Issues Affecting The Single Transferable Vote - Tactical Voting

Tactical Voting

According to the Gibbard-Satterthwaite theorem tactical voting is possible in all non-dictatorial deterministic voting systems. A number of methods of tactical or strategic voting exist that can be used in elections conducted using STV. In general these methods are only effective in marginal districts and only affect the allocation of a single seat per district.

One potential strategy in STV involves casting a first preference vote for a candidate that has no chance of winning. This vote will then be transferred at full value and carry more weight in determining the winners at the later stages of the count. This strategy can be effective due to two features of some STV counting systems.

1. Simple Gregorian transfer (SGT) systems such as ERS97 and Newland-Britton Northern Ireland consider for transfer only the last parcel of votes received by an elected candidate.
2. Both simple Gregorian transfer systems and weighted inclusive Gregorian transfer (WIGT) systems such as BC-STV do not transfer votes to already elected candidates.

For example in an election in which 5 candidates are contesting 3 seats the following 400 votes are cast:

105: A>D>B

90: B

80: C>B

75: D>B

50: E>A>C>D

The quota is 100 and A is elected at stage 1. E is the first candidate to be eliminated. Under SGT the 50 E>A>C>D votes ‘skip’ candidate A and transfer to candidate C; who is then elected. These votes then transfer again to candidate D who wins the final seat. The SGT winners are A, C and D.

Under WIGT the 50 E>A>C>D votes again 'skip' candidate A and transfer to C who wins the second seat. All C's votes (both the E>A>C>D votes and the C>B votes) then transfer at reduced value. As a result of this transfer candidate B wins the third seat. The WIGT winners are A, C and B.

The Meek system, which is immune to this strategy, elects A, B and D.

Under SGT the E>A>C>D voters effectively determine the winners of the two final seats. Under WIGT as a result of the E>A>C>D votes failing to contribute to the election of candidate A and transferring at an increased value, C wins a seat as opposed to D. SGT systems are more vulnerable to this form of tactical voting than WIGT systems.

While this appears an effective strategy in theory there are a number of problems with it in practice. Firstly accurate information is needed about how other voters are going to vote, in practice this information is difficult to obtain. Secondly if all voters vote for candidates they believe will lose, these candidates will win. In general, this strategy will not work if everyone uses it. Little evidence exists of the use of this strategy in the real world.