Why Language Models Hallucinate
Authors: name: Adam Tauman Kalai; name: Ofir Nachum; name: Santosh S. Vempala; name: Edwin Zhang
Published: 2025-09-04 21:26:31

Abstract: Like students facing hard exam questions, large language models sometimes
guess when uncertain, producing plausible yet incorrect statements instead of
admitting uncertainty. Such "hallucinations" persist even in state-of-the-art
systems and undermine trust. We argue that language models hallucinate because
the training and evaluation procedures reward guessing over acknowledging
uncertainty, and we analyze the statistical causes of hallucinations in the
modern training pipeline. Hallucinations need not be mysterious -- they
originate simply as errors in binary classification. If incorrect statements
cannot be distinguished from facts, then hallucinations in pretrained language
models will arise through natural statistical pressures. We then argue that
hallucinations persist due to the way most evaluations are graded -- language
models are optimized to be good test-takers, and guessing when uncertain
improves test performance. This "epidemic" of penalizing uncertain responses
can only be addressed through a socio-technical mitigation: modifying the
scoring of existing benchmarks that are misaligned but dominate leaderboards,
rather than introducing additional hallucination evaluations. This change may
steer the field toward more trustworthy AI systems.