What does the Mental Health Act mean by “mental disorder”?

The legal definition of “mental disorder” for the purposes of the Mental Health Act has evolved over the years to reflect changes in society and the liberalisation of psychiatry.

The Mental Health Act 1959 was the first “modern” Act designed to deal with people with mental disorders. It replaced frankly archaic Acts such as the Lunacy Act 1890 and the Mental Deficiency Acts of 1913 and 1938. This was replaced in turn by the Mental Health Act 1983, which was further amended by the Mental Health Act 2007, which radically modified the definition of “mental disorder”.

This is an overview of those changes.