Rethinking Information Structures in RLHF: Reward Generalization from a Graph Theory Perspective
Tianyi Qiu, Fanzhi Zeng, Jiaming Ji, Dong Yan, Kaile Wang, Jiayi Zhou, Yang Han, Josef Dai, Xuehai Pan, Yaodong Yang
There is a trilemma in reinforcement learning from human feedback (RLHF): the incompatibility between highly diverse contexts, low labeling cost, and reliable alignment performance. We mitigate such incompatibility through the design of dataset information structures during reward modeling, and introduce the Induced Bayesian Network (IBN), the first theory of reward generalization capable of generating substantial verified predictions on large language models (LLMs). Specifically, we first reexamine the RLHF process and propose a theoretical framework portraying it as an autoencoding process over text distributions. Our framework formalizes the RLHF objective of ensuring distributional consistency between human preference and LLM behavior. Then, based on this framework, we introduce the IBN to analyze generalization in the reward modeling stage of RLHF. Drawing from random graph theory and causal analysis, it enables empirically grounded derivation of generalization error bounds, a key improvement over classical theories of generalization. Finally, an insight from our analysis is the superiority of the tree-based information structure in reward modeling, compared to chain-based baselines in conventional RLHF methods. With IBN, we derive that in complex contexts with limited data, the tree-based reward model (RM), trained on a tree-structured preference dataset, induces up to $\Theta(\log n/\log\log n)$ times less variance than the baseline, where $n$ is the dataset size. As validation, we demonstrate that on three NLP tasks, the tree-based RM achieves 65% win rate on average against chain-based baselines. It shows that alignment performance can be gained for free via the design of dataset information structure, without the need for any other changes.