Clinical lycanthropy is a mental disorder where a person believes that they sometimes transform into a wolf or other animal. In some cases the term is applied to patients who believe they are sometimes taken over by a non-animal spirit, such as a demon.
Often, in cases of clinical lycanthropy, a patient has strong repressed desires and emotions, related to sexuality, violence, hedonistic urges, or something else which they cannot express in their "normal" life. As a result, the "werewolf" or "demon" self emerges to express these desires that the "normal" self cannot or will not. There may even be visual or auditory hallucinations.
Some clinical lycnathropes claim that they have raped or murdered people or animals in their "werewolf state", even if they've actually done nothing at all.
Clinical lycanthropes studied in modern times often were schizophrenics, whose symptoms manifested partly as a belief that they transformed into something else.
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Another related disease, hemolytic porphyria may have led to some accusations of lycanthropy in past ages. This particular disease caused extreme photosensitivity, deformations of the face, teeth, ears and other body parts, and was sometimes accompanied by mental illness in the sick person. Supposedly, it could cause cravings for red meat and even blood, as the person's body had trouble getting enough of certain nutrients. Since sunlight could cause terrible burns or scarring, victims may have tended to go out at night, thus feeding accusations and suspicions.