When Self-Consistency Backfires: Majority Vote Hurts the Majority of Hard Science Problems for Small LLMs

Abstract

Self-consistency via majority vote reduces per-problem accuracy on most GPQA Diamond problems for small instruction-tuned models: 56.6% of problems for Qwen2.5-7B and 65.7% for Llama-3-8B. The obvious remedy is a verifier-free confidence gate. This version reports that the most natural repair also fails, and separates three signal failures that v1 treated as one. A token-entropy gate fails for a measurement reason: averaged over a chain of some 602 tokens, the statistic is a measurement of the prose rather than of confidence in the answer. On Qwen2.5-7B-Instruct-Turbo, 198 problems at 64 samples each, a sample whose answer contradicts its own problem's plurality still emits that answer at a median margin of 20.52 nats, with 75.7% above 10 nats. Both quantities were pre-registered and tested once on 69 problems no exploratory analysis had read; both passed. The unit is the whole result: pooled across the benchmark the margin separates correct from incorrect samples by +0.0604 on the fraction above 10 nats [+0.0183, +0.1017], excluding zero; per-problem and paired it does not, at -0.0168 [-0.0527, +0.0182], crossing zero. The claim is not that token log-probabilities carry no information, but that a signal with real across-question discrimination is close to useless for the within-question decision a router faces. The plurality-agreement gate's failure remains without a mechanism, and we report it as an open problem. These new claims rest on one model: a registered second-model replication was sampled and could not be evaluated, and we report that rejection rather than the result. We separately report that on hosted serverless inference at a small budget, three reasoning-native models could not be evaluated, for three separately measured reasons; all three are downloadable, so this bounds what a metered per-token API buys rather than what is knowable.