Rewarding Intellectual Humility Learning When Not To Answer In Large Language Models

TL;DR

This paper explores a reinforcement learning approach that explicitly rewards models for abstaining from answering to reduce hallucinations, demonstrating its effectiveness across multiple benchmarks and model sizes.

Contribution

It introduces a verifiable reward framework for training LLMs to abstain, improving factual accuracy and reliability in open-ended and multiple-choice tasks.

Findings

Moderate abstention rewards reduce incorrect answers.

Larger models are more robust to abstention incentives.

Supervised abstention training helps mitigate exploration limitations.

Abstract

Large Language Models (LLMs) often produce hallucinated or unverifiable content, undermining their reliability in factual domains. This work investigates Reinforcement Learning with Verifiable Rewards (RLVR) as a training paradigm that explicitly rewards abstention ("I don't know") alongside correctness to promote intellectual humility. We fine-tune and evaluate Granite-3.3-2B-Instruct and Qwen-3-4B-Instruct on the MedMCQA and Hendrycks Math benchmarks using a ternary reward structure ($-1$, r_abs, 1) under varying abstention reward structures. We further study the effect of combining RLVR with supervised fine-tuning strategies that teach abstention prior to reinforcement learning. Our results show that moderate abstention rewards (r_abs $\approx -0.25$ to 0.3) consistently reduce incorrect responses without severe accuracy degradation on multiple-choice tasks, with larger models exhibiting greater robustness to abstention incentives. On open-ended question answering, we observe limitations due to insufficient exploration, which can be partially mitigated through supervised abstention training. Overall, these findings demonstrate the feasibility and flexibility of verifiable reward design as a practical approach for hallucination mitigation in language models. Reproducible code for our abstention training framework is available here https://github.com/Mystic-Slice/rl-abstention.