Loose lips sink ships: Mitigating Length Bias in Reinforcement Learning from Human Feedback

Abstract: Reinforcement learning from human feedback serves as a crucial bridge, aligning LLMs with human and societal values. This alignment requires a vast corpus of human feedback to learn a reward model, which is subsequently used to finetune LLMs. However, we have identified that the reward model often finds shortcuts to bypass its intended objectives, misleadingly assuming that humans prefer longer responses. The emergence of length bias often induces the model to favor longer outputs, yet it doesn't equate to an increase in helpful information within these outputs. In this paper, we propose an innovative solution, applying the Product-of-Experts (PoE) technique to separate reward modeling from the influence of sequence length. In our framework, the main expert concentrates on understanding human intents, while the biased expert targets the identification and capture of length bias. To further enhance the learning of bias, we introduce perturbations into the bias-focused expert, disrupting the flow of semantic information. Experimental results validate the effectiveness of our approach, indicating that LLM performance is improved, irrespective of sequence length.