  Inverse reinforcement learning (IRL) aims to infer an agent's preferences
(represented as a reward function $R$) from their behaviour (represented as a
policy $\pi$). To do this, we need a behavioural model of how $\pi$ relates to
$R$. In the current literature, the most common behavioural models are
optimality, Boltzmann-rationality, and causal entropy maximisation. However,
the true relationship between a human's preferences and their behaviour is much
more complex than any of these behavioural models. This means that the
behavioural models are misspecified, which raises the concern that they may
lead to systematic errors if applied to real data. In this paper, we analyse
how sensitive the IRL problem is to misspecification of the behavioural model.
Specifically, we provide necessary and sufficient conditions that completely
characterise how the observed data may differ from the assumed behavioural
model without incurring an error above a given threshold. In addition to this,
we also characterise the conditions under which a behavioural model is robust
to small perturbations of the observed policy, and we analyse how robust many
behavioural models are to misspecification of their parameter values (such as
e.g.\ the discount rate). Our analysis suggests that the IRL problem is highly
sensitive to misspecification, in the sense that very mild misspecification can
lead to very large errors in the inferred reward function.
