"In clinical psychology, folie á deux ("madness of two") refers to a mental illness shared by two or more people with close emotional ties to each other. An example would be a cult leader who believes the world is coming to an end and who convinces his followers to commit mass suicide together before it happens. The primary illness is in the leader; in the others, the delusional system is "induced" and would not have happened without the leader's influence. The phenomenon is also known as Induced Delusional Disorder."

From: http://www.familycourtchronicles.com...hy/folieadeux/