Mental Illness: All states require the presence of a mental illness, but definitions differ. Psychosis usually is included, but personality disorder is not. Drug or alcohol abuse or both may be allowed. Mental illness alone is not sufficient for commitment but requires at least one of the following two additional conditions.

Dangerousness: To self or others: Most states require that the patient be dangerous but differ in the degree of urgency—an imminent danger (e.g., likely to hurt himself in the next 24 hours) versus a relative danger (e.g., physically deteriorating through depressive withdrawal). Dangerousness is the most common reason for commitment in most states. Two major problems with the dangerousness standard are (a) psychiatrists have difficulty accurately predicting future dangerous behavior except in the most obvious cases; and (b) it has been uncertain what level of proof the law requires—states vary widely, from the lowest civil standard of “preponderance of the evidence” (51% certainty) to the most strict criminal standard of “beyond a reasonable doubt”
(95% certainty). This latter issue appears to have been resolved by the U.S. Supreme Court decision (Addington v Texas, 1979) causing Federal courts to favor “clear and convincing evidence” (75% certainty).

Disabled and in need of treatment: Although diminished in degree, most states allow commitment solely on the grounds that a person is significantly handicapped by a mental illness, is unable to provide for his own basic needs (the parens patriae provision), and is in need of, and would benefit from, treatment. This is qualified in many states by a requirement proceeding from Lake v Cameron (1966) that hospitalization be the “least restrictive” of the treatment options available.

Some states now allow commitment of the developmentally disabled, substance users, and mentally disabled minors. Special due-process provisions apply.