Duverger's Law's Converse
The converse of Duverger's Law is not always valid; two-party politics may emerge even when the plurality vote is not used. This is particularly true in the case of countries using systems that even if they do not use the plurality vote, do not fully incorporate PR either. For instance, Malta has a single transferable vote (STV) system and (what seems to be) stable two-party politics.
In Australian Senate, there is proportional voting but even though smaller parties have been able to win seats, there is still a trend towards the major parties, whose dominance in the lower house effectively promotes their upper house candidates.
Some systems are even more likely to lead to a two-party outcome: for example, elections in Gibraltar use a partial block vote system in a single constituency, so the third most popular party is unlikely to win any seats.
In recent years some researchers have modified Duverger's Law by suggesting that electoral systems are an effect of party systems rather than a cause. It has been shown that changes from a plurality system to a proportional system are typically preceded by the emergence of more than two effective parties, and are typically not followed by a substantial increase in the effective number of parties.
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