A strand of mechanistic approaches to deception in large language models (LLMs) relies on "lie detectors" - truth probes aimed to identify model outputs that are internally represented as false. This lie detector approach to LLM deception implicitly assumes that deception is coextensive with lying. This paper challenges that assumption by experimentally investigating whether LLMs can produce deceptive statements that are not false and whether truth probes (if naively applied as deception detectors) fail to detect such behavior. In doing so, we make four core contributions: First, we provide a dataset of manually written examples of deceptive non-falsities grounded in linguistic theory. Second, we show across six open-source LLMs of different sizes and families that some models can select these deceptive non-falsities in a forced-choice task. Third, we demonstrate that while truth probes trained on a standard true-false dataset perform well at detecting false outputs, they perform significantly worse at detecting deceptive non-falsities. We view this as substantiating the worry that naive applications of truth probes as deception detectors suffer from a critical blind spot. Fourth, we explore whether training deception probes in dialogical contexts or including misleading non-false statements in the training improves the deception detection rate.

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