  Preference based Reinforcement Learning (PbRL) removes the need to hand
specify a reward function by learning a reward from preference feedback over
policy behaviors. Current approaches to PbRL do not address the credit
assignment problem inherent in determining which parts of a behavior most
contributed to a preference, which result in data intensive approaches and
subpar reward functions. We address such limitations by introducing a credit
assignment strategy (Hindsight PRIOR) that uses a world model to approximate
state importance within a trajectory and then guides rewards to be proportional
to state importance through an auxiliary predicted return redistribution
objective. Incorporating state importance into reward learning improves the
speed of policy learning, overall policy performance, and reward recovery on
both locomotion and manipulation tasks. For example, Hindsight PRIOR recovers
on average significantly (p<0.05) more reward on MetaWorld (20%) and DMC (15%).
The performance gains and our ablations demonstrate the benefits even a simple
credit assignment strategy can have on reward learning and that state
importance in forward dynamics prediction is a strong proxy for a state's
contribution to a preference decision. Code repository can be found at
https://github.com/apple/ml-rlhf-hindsight-prior.
