As the UK votes on voting, a Guardian article explains:

A theorem (proved by Allan Gibbard and Mark Satterthwaite) tells us about elections designed to find a single winner, as is the case when a constituency elects its MP. The theorem says that, if there are three or more candidates, any voting system which is not a dictatorship and which allows the possibility of any candidate winning, is susceptible to tactical voting (where voters have an incentive to vote in a way that doesn’t reflect their personal preferences).

Advertisements