ToM-RL: Reinforcement Learning Unlocks Theory of Mind in Small LLMs

Recent advancements in rule-based reinforcement learning (RL), applied during the post-training phase of large language models (LLMs), have significantly enhanced their capabilities in structured reasoning tasks such as mathematics and logical inference. However, the effectiveness of RL in social reasoning, particularly in Theory of Mind (ToM), the ability to infer others' mental states, remains largely unexplored. In this study, we demonstrate that RL methods effectively unlock ToM reasoning capabilities even in small-scale LLMs (0.5B to 7B parameters). Using a modest dataset comprising 3200 questions across diverse scenarios, our RL-trained 7B model achieves 84.50\% accuracy on the Hi-ToM benchmark, surpassing models like GPT-4o and DeepSeek-v3 despite significantly fewer parameters. While smaller models ($\leq$3B parameters) suffer from reasoning collapse, larger models (7B parameters) maintain stable performance through consistent belief tracking. Additionally, our RL-based models demonstrate robust generalization to higher-order, out-of-distribution ToM problems, novel textual presentations, and previously unseen datasets. These findings highlight RL's potential to enhance social cognitive reasoning, bridging the gap between structured problem-solving and nuanced social inference in LLMs.