Title: The Hidden Link Between RLHF and Contrastive Learning
Primary area: alignment, fairness, safety, privacy, and societal considerations
Keywords: RLHF; Contrastive Learning; Mutual Information

Abstract: Alignment of large language models (LLMs) with human values has recently
garnered significant attention, with prominent examples including the canonical
yet costly Reinforcement Learning from Human Feedback (RLHF) and the simple
Direct Preference Optimization (DPO). In this work, we demonstrate that both
RLHF and DPO can be interpreted from the perspective of mutual information
(MI) maximization, uncovering a profound connection to contrastive learning.
Within this framework, both RLHF and DPO can be interpreted as methods that
performing contrastive learning based on the positive and negative samples derived
from base model, leveraging the Donsker–Varadhan (DV) lower bound on MI
(equivalently, the MINE estimator). Such paradigm further illuminates why RLHF
may not intrinsically incentivize reasoning capacities in LLMs beyond what is
already present in the base model. Building on the perspective, we replace the
DV/MINE bound with the Jensen–Shannon (JS) MI estimator and propose the
Mutual Information Optimization (MIO). Comprehensive theoretical analysis and
extensive empirical evaluations demonstrate that MIO mitigates the late-stage
decline in chosen-likelihood observed in DPO, achieving competitive or superior
performance across various challenging reasoning and mathematical benchmarks

TL;DR: This paper reveals the link between RLHF and contrastive learning, explains why RL fails when the policy model assigns very low probability, and introduces a new algorithm MIO.