See Insanity and diminished capacity
A "diminished capacity" plea differs in important ways from "not guilty by reason of insanity." "Reason of insanity" is an affirmative defense to crimes. That is, a successful plea of insanity will, in most states, result in a verdict of "not guilty" and commission of the defendant to a mental institution. "Diminished capacity," on the other hand, merely results in the defendant being convicted of a lesser offense.
The basis for this is found in the American criminal law system. Most crimes in the United States are defined in a way that implicitly distinguishes an actus reus -- a guilty act -- from a mens rea -- or guilty mental state. For example, the New York Penal Code defines second-degree murder as causing the death of a person (guilty act) with the intent to cause the death of a person (mental state). N.Y. Penal § 125.25(1) By contrast, second-degree manslaughter is defined as causing the death of a person (guilty act) recklessly (mental state). N.Y. Penal 125.15(1).
The diminished capacity plea is based in the belief that certain people, because of mental impairment or disease, are simply incapable of reaching the mental state required to commit a particular crime. In the example of murder and manslaughter, a diminished capacity defense contends that a certain defendant is incapable of intending to cause a death, and therefore must have at most caused such a death recklessly. Thus, a successful plea of diminished capacity in a murder trial would likely result in the charge being reduced to manslaughter.
A spotted history
California allowed a plea of diminished capacity beginning in the 1950s. But the plea came under intense scrutiny during as the so-called "Twinkie defense," in the 1979 case California v. White. Dan White, a former city supervisor, shot and killed the mayor of San Francisco, George Moscone, and another city supervisor, Harvey Milk. The crime displayed a high degree of premeditation: White packed extra bullets, climbed through a City Hall window to avoid metal detectors, and shot the two men nine times.
White's attorneys argued diminished capacity. They claimed that a diet of only junk food had created a chemical imbalance in White's brain (the "Twinkie defense"), and that he was depressed over his loss of his city supervisor position. Therefore, he was unable to premeditate murder, one of the requirements for first-degree murder.
The jury convicted White of voluntary manslaughter -- the least serious charge for homicide. This caused an uproar against the diminished capacity plea in California, and in 1982, voters overwhelmingly approved a proposition to eliminate the defense.
The federal rule of diminished capacity -- sentencing
In a federal case the rules of evidence, procedure and sentencing are set at the federal level. The rules of procedure are the Federal Rules of Criminal Procedure, the rules of evidence are the Federal Rules of Evidence, and the sentencing rules are the United States Sentencing Guidelines (USSG).
The USSG sets minimum sentencing for certain federal crimes, and also sets the guidelines by which judges may stray from these minimums. The USSG states that a federal court may depart downward from the minimum sentence on the basis of diminished capacity if the offense was nonviolent. U.S.S.G. § 5K2.13. See also United States v. Cook, 53 F.3d 1029 (9th Cir. 1994).
U.S.S.G. § 5K2.0 allows departure from the sentencing minimums for "extraordinary mental condition." Unlike 5K2.13, this section does not explicitly limit the departure to non-violent crimes. However, it was not until very recently that the Court of Appeals for the Ninth Circuit explicitly recognized that 5K2.0 creates a "diminished capacity" argument for violent crimes.
In United States v. Green (9th Cir. Sept. 8, 1997), the Ninth Circuit ruled that the trial court had the discretion to depart downward in sentencing based on diminished capacity in the case of the defendant, a man who pled guilty to two counts of bank robbery.
A successful plea of diminished capacity would not earn a "not guilty" verdict, but merely a reduced sentence, under the federal sentencing guidelines.