Vote-Splitting / Spoiler Examples¶
Dramatic single-winner elections where Choose-One Plurality elects the wrong winner because a majority bloc splits its vote across similar candidates — and STAR elects the candidate the majority actually preferred.
→ How often does this happen in real elections? There's a published count — 11.9% of 2022 US primaries — and a careful reading of what it does and doesn't prove: How often does vote splitting actually happen?
Each file declares a blocs: group, so the engine's [Vote-splitting check] confirms the spoiler automatically. Run any of them through the STAR engine:
Why these are LH-only (house principle). The teaching contrast here is Plurality vs STAR, not BV vs LH — BetterVoting and the LH engine agree on these results, so a BV screenshot would just duplicate the LH numbers. Two-view (BV beside LH) is reserved for cases where the two diverge (the bug is the lesson — see Flat scores, ties & tie-breaking — and the BetterVoting bugs,
Runoff_07) or where reading BV's own UI is the point.
python STARVote_LH_tabulation_engine/starvote_larry_hastings.py \
split_voting/01_political_left_split.yaml
| File | Story | Choose-One winner | STAR winner |
|---|---|---|---|
01_political_left_split.yaml |
A 66% progressive coalition splits 3 ways | Conservative (34%) — the spoiler | Labour — the coalition's consensus |
02_icecream_chocolate_split.yaml |
Chocolate lovers (66%) split across 3 chocolates | Vanilla (34%) | MilkChoco |
03_lunch_veggie_vs_meat.yaml |
A 70% veggie majority splits across 3 dishes | BeefBurger (30%) | VeggieCurry |
06_sub_majority_not_spoiled.yaml |
The control case — a 41% winner that nothing spoiled | Aspen (41%) | Aspen — same winner |
In the first three the bloc is an outright majority, yet Choose-One hands the win to the lone outsider. STAR lets each voter score every candidate, so the bloc isn't split — and the automatic runoff elects the majority's compromise pick.
06 is the deliberate control: the same sub-majority arithmetic (a 41% winner, losers combining to a majority) with no spoiler underneath it — STAR, Ranked Robin and RCV-IRV all confirm the plurality winner. Keep it next to 01: identical warning signs, opposite verdicts, which is exactly why "the winner got under 50%" is a screening flag and not a finding.
Teaching tip: show the Choose-One first choices line first (the wrong winner) and let it sting, then reveal the STAR result. The
[Vote-splitting check]block states the verdict in numbers.