Folie à deux is not a common condition. The term was coined in 1877 by Lasèque and Falret1 to connote contagious insanity or the psychosis of association. It has been defined by Gralnick2 as "a psychiatric entity characterized by the transference of delusional ideas and/or behavior from one person to one or more others who have been in close association with the primarily affected patient." The first recorded case was described by Berlyn in 1819.3
Heredity has often been cited as a factor in psychiatric disease. In folie à deux, length of association between the affected patients is probably a more important factor than heredity. It could certainly account for the relative frequency with which folie à deux occurs in families just as well as a theoretical hereditary tendency. However, Craike and Slater4 have reported folie à deux in uniovular twins, sisters who had been separated