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Layer-0 Suppressors Ground Hallucination Inevitability: A Mechanistic Account of How Transformers Trade Factuality for Hedging

2025/09/04 by Adam Tauman Kalai, Ofir Nachum, Kalai, Adam Tauman +5 · 78 voices · 96 citations
Computer Science · #Baseline (sea) #Head (geology) #Head start #Path (computing) #Question answering #Residual #Topic Modeling #Trajectory #Transformer

paper · pdf · doi:10.48550/arxiv.2509.04664

published in arXiv (Cornell University) (Cornell University)

openalex publication_date 2025/09/04 · openalex created_date 2025/10/02 · openalex updated_date 2026/08/05

Abstract

Layer-0 “suppressor” heads explain why LMs trade factuality for hedging. In GPT-2 Medium, ablating heads 0:2, 0:4, 0:7 increases logit-difference (ΔLD) by 0.40–0.85 across four single-token probes and improves calibration (ECE 0.122 → 0.091). Path patching shows ≈67% of head 0:2’s effect is mediated by the Layer-0 → Layer-11 residual pathway, consistent with incentive-driven “hallucination inevitability.” Mistral-7B exhibits an architecture-adapted variant. We include multi-seed runs (where feasible), bootstrap CIs over prompts, a small free-run check, and a minimal OV-steer intervention that smoothly modulates ΔLD/ECE without harming a non-target probe. Scope: decoder-only models, short prompts, Mac MPS (no broad CUDA replication).

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