What factors influence the number of political parties?

Question

While I know that the question could disintegrate into a list of things type of question I would really see answers that respect the back-it-up-rule and which would go deeper than only listing factors.

Now the question: What factors influence the number of political parties?

I'm interested into which factors have been scientifically proven to influence the number of political parties in a geographical area. Additionally and to make the question narrower, I would like to see explanations as to why these factors influence the number of political parties.

Answer 1

The main factor that determines the number of parties is the voting system.

As for research supporting the claim I will cite this paper, which says:

One of Duverger's most famous claims is that, in a law-like relationship, the plurality rule favors a two-party system while proportional systems lead to multipartyism (Duverger 1955). This raises the question of what is to ‘count’ as a party, in particular how to count very small parties. In recent years Lijphart (1994) reexamined the evidence for this thesis. The study compared 27 advanced industrialized democracies in 1945-90 based on the Laakso and Taagepera measure of the 'effective number of parliamentary parties' (ENPP), which takes account not only of the number of parties but also the relative size of each. Lijphart found that the ENPP was 2.0 in plurality systems, 2.8 in majority and 3.6 in proportional systems. Within proportional systems he found that the minimum threshold of votes also has an effect on the inclusion of minor parties.

This other paper mentions it along with electoral volatility:

The number of parties that compete for elections has been found to be positively associated with electoral volatility (Pedersen, 1979; Crewe, 1985; Bartolini and Mair, 1990), and it is well known that the electoral system is one of the main determinants of the number of parties that compete in elections (e.g. Rae, 1971; Taagepera and Shugart, 1989; Cox, 1997).

The reason why systems like first-past-the-post tend to produce two party systems is quite obvious. If in each district seats are assigned only to the party with the highest number of votes it becomes a lot harder for smaller parties to obtain any seats at all. The only exception are regional parties that are strong in a certain region.

Edit: fixed typos

Answer 2

As of Giovanni Sartori's Comparative Constitutional Engineering, the number of political parties are influenced both by electoral system and by outlawing some political parties.

Citing the example of the post-World War II Germany, Sartori describes the limited number of parties in a usually more fragmented proportional representation system as a results of a the ban of the of right-wing and left-wing extremist political parties by the Constitutional Court. That choice is not determined by the representation system alone as a previous answer (correctly) suggests, but by political decisions enforced by law on how to limit parties and subjects who have to take the responsibility of the government.

A previous answer highlights a similar suppression of subjects not by law but by brutalities in non-democratic regimes. The point is that both representational system and the identification of the subjects that are fit for command by ruling powers are essentials in defining the number of political parties, and consequently the governance of a country.

Answer 3

I base this answer on a mixture of common sense and this publication by Human Rights Watch.

One of the most obvious factors impacting the number of political parties for an area is political repression by the incumbent government, such as in Zimbabwe:

ZANU-PF has used the police and other state agencies to arbitrarily arrest and "disappear" more than 40 MDC members and human rights activists.

(An enforced disappearance is detention by authorities who refuse to acknowledge that they are holding the person or to reveal the person's fate or whereabouts, placing that person outside the protection of the law, and is a serious violation of international law. See International Convention for the Protection of All Persons from Enforced Disappearance, G.A. res. 61/177, U.N. Doc. A/RES/61/177 (2006), adopted Dec. 20, 2006.)

It is not outside common sense reasoning that an area which has considerable political repression such as arrest, intimidation and/or assassination of members of opposing political parties, or the incumbent government making an opposition party illegal, would through fear result in fewer opposition parties being formed.

Answer 4

5 years later... Newton et. Al, 2010: number of parties linked to number of cleavages -> if there are 2 main cleavages in a country (e.g. class and religion), there will be 3 main parties; 3 main cleavages, 4 main parties; etc.

i'm not convinced by this theory though - and the rationale behind this 'rule' is unclear