Party System Fragmentation

A party system is fragmented if it contains more than two parties, none of which comes close to obtaining an absolute majority in the representative assembly. Party system fragmentation thus has two aspects: the number of parties in the system and their relative size. A pure two-party system fails to meet both of these defining criteria. A system with many parties of which one is large enough to approach an absolute majority on its own also cannot be considered fragmented. In empirical terms, fragmentation in practice refers to those parties that gain representation. Parties without official representation are often difficult to observe and document. Moreover, most theories of the causes and consequences of party system fragmentation deal with parties with seats in a representative body. If the representative assembly of a country contains many fairly small parties and no party approaching majority status, the party system of that country can be considered fragmented. Several measures are available to determine varying degrees of fragmentation; they are strongly intercorrelated and can therefore be used interchangeably. The Laakso-Taagepera Index of the Effective Number of Parties is the most widely used measure. It is calculated as follows:

where N stands for the effective number of parties and pi denotes the fraction of the seats held by each party i in the assembly. All parties are accorded some weight, but the index weights the largest parties most. Theoretically, if there are 2 parties of exactly the same size, the effective number of parties is 2.0. Three parties of equal size render the index value 3.0, 10 such parties 10.0, and so on. The party systems of the United States, Sweden, and Finland can be used as empirical illustrations. The mean value of the Laakso-Taagepera Index for the U.S. House of Representatives since 1960 is 1.9. For Sweden, with a multiparty system where the Social Democrats have held around or more than 40% of the Riksdag seats, the corresponding figure is 3.6. In Finland, no party normally wins more than a quarter of the parliamentary seats; the mean effective number of parties is 5.2. The three cases represent a two-party system, a moderately fragmented multiparty system, and a highly fragmented system, respectively.

The numerical aspects of party systems have always been an important factor in the comparative literature on political parties. Claims that systems with many parties were less governable than two-party systems were presented already in the 19th century. As the interwar years saw the demise of democracy in a large number of European states, the criticism against multiparty government gained in prominence. Nevertheless, it is only with the emergence of comprehensive comparative [Page 1823]databases during recent decades that robust empirical generalizations have become possible.

The dynamics of electoral systems go a long way toward explaining why some countries have fragmented party systems while others do not. Single-member plurality (SMP) systems effectively reduce the number of parties that gain representation, while proportional (PR) list systems are conducive to fragmentation, especially when electoral districts are large. Among democratic countries with SMP systems, the mean effective number of parties since 1960 is 2.1, while the corresponding figure for democracies with PR list systems is 3.9. Countries with other electoral formulas, including mixed-member systems, usually attain values between these extremes. It is very difficult for parties other than the two largest ones to win seats in SMP systems, while proportional systems frequently allow even quite small parties to gain representation. Voters' awareness of this mechanism reinforces its effect. If voters deem it highly unlikely that a vote for a minor party will result in the party's winning a seat—as is the case in SMP systems—their propensity to vote for it becomes smaller. By the same token, a higher probability of a small party's winning representation in PR systems will mean that voters are more likely to stay with their preferred party even if it is a minor one.

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