Single-member First-past-the-post

First-past-the-post is a pretty simple system:

1. Each voter chooses one, and only one, candidate
2. The winner is the candidate chosen by the most voters

Reformers like to bag out on this system, but I don't think that it's all that bad as a single-winner system. The system has some pretty attractive properties:

• Voting and counting are both extremely simple.
• The candidate who is most people's favourite will always win.
• Casting a vote for your favourite candidate is never worse than not voting at all.

That being said, the system has two related flaws:

• Two or more similar candidates can split the vote, causing a third, less popular, candidate to win.
• If a voter likes multiple candidates, they have no way to express that. They may be faced with a dilemma of voting for the candidate they most prefer or supporting a more electable compromise.

Majoritarianism

Suppose that there are several candidates and 100 voters. Fifty-one voters prefer one of them, who we'll call Alex, to all others. Assuming that they all vote for Alex, there is nothing that the other 49 voters can do to force the election of anyone else. If they all vote for one candidate, that candidate will have 49 votes - 2 less than Alex. And if they split up, none of the other candididates will have even that many as a voter can only vote for one candidate.

More generally, whenever one candidate is a majority's most preferred candidate and that majority votes sincerely, that candidate will have a majority of the vote and the remaining candidates split up a minority of the vote. Thus, the majority can enforce their wishes simply by voting sincerely. No other group can succeed by gaming the system.

However, there are more general senses of a majority preference. First-past-the-post does not always reflect these. Suppose that in an election, voters have the following preferences:

Assuming everyone votes sincerely, Christina wins with 36 votes. However, 64 voters prefer both Alex and Bob to Christina: if only one of them had run, they'd win by a landslide. However, because both did, they split the vote and allowed Christina to run up the middle. Unless the majority coordinates on one of them, neither will win.

Tactical voting

Under first-past-the-post, the only effect of voting for someone is to increase the number of votes that that candidate has. If the candidate was already winning or was well behind, the vote has no effect on the outcome. However, if two candidates are close and you vote for one of them, then you vote may prove decisive. Thus, voting for someone cannot cause a less-preferred outcome than not voting at all, but voting for one of the front-runners is generally far more effective than voting for a less popular candidate. This is called a compromising tactic. No electoral system is immune to tactical voting, but first-past-the-post is one of the more vulnerable methods because it is very easy to waste your vote on an unpopular candidate.

In the following table, even if everyone who likes Christina voted for her, she'd be nowhere near winning. There are three groups of voters who support her. The first are utterly indifferent between the others. Thus, they have no reason to make one of the other candidates beat the other, so may as well vote for Christina. The second are indifferent between Christina and Alex, but detest Bob. As the can't vote for both, they would be best off voting the one that is more viable, who is Alex in this case. The third prefers Christina to Bob and Bob to Alex. These voters face a dilemma: do they support their favourite, or do they compromise? If they vote sincerely and the group indifferent between Christina and Alex vote sensibly, then Alex wins with 47 votes to Bob's 43. However, if they compromise for Bob, then Bob wins with 48 votes for Alex's 47.

Additionally, those who genuninely prefer Alex or Bob to everyone else has no reason to vote for anyone else, as their favourite candidate is already viable.

For another example, suppose that a reliable poll series showed three candidates hovering around a third of the vote. In this case, each candidate has a roughly equal chance of winning, so no voter has any real incentive to vote for anybody other than their favourite. Of course, if two of the candidates are ideologically close, then it may make sense for one of them to not run and instead encourage their supporters to instead vote for other. Some supporters of the withdrawn candidate might get discouraged from voting and others may actually prefer the more distant rival. However, the pre-electoral coaltion may increase the remaining candidate's chances significantly. Thus, first-past-the-post can lead to a two party system.

However, in countries such as Canada, the United Kingdom and India, third parties have long had a presence in the political system and often get millions of votes. This suggests that not every voter votes tactically: some will vote for their favourite no matter what. It also shows that the method only tends to weaken third-parties: it does not eliminate them.

Summary

First-past-the-post has votes vote for one and only one of possibly several candidates. When one candidate is a majority's favourite, when all the alterntives are similarly popular or when voters only have one candidate that they like and are indifferent between the rest, this system works well. In other cases, however, it can behave poorly unless voters and parties are willing to identify and support a compromise, rather than just their favourite.