A mental disorder or mental illness is a psychological or behavioral pattern that occurs in an individual and is thought to cause distress or disability that is not expected as part of normal development or culture. The recognition and understanding of mental disorders has changed over time and across cultures. Definitions, assessments, and classifications of mental disorders can vary, but guideline criteria listed in the ICD, DSM and other manuals are widely accepted by mental health professionals. Categories of diagnoses in these schemes may include dissociative disorders, mood disorders, anxiety disorders, psychotic disorders, eating disorders, developmental disorders, personality disorders, ambulatory disorders and many other categories. In many cases there is no single accepted or consistent cause of mental disorders, although they are often explained in terms of a diathesis-stress model and biopsychosocial model. Mental disorders have been found to be common, with over a third of people in most countries reporting sufficient criteria at some point in their life. Services for mental disorders may be based in hospitals or in the community. Mental health professionals diagnose individuals using different methodologies, often relying on case history and interview. Psychotherapy and psychiatric medication are two major treatment options, as well as supportive interventions and self-help. Treatment may be involuntary where legislation allows. Several movements campaign for changes to services and attitudes.

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