Conversion syndrome (a later term for hysteria) is a diagnosis popular in the late 19th century in which symptoms which cannot be explained medically by physical findings are attributed to psychological conflict (the conversion alluded to is the conversion of a psychological conflict into a bodily disturbance). Those who studied it included Pierre Janet , J. M. Charcot, Josef Breuer, and Sigmund Freud. It was primarily diagnosed in women: this sexual disparity is usually explained today sociologically, as is the drastically reduced incidence of the syndrome. It is still occasionally diagnosed today, though generally the term is used without reference to any specific underlying mechanism.

Freud theorized that conversion syndrome stems from an internalised emotion of anger, disgust or unresolved conflict. Freud claimed that free association through hypnosis resolved difficulties arising from conversion syndrome by identifying the key reasons for emotional conflict in the sufferer.