Folie à deux homicide and the two-factor model of delusions

Introduction. Cases of folie à deux resulting in homicide challenge traditional definitions of delusions. Secondaries who abandon their beliefs soon after separation from a primary raise doubts about the status of their delusional beliefs at the time of the offence. In this paper, we apply the ''two-factor'' model of delusions to a series of cases of folie à deux resulting in homicide. Method. A retrospective analysis of five cases. Results. The primary affected person appeared to be the source of the delusional content in each of the cases. Impairment of belief evaluation was evident to some degree in all patients. There appeared to be a range of underlying causes of impaired belief evaluation. The transitory nature of the reported beliefs in some secondary cases suggested that they were due to motivational rather than neuropathological mechanisms. Social isolation contributed to the inability to critically appraise beliefs in most cases. Conclusion. The two-factor model is a useful method to contrast the emergence of a shared delusional belief in primary and secondary patients with folie à deux. Folie à deux demonstrates the need to consider the exogenous source of delusional content in many patients.