Why do some countries end up with only two big parties?
Split 100,000 moderate voters between two parties and an 80,000-strong rival can win, so voters learn not to split.
▶ Start the storyMuch of it comes down to how votes are counted. Duverger's law holds that in political systems with single-member districts and the first-past-the-post voting system, only two powerful political parties tend to control power every election cycle. The reasoning is that citizens do not vote for small parties because they fear splitting votes away from a major party and allowing their opponents to win.
The arithmetic behind that fear comes from Maurice Duverger himself. Take an election for a single seat in which 100,000 moderate voters and 80,000 radical voters take part. If two moderate parties and one radical party run candidates and every voter votes, the radical candidate would tend to win unless one of the moderate candidates gathered fewer than 20,000 votes. Seeing the risk, moderate voters lean toward the moderate candidate they think likelier to gain more votes. Either the two moderate parties must merge, or one must fail, as voters gravitate to the two strongest parties. Duverger called this trend polarization.
Step 1: 100,000 moderates, 80,000 radicals
One seat, three candidates: two moderate, one radical
Step 2: The moderate vote splits
The radical tends to win unless one moderate gets under 20,000
Step 3: Voters gravitate to the stronger moderate
Backing the likelier moderate stops the radical
Step 4: Merge or fade
Either the two moderate parties merge or one fails
Elsewhere the picture differs. In countries with proportional representation or two-round elections, such as France, Sweden, New Zealand or Spain, there is no two-party duopoly on power, and there are usually more than two significant parties.
The cost to small parties can be steep. The Green Party of Canada received about 5% of the popular vote from 2004 to 2011 but won only one seat out of 308 in the House of Commons in that span. In the 1992 U.S. presidential election, Ross Perot's candidacy received zero electoral votes despite receiving 19% of the popular vote.
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Recap
Under most-votes-wins elections in single-seat districts, voters tend to desert small parties, so two big parties tend to remain.
💡 A trick to remember it · Two kids can share a seesaw, but a third makes someone fall off: single-seat elections tend to seat only two big parties.
Surprising fact · In Duverger's example, 100,000 moderate voters can lose to 80,000 radical ones if two moderate parties split the vote.
Sources (1)
No source, no claim. Every fact in this lesson (19 claims) cites at least one of these.