Title: Is Optimal Transport Necessary for Inverse Reinforcement Learning?
Primary area: reinforcement learning
Keywords: reinforcement learning, reward assignment, inverse rl, optimal transport

Abstract: Inverse Reinforcement Learning (IRL) aims to recover a reward function from expert demonstrations. Recently,  Optimal Transport (OT) methods have been successfully deployed to align trajectories and infer rewards. While OT-based methods have shown strong empirical results, they are conceptually complicated, requiring the solution of the OT optimization problem. In this work, we challenge the necessity of OT in IRL by proposing two simple, heuristic alternatives: (1) Minimum-Distance Reward, which assigns rewards based on the nearest expert state regardless of temporal order; and (2) Segment-Matching Reward, which incorporates lightweight temporal alignment by matching agent states to corresponding segments in the expert trajectory. These methods circumvent optimization, exhibit linear-time complexity, and are easy to implement. Through extensive evaluations across 32 online and offline benchmarks with three reinforcement learning algorithms, we show that our simple rewards match or outperform recent OT-based approaches. Our findings suggest that the core benefits of OT may arise from basic proximity alignment rather than its optimal coupling formulation, advocating for reevaluation of optimal transport in future IRL design.