A mental disorder, also called a mental illness or psychiatric disorder, is a behavioral or mental pattern that causes significant distress or impairment of personal functioning. Such features may be persistent, relapsing and remitting, or occur as single episodes. Many disorders have been described, with signs and symptoms that vary widely between specific disorders. Such disorders may be diagnosed by a mental health professional, usually a clinical psychologist or psychiatrist.

The causes of mental disorders are often unclear. Theories may incorporate findings from a range of fields. Mental disorders are usually defined by a combination of how a person behaves, feels, perceives, or thinks. This may be associated with particular regions or functions of the brain, often in a social context. A mental disorder is one aspect of mental health. Cultural and religious beliefs, as well as social norms, should be taken into account when making a diagnosis.

Services are based in psychiatric hospitals or in the community, and assessments are carried out by mental health professionals such as psychiatrists, psychologists, psychiatric nurses and clinical social workers, using various methods such as psychometric tests but often relying on observation and questioning. Treatments are provided by various mental health professionals. Psychotherapy and psychiatric medication are two major treatment options. Other treatments include lifestyle changes, social interventions, peer support, and self-help. In a minority of cases, there might be involuntary detention or treatment. Prevention programs have been shown to reduce depression.

In 2019, common mental disorders around the globe include depression, which affects about 264 million, bipolar disorder, which affects about 45 million, dementia, which affects about 50 million, and schizophrenia and other psychoses, which affects about 20 million people. Neurodevelopmental disorders include intellectual disability and autism spectrum disorders which usually arise in infancy or childhood. Stigma and discrimination can add to the suffering and disability associated with mental disorders, leading to various social movements attempting to increase understanding and challenge social exclusion.

Signs and Symptoms

The onset of psychiatric disorders usually occurs from childhood to early adulthood. Impulse-control disorders and a few anxiety disorders tend to appear in childhood. Some other anxiety disorders, substance disorders and mood disorders emerge later in the mid-teens. Symptoms of schizophrenia typically manifest from late adolescence to early twenties. The likely course and outcome of mental disorders vary and are dependent on numerous factors related to the disorder itself, the individual as a whole, and the social environment. Some disorders may last a brief period of time, while others may be long-term in nature.

All disorders can have a varied course. Long-term international studies of schizophrenia have found that over a half of individuals recover in terms of symptoms, and around a fifth to a third in terms of symptoms and functioning, with many requiring no medication. While some have serious difficulties and support needs for many years, "late" recovery is still plausible. The World Health Organization concluded that the long-term studies' findings converged with others in "relieving patients, carers and clinicians of the chronicity paradigm which dominated thinking throughout much of the 20th century."

A follow-up study by Tohen and co-workers revealed that around half of people initially diagnosed with bipolar disorder achieve symptomatic recovery (no longer meeting criteria for the diagnosis) within six weeks, and nearly all achieve it within two years, with nearly half regaining their prior occupational and residential status in that period. Less than half go on to experience a new episode of mania or major depression within the next two years.