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A Lie Detector Test for Language Models: Reading Knowledge a Model Won't Reveal
Summary
The paper presents Probe of Internal Recognition (PIR), a reference‑free technique that reads a language model’s internal activations to detect which answer it recognizes, achieving 70‑87% balanced accuracy across eight LLMs. PIR reliably distinguishes deliberate concealment from lack of knowledge, enabling audits of sandbagging and unlearning.
- PIR reads hidden activations to identify the internally recognized multiple‑choice answer, reaching 70‑87% balanced accuracy without any reference model.
- The method remains effective under prompted deception, trained sandbagging, and external checkpoint restrictions, with recognition rates of 85‑93%.
- PIR can tell whether a model is refusing to answer or truly lacks the knowledge, supporting sandbagging audits and verification of unlearning.
- The approach extends to free‑form generation by probing token‑level internal signals, not just MCQs.
Model auditors and developers need a way to verify whether an LLM is deliberately withholding knowledge versus not possessing it.
8/10


