"Folie à deux" is characterized by the communication of delusional ideas from one subject to other ones who have been living closely with him for a long time, usually his relatives. Before illness shows, there is a leader-follower relationship between partners, who are lacking of external sources of pleasure and are socially or culturally isolated. The dominating partner usually is a paranoic subject who strongly needs his ideas to be accepted by other people, while the dominated partner is a dependent subject who shares the other one's ideas in order to get pleasure from him and who is not able to criticize these ideas because of the lack for external influences. The two partners project hostility into the external world because, if the dominated partner does not share the other one's ideas, hostility will be projected into him and he will become very anxious. Interpersonal mechanisms in "folie à deux" are similar to those which take place in brainwashing, hypnosis and psychotherapy.