According to a classic position in analytic philosophy of mind, we must interpret agents as largely rational in order to be able to attribute intentional mental states to them. However, adopting this position requires clarifying in what way and by which criteria agents can still be irrational. In this paper I will offer one such criterion. More specifically, I argue that the kind of rationality methodologically required by intentional interpretation is to be specified in terms of psychological efficacy. Thereby, this notion can be distinguished from a more commonly used notion of rationality and hence cannot be shown to be undermined by the potential prevalence of a corresponding kind of irrationality.