  The standard problem setting in Dec-POMDPs is self-play, where the goal is to
find a set of policies that play optimally together. Policies learned through
self-play may adopt arbitrary conventions and implicitly rely on multi-step
reasoning based on fragile assumptions about other agents' actions and thus
fail when paired with humans or independently trained agents at test time. To
address this, we present off-belief learning (OBL). At each timestep OBL agents
follow a policy $\pi_1$ that is optimized assuming past actions were taken by a
given, fixed policy ($\pi_0$), but assuming that future actions will be taken
by $\pi_1$. When $\pi_0$ is uniform random, OBL converges to an optimal policy
that does not rely on inferences based on other agents' behavior (an optimal
grounded policy). OBL can be iterated in a hierarchy, where the optimal policy
from one level becomes the input to the next, thereby introducing multi-level
cognitive reasoning in a controlled manner. Unlike existing approaches, which
may converge to any equilibrium policy, OBL converges to a unique policy,
making it suitable for zero-shot coordination (ZSC). OBL can be scaled to
high-dimensional settings with a fictitious transition mechanism and shows
strong performance in both a toy-setting and the benchmark human-AI & ZSC
problem Hanabi.
