In general, the number of disorders has increased (to the current 256 classification codes), partly because disorder categories have been split and partly because 'new' disorders have appeared.

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The publication of the Third Edition of the Diagnostic and Statistical Manual for Mental Disorders (DSM-III) in 1980 (when conditions such as ADHD made their debut on its pages) substantially altered the way that we view many problems. The precise diagnostic criteria presented in the manual allowed for new types of research on the causes and consequences of mental disorders.

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