In Chapters 1–3, I explored the differing origins of delusional beliefs with the express purpose of arguing that the underlying mechanisms of delusional beliefs are what distinguishes different sets of delusions from one another. This notion is a departure from the now well-established demarcation of delusions into primary and secondary delusions and overvalued ideas. It emphasizes underlying mechanisms, pointing to the role of impairment of face processing, for example, in delusional misidentification syndromes and the place of the evolutionary biology of mate selection and the differential energetics of mating strategies in morbid jealousy. Finally, I argued that in folie a deux, the social psychology of persuasive communication and the normative mechanisms for transferring beliefs between people and in social groups are a prerequisite for understanding its origins.