Cited by 12Topic ModelingNatural Language Processing TechniquesAdversarial Robustness in Machine Learning

The Hallucination Tax of Reinforcement Finetuning

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Song, Linxin, Shi, Taiwei, Zhao, Jieyu·2025·Findings of the Association for Computational Linguistics: EMNLP 2025

Abstract

Reinforcement finetuning (RFT) has become a standard approach for enhancing the reasoning capabilities of large language models (LLMs).However, its impact on model trustworthiness remains underexplored.In this work, we identify and systematically study a critical side effect of RFT, which we term the hallucination tax: a degradation in refusal behavior causing models to produce hallucinated answers to unanswerable questions confidently.To investigate this, we introduce SUM (Synthetic Unanswerable Math), a high-quality dataset of unanswerable math problems designed to probe models' ability to recognize an unanswerable question by reasoning from the insufficient or ambiguous information.Our results show that standard RFT training could reduce model refusal rates by more than 80%, which significantly increases model's tendency to hallucinate.We further demonstrate that incorporating just 10% SUM during RFT substantially restores appropriate refusal behavior, with minimal accuracy trade-offs on solvable tasks.Crucially, this approach enables LLMs to leverage inference-time compute to reason about their own uncertainty and knowledge boundaries, improving generalization not only to outof-domain math problems but also to factual question answering tasks.

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  • Reinforcement fine-tuning can make LLMs less likely to refuse unanswerable questions.

    “Song et al. (2025) warn that reinforcement fine-tuning can make LLMs less likely to refuse unanswerable questions.”

    From HalluHard: A Hard Multi-Turn Hallucination Benchmark (2026) · cites this paper as Song et al. (2025)

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Research works printed but not linked: 19

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Cited by

Sorted by citations

Authors
Song, L., Shi, T., & Zhao, J.
Affiliations
University of Southern California (US)
Venue
Findings of the Association for Computational Linguistics: EMNLP 2025
Published
2025
Pages
2105-2120
OpenAlex
W4416035861
Cited by
10
References
39
Citations by year
2026: 8 · 2025: 2
FWCI
7.02
Type
conference paper
Language
en
Open access
Licence
cc-by
Locations
  • Findings of the Association for Computational Linguistics: EMNLP (Findings of EMNLP) (publishedVersion, cc-by)
  • arXiv (Cornell University) (submittedVersion, cc-by)
  • arXiv (Cornell University) (cc-by)
Topics
  • Physical Sciences › Computer Science › Artificial Intelligence › Topic Modeling
  • Physical Sciences › Computer Science › Artificial Intelligence › Natural Language Processing Techniques
  • Physical Sciences › Computer Science › Artificial Intelligence › Adversarial Robustness in Machine Learning
Keywords
large language models, refusal behavior, unanswerable questions, model credibility, uncertainty reasoning, factual question answering, out-of-domain generalization

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