Mental state is generally a literary or legal term, and is only used in psychiatry or psychology as the mental state examination, where it refers to the condition of someone's mind. Here there is an assessment of thought processes, memory, mood, cognitive state, and energy level. Where a mental state is shared by a large proportion of the members of a group or society, it can be called a collective mental state. Gustave Le Bon proposed that mental states are passed by contagion, while Sigmund Freud wrote of war fever in his work Group Psychology and the Analysis of the Ego (1922), a perfect example of the collective mental state. Franz Borkenau wrote of collective madness, while many writers have discussed collective depression. Psychosis can be passed from one individual to another as induced psychosis or folie a deux, but rarely involves more than two people. Where the mental state involves a large population, it is more appropriate to use plain English rather than psychiatric or psychological terminology.