What are the differences between the pleas of diminished capacity, not guilty by reason of insanity, and guilty but mentally ill?

Diminished capacity applies when mental impairment such as mental retardation prevents the defendant from forming the specific mental state required for a crime, so the defendant is usually convicted of a lesser offense like manslaughter. Not guilty by reason of insanity applies when a mental disease such as delusional disorder was present at the time of the act, resulting in acquittal. Guilty but mentally ill applies when the person has a psychiatric disorder such as pyromania, substance abuse, or a sexual offense; the crime occurred because of the illness, but the person is still held responsible for the behavior.

The three pleas differ mainly in how mental impairment affects criminal responsibility and the verdict. Diminished capacity asserts that a condition such as mental retardation made the defendant unable to form the specific mental state, such as intent, required for a particular offense like first-degree murder; the typical outcome is conviction on a lesser charge such as manslaughter. Not guilty by reason of insanity is entered when a mental disease, for example a delusional disorder, was present at the time of the alleged criminal act, meaning the defendant is not held criminally responsible. Guilty but mentally ill is used when the person shows clinical symptoms of a DSM-IV-TR psychiatric disorder such as pyromania, substance abuse, or sexual offenses; in this plea, the criminal act is attributed to the illness, yet the client is still considered responsible for the behavior. For instance, a substance abuser who robs a store to obtain money for drugs may plead guilty but mentally ill rather than be acquitted.