Capgras delusion is the phenomenon in which a person believes that a close friend or family member has been replaced by an identical looking impostor. This could be tied in to the old belief that babies were stolen and replaced by changelings in medieval folklore, as well as the modern idea of aliens taking over the bodies of people on earth to live amongst us for reasons unknown. This delusion is most common in people with schizophrenia but it can occur in other disorders.
Fregoli delusion is a rare brain phenomenon in which a person holds the belief that different people are, in fact, the same person in a variety of disguises. It is often associated with paranoia and the belief that the person in disguise is trying to persecute them. The condition is named after the Italian actor Leopoldo Fregoli who was renowned for his ability to make quick changes of appearance during his stage act. It was first reported in 1927 in the case study of a 27-year-old woman who believed she was being persecuted by two actors whom she often went to see at the theatre. She believed that these people “pursued her closely, taking the form of people she knows or meets”.