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We all know the situation could arise in the U.S. where one candidate wins the popular vote but another one the electoral college. Given that the same could arise in the United Kingdom and other countries with a first-past-the-post or "winner takes it all" electoral system, what are the genuine disadvantages of first-past-the-post electoral systems? Besides the given examples?

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    Is it fair to inquire about the disadvantages without taking into account the advantages? – corsiKa Dec 4 '12 at 22:00
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    I could have reformulated the question, but at least for me the disadvantages were more important as I did already look up the advantages (the very few that exist) – Sven Clement Dec 4 '12 at 22:02
  • You may find this cat's explanation interesting: youtube.com/watch?v=HiHuiDD_oTk – Peter David Carter May 19 '16 at 12:09
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Simple plurality voting has very little in its favor in any election with more than two candidates, but the top disadvantage is vote splitting. FPTP only allows voters to vote for a single person, and since the vote can't be transferred, if that person doesn't win that means the voter might as well have not voted at all.

Imagine a situation like the following:

  • Person A is liked by 60% of the electorate
  • Person B is liked by the same 60% of the electorate
  • Person C is liked by the remaining 40% of the electorate

Assume that the 60% don't like C, and the 40% don't like A and B (it doesn't really matter either way, since FPTP doesn't care who you like after #1). It's unclear if A or B should win, but clearly C shouldn't -- a majority of people don't like him. However, if half the 60% vote for person A and the other half vote for person B, the results are:

  • Person A gets 30% of the vote
  • Person B gets 30% of the vote
  • Person C gets 40% of the vote and wins

Because person A and person B were so similar, they split the vote and neither got enough to win; they would've been better served if one of them had dropped out.

Very similar to (but probably more ridiculous than) vote splitting is the spoiler effect. This happens when a similar minor candidate gets very little of the vote, but it's enough to tip the election the other way. For example:

  • Person A gets 47% of the vote
  • Person B gets 4% of the vote entirely from voters who would've voted for person A
  • Person C gets 49% of the vote

Person C wins by 2 points, but it's person B's fault -- if he hadn't run, person A would've won. Worst of all, everyone who voted for person B is kicking themselves because the person they liked least is now the winner. One of the desirable qualities of a voting system (according to Kenneth Arrow) is independence of irrelevant alternatives -- the introduction of a new candidate shouldn't change the winner unless it's to that candidate. Plurality voting is particularly bad at this.

The result of all this is that FPTP bodies tend towards dual party systems after a while; the similar candidates fight it out in primaries so that in the real election, there's only two major choices and the effect of vote splitting is minimized. Nonetheless, spoilers can still cause problems. FPTP doesn't gather sufficient information to be certain when spoilers have changed a result, but in Gaming the Vote Poundstone estimates that at least five U.S. presidential elections have been decided by spoilers

  • And of course the American primaries still use a FPTP system, too, so these effects are apparent there. One could argue that the reason Trump won the Republican nomination in 2016 is because there were so many other candidates splitting the votes amongst themselves for so long that Trump established dominance with just the remaining minority; though one could argue against it as well, as there's not much data on who, say, Rubio supporters would have gravitated towards if he'd dropped out earlier. One might wonder if a similar effect will happen with the Democrats now. – zibadawa timmy Jun 29 at 18:31
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First-past-the-post voting tends to result in a smaller number of viable candidates. In the traditional two-party system of Anglosphere politics, every candidate except those from the top two parties is effectively a spoiler; they weaken the position of whichever major party candidate is more ideologically similar to them, giving the other major party candidate an advantage. As a result, it is in people's interest to vote for a major party candidate even if they are more aligned with another candidate. This lessens voters' ability to support their preferred policies through voting.

A great explanation of this is here.

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    The primary system used by America makes the polarization much worse because primary voters - who are self-selecting members of the party - are on one end of the political spectrum and do not vote strategically. In a system where party bosses make the decision they have a strong motivation to choose a centrist candidate who can appeal to swing voters. See the Median Voter Theorem – Readin Dec 12 '17 at 4:26
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First-past-the-post systems are polarizing. Because of the advantages of tactical voting, most people concentrate on a popular candidate who is good enough. So parties build themselves around such candidates. Then the parties tend to be left in control of the nominating process that winnows down to two candidates per post.

In the USA, this results in a two-party system driven by the presidential candidates. Tactical voting dictates that there be only two presidential candidates, so there are two parties. And the candidates for other races tend to either be for one candidate or the other, so they join the two parties as well.

The net result is that the US is divided between two polarizing parties. Republicans generally need to be business-friendly, low-tax, pro-gun, pro-life, pro-military, tough on crime, etc. Democrats are pro-union, environmentalists, anti-gun, pro-choice, for social services, for criminal defendants, etc. Occasionally candidates realign these divisions. For example, prior to 1980, the Democrats were the pro-life party and the Republicans were the pro-choice party. The pro-life Reagan changed that.

This is less of a problem in parliamentary countries like the UK. There still tend to only be one or two viable candidates per seat, but it doesn't matter as much as other seats can have candidates from different parties. Since they don't have a direct election for a national office, they aren't as limited to two parties nationally.

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