In folie à deux, a ‘primary’ patient transmits a delusional belief to one or more ‘secondary’ patients who then adopt and share the belief. This paper applies the two-factor theory of delusion to retrospectively analyse published cases of folie à deux. Lessons from this retrospective analysis include, firstly, that two-factor theorists need to shift their focus from endogenous processes to consider the exogenous source of delusional content in most secondaries. Secondly, secondaries who come to share the belief via normal processes of social contagion only qualify as delusional by virtue of the abnormal persistence of their belief, albeit a temporary persistence in those secondaries who abandon their belief soon after separation from the primary. Underpinning this abnormal persistence may be a form of inhibitory failure—a difficulty with inhibiting a belief to allow reasoning for and against it being true.