MaPPO: Maximum a Posteriori Preference Optimization with Prior Knowledge
Authors: name: Guangchen Lan, name: Sipeng Zhang, name: Tianle Wang, name: Yuwei Zhang, name: Daoan Zhang, name: Xinpeng Wei, name: Xiaoman Pan, name: Hongming Zhang, name: Dong-Jun Han, name: Christopher G. Brinton

As the era of large language models (LLMs) on behalf of users unfolds,
Preference Optimization (PO) methods have become a central approach to aligning
LLMs with human preferences and improving performance. We propose Maximum a
Posteriori Preference Optimization (MaPPO), a framework for learning from
preferences that explicitly incorporates prior reward knowledge into the
optimization objective. While existing methods such as Direct Preference
Optimization (DPO) and its variants treat preference learning as a Maximum
Likelihood Estimation (MLE) problem, MaPPO extends this paradigm by integrating
prior reward estimates into a principled Maximum a Posteriori (MaP) objective.
This not only generalizes DPO and its variants, but also enhances alignment by
mitigating the oversimplified binary classification of responses. More
importantly, MaPPO introduces no additional hyperparameter, and supports
preference optimization in both offline and online settings. In addition, MaPPO
can be used as a plugin with consistent improvement on DPO variants, including
widely used SimPO, IPO, and CPO. Extensive empirical evaluations of different
model sizes and model series on three standard benchmarks, including MT-Bench,
AlpacaEval 2.0, and Arena-Hard, demonstrate consistent improvements in
alignment performance without sacrificing computational efficiency.