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Does Fine-Tuning LLMs on New Knowledge Encourage Hallucinations?

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arxiv 2405.05904 v3 pith:QQJX4ADB submitted 2024-05-09 cs.CL

Does Fine-Tuning LLMs on New Knowledge Encourage Hallucinations?

classification cs.CL
keywords knowledgefine-tuningmodelfactualexampleslanguagelargemodels
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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When large language models are aligned via supervised fine-tuning, they may encounter new factual information that was not acquired through pre-training. It is often conjectured that this can teach the model the behavior of hallucinating factually incorrect responses, as the model is trained to generate facts that are not grounded in its pre-existing knowledge. In this work, we study the impact of such exposure to new knowledge on the capability of the fine-tuned model to utilize its pre-existing knowledge. To this end, we design a controlled setup, focused on closed-book QA, where we vary the proportion of the fine-tuning examples that introduce new knowledge. We demonstrate that large language models struggle to acquire new factual knowledge through fine-tuning, as fine-tuning examples that introduce new knowledge are learned significantly slower than those consistent with the model's knowledge. However, we also find that as the examples with new knowledge are eventually learned, they linearly increase the model's tendency to hallucinate. Taken together, our results highlight the risk in introducing new factual knowledge through fine-tuning, and support the view that large language models mostly acquire factual knowledge through pre-training, whereas fine-tuning teaches them to use it more efficiently.

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Forward citations

Cited by 12 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

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    cs.LG 2026-06 conditional novelty 6.0

    Hallucination in LLMs is driven by an oracle-invisible “decoding risk” term that grows with scale and causally compounds errors within a response.

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    EHRAG constructs structural hyperedges from sentence co-occurrence and semantic hyperedges from entity embedding clusters, then applies hybrid diffusion plus topic-aware PPR to retrieve top-k documents, outperforming ...

  3. Why Fine-Tuning Encourages Hallucinations and How to Fix It

    cs.CL 2026-04 unverdicted novelty 6.0

    Supervised fine-tuning increases LLM hallucinations via interference among overlapping semantic representations; self-distillation mitigates this by regularizing output-distribution drift while freezing parameters pre...

  4. Understanding New-Knowledge-Induced Factual Hallucinations in LLMs: Analysis and Interpretation

    cs.CL 2025-11 unverdicted novelty 6.0

    Fine-tuning on new knowledge induces propagating hallucinations in LLMs by weakening attention to key entities, with mitigation via reintroducing known knowledge during later training stages.

  5. Data Quality over Capacity: Internalizing Documents into LoRA Adapters for Closed-Book QA

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    For closed-book QA via LoRA adapters, once capacity is sufficient, training-data quality—especially answer brevity—outperforms rank, learning rate, and architecture changes, lifting 15-doc accuracy from 57.7% to 85.7%.

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  9. Reinforcement Learning Improves Traversal of Parametric Knowledge in LLMs

    cs.CL 2025-11 reject novelty 5.0

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  10. A Survey on Hallucination in Large Language Models: Principles, Taxonomy, Challenges, and Open Questions

    cs.CL 2023-11 unverdicted novelty 5.0

    The paper surveys hallucination in LLMs with an innovative taxonomy, factors, detection methods, benchmarks, mitigation strategies, and open research directions.

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    cs.NE 2026-05 unverdicted novelty 3.0

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