Why are small radical parties often disadvantaged by the single transferable vote system?

Under the single transferable vote (STV) system, voters rank candidates by preference, and votes are transferred based on these rankings to fill seats. While this method is designed to provide proportional representation, it often disadvantages small radical parties while benefiting minor centrist parties.123

The primary reason for this disparity lies in the nature of vote transfers. Because STV relies on the aggregation of ranked preferences, candidates from centrist parties are more likely to be selected as second or third choices by voters from larger, mainstream parties. When a mainstream candidate has a surplus of votes or a minor candidate is eliminated, these transferred votes frequently flow to centrist alternatives, helping them reach the electoral quota.132

In contrast, radical or extremist parties often lack this broad appeal across the political spectrum. Their candidates are less likely to be ranked as secondary preferences by supporters of other parties, meaning they rarely benefit from the transfer of surplus or eliminated votes. Consequently, even when radical parties receive a share of the national vote similar to that of centrist parties, they often secure fewer seats in the legislature.32