdown and Regime Transitions: a New Data Set.” Perspectives on Politics12 (2): 313–31.
20.
GeysBennyBrunoHeyndelsVermeirJan. 2006. “Explaining the Formation of Minimal Coalitions: Anti‐System Parties and Anti‐Pact Rules.” European Journal of Political Research45 (6): 957–84.
21.
GolosovGrigorii V. 2010. “The Effective Number of Parties: a New Approach.” Party Politics16 (2): 171–92.
22.
GolosovGrigorii V. 2016. “Why and How Electoral Systems Matter in Autocracies.” Australian Journal of Political Science51 (3): 367–85.
23.
GolosovGrigorii V. 2025. “Electoral Rules and the Two-Party System: a Methodological Inquiry with Reference to Duverger’s Law.” International Political Science Review46 (3): 388–405.
24.
GrofmanBernard. 2006. “The Impacts of Electoral Laws on Political Parties.” In The Oxford Handbook of Political Economy, edited by WeingastBarry R.DonaldWittman, 102–18. Oxford University Press.
25.
GrofmanBernardKlineReuben. 2012. “How Many Political Parties are There, Really? A New Measure of the Ideologically Cognizable Number of Parties/Party Groupings.” Party Politics18 (4): 523–44.
26.
GrofmanBernardAlessandroChiaramonteRobertoD’Alimonte, et al.2004. “Comparing and Contrasting the Uses of Two Graphical Tools for Displaying Patterns of Multiparty Competition: Nagayama Diagrams and Simplex Representations.” Party Politics10 (3): 273–99.
27.
HolbrookThomas M.Van DunkEmily. 1993. “Electoral Competition in the American States.” American Political Science Review87 (4): 955–62.
28.
KendallMaurice G.StuartAlan. 1950. “The Law of the Cubic Proportion in Election Results.” British Journal of Sociology1 (3): 183–96.
29.
KirkpatrickEvron M.1971. “Toward A More Responsible Two-Party System: Political Science, Policy Science, or Pseudo-science?” American Political Science Review65 (4): 965–90.
30.
KollmanKenAllenHickenDanieleCaramani, et al.2024. Constituency-Level Elections Archive. Center for Political Studies, University of Michigan. https://www.electiondataarchive.org
31.
LaaksoMarkkuTaageperaRein. 1979. “Effective’ Number of Parties: a Measure with Application to West Europe.” Comparative Political Studies12 (1): 3–27.
32.
LagoIgnacioMartínez i ComaFerran. 2024. “Assessing Electoral Coordination: a Quasi-Experimental Design of Changes in District Magnitude over Time.” Party Politics13540688241291697.
MassicotteLouisBlaisAndré. 1999. “Mixed Electoral Systems: a Conceptual and Empirical Survey.” Electoral Studies18 (3): 341–66.
39.
McDonaldMichael D.BudgeIan. 2005. Elections, Parties, Democracy: Conferring the Median Mandate. Oxford University Press.
40.
MerrillSamuelGrofmanBernard. 1999. A Unified Theory of Voting: Directional and Proximity Spatial Models. Cambridge University Press.
41.
MolinarJuan. 1991. “Counting the Number of Parties: an Alternative Index.” American Political Science Review85 (4): 1383–91.
42.
MurrAndreas E.StegmaierMaryLewis-BeckMichael S.. 2021. “Vote Expectations Versus Vote Intentions: Rival Forecasting Strategies.” British Journal of Political Science51 (1): 60–67.
43.
NwokoraZimPelizzoRiccardo. 2014. “Sartori Reconsidered: toward a New Predominant Party System.” Political Studies62 (4): 824–42.
44.
OrdeshookPeter C.ShvetsovaOlga V.. 1994. “Ethnic Heterogeneity, District Magnitude, and the Number of Parties.” American Journal of Political Science38 (1): 100–23.
45.
OshriOdeliaOmerYairLeonieHuddy. 2022. “The Importance of Attachment to an Ideological Group in Multi-Party Systems: Evidence from Israel.” Party Politics28 (6): 1164–75.
46.
OtjesSimon. 2025. “Electoral Systems and Assembly Size.” Electoral Studies95: 102922.
47.
PechGerald. 2004. “Coalition Governments Versus Minority Governments: Bargaining Power, Cohesion and Budgeting Outcomes.” Public Choice121 (1): 1–24.
RaeDouglas W.1971. The Political Consequences of Electoral Laws. Yale University Press.
50.
RanneyAustin. 1976. “Parties in State Politics.” In Politics in the American States: A Comparative Analysis, edited by HerbertJacobsVines KennethN.. 3rd ed., 51–92. Little, Brown.
51.
ReedSteven. 1990. “Structure and Behavior: Extending Duverger’s Law to the Japanese Case.” British Journal of Political Science29 (3): 335–56.
52.
RichTimothy S.2013. “The Electoral Incentives for Legislator Perceptions in Mixed Systems: Integrating Evidence from East Asia.” Representation49 (1): 17–31.
53.
RikerWilliam H. 1982. “The Two-Party System and Duverger’s Law: an Essay on the History of Political Science.” American Political Science Review76 (4): 753–66.
54.
RoyedTerry J. 1996. “Testing the Mandate Model in Britain and the United States: Evidence from the Reagan and Thatcher Eras.” British Journal of Political Science26 (1): 45–80.
55.
SartoriGiovanni. 1986. “The Influence of Electoral Systems: Faulty Laws or Faulty Method?” In Electoral Laws and their Political Consequences, edited by BernardGrofmanArendLijphart, 43–68. Agathon Press.
56.
SchedlerAndreas. 2006. “The Logic of Electoral Authoritarianism.” In Electoral Authoritarianism: The Dynamics of Unfree Competition, edited by AndreasSchedler, 1–23. Lynne Rienner.
57.
ShugartMatthew S.TaageperaRein. 2017. Votes from Seats: Logical Models of Electoral Systems. Cambridge University Press.
58.
SuttonPaul. 1999. “Democracy in the Commonwealth Caribbean.” Democratization6 (1): 67–86.
59.
TaageperaRein. 1997. “Effective Number of Parties for Incomplete Data.” Electoral Studies16 (2): 145–51.
60.
TaageperaRein. 1999. “Supplementing the Effective Number of Parties.” Electoral Studies18 (4): 497–504.
61.
TaageperaRein. 2004. “Extension of the Nagayama Triangle for Visualization of Party Strengths.” Party Politics10 (3): 301–06.
62.
TaageperaReinNemčokMiroslav. 2021. “Majoritarian and Consensual Democracies: the Seat Product Connection.” Party Politics27 (3): 393–406.
63.
TufteEdward R. 1973. “The Relationship Between Seats and Votes in Two-Party Systems.” American Political Science Review67 (2): 540–54.
64.
WareAlan. 2009. The Dynamics of Two-Party Politics: Party Structures and the Management of Competition. Oxford University Press.
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