SV (Supplementary Vote)

Single-winner ballot, simplified AV. Voters mark a first and second choice only. If no candidate has >50% of first choices, all but the top two are eliminated and their votes transferred (only if the second choice was one of the top two).

Used for directly-elected mayors in England (London, and other city mayors) from their creation until 2022, when the Elections Act 2022 reverted mayoral (and Police & Crime Commissioner) elections to FPTP. Labour has signalled it may reintroduce SV or a similar system.

Disadvanages of SV

Only two preferences

  • Unlike full AV, a vote for a third-placed candidate is wasted if that candidate isn’t in the final two
    • narrower “genuine choice” than a full ranking

Can still elect a non-majority winner

  • If many second preferences are exhausted (voter’s second choice wasn’t in the top two), the winner may still have less than 50% of the original total vote
    • e.g. London Mayor 2016: Sadiq Khan won with 56.9% of the final round but only ~44% of first preferences overall (once redistributed)

Complexity vs. simplicity trade-off

  • More complex than FPTP for voters and counters
  • But less rigorous than full AV/STV, so criticised from both directions
  • Reverted to FPTP in 2022 partly on “simplicity and cost” grounds

Reduced choice of representative

  • Single-winner, so no equivalent to STV’s “approach any of your reps” benefit
  • All the FPTP constituency-link caveats apply, just for a bigger office

Advantages of SV

Simpler than full AV

  • Only two rankings to complete, quicker to count than exhaustive preferential systems like STV
  • Retains most of AV’s “avoids a widely-disliked winner” benefit for the common two-horse-race case

Majority-ish legitimacy

  • Winner typically has a majority of the final two-candidate contest, stronger mandate than a FPTP plurality winner
    • relevant for a high-profile single office like a mayor

Reduces tactical voting for high-profile races

  • Voters can back a smaller-party favourite as first choice and a likely-winner as second, without feeling their vote is wasted

Suited to single, high-profile offices

  • Designed specifically for executive mayoral-style elections rather than legislatures, where a single accountable figurehead matters more than proportional representation of parties
Links to this page