Clinical Lycanthropy is a mental illness (specifically, a psychiatric syndrome) that involves a strong delusional belief in the afflicted person that they have transformed into an animal, often a wolf. The syndrome is diagnosed by either a lucid patient indicating that he or she has at one time transformed into an animal, or by observing certain animal behaviors in the patient that would indicate a belief that he or she is currently transformed into some animal. The belief is caused by a form of psychosis or dementia, and causes the person to whole-heartedly believe that they have become a certain animal.
The only observable change will be in their behavior – they will indeed act like whatever animal they believe to have been turned into. Clinical Lycanthropy differs from the mythology of werewolves in that the afflicted person does not actually appear to have transformed into anything physically. They may only act differently, expressing animal behaviors such as how an animal moves or makes noise. The syndrome is so rare that very few modern examples are available for study (under 40 cases in record), and so the causes of the condition are not very well understood at this point in time.
The term lycanthropy is derived from Greek mythology and the story of King Lycaon, who supposedly fed human flesh to the god Zeus and was then punished by being turned into a wolf. Werewolf myths have persisted through our history, probably in most part due to cases of lycanthropy. On the other hand, there may also be a cultural basis for the wolf being the most frequent form of animal delusion. It appears that lycanthropy feeds the werewolf myth while the werewolf myth has a hand in determining exactly what animal form a person’s delusion will end up taking.