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The diminished responsibility defense, often referred to as diminished capacity in the United States, is a partial defense arguing that the defendant should not be held fully responsible for his crime because of his mental condition. In order to demonstrate diminished capacity, the defense must demonstrate that the defendant was suffering from an "abnormality of the mind" at the time of the crime, and that this abnormality substantially impaired his responsibility for the crime he committed. The diminished capacity defense is most famously used to reduce liability in killings, usually from murder to voluntary manslaughter.
Abnormality of mind can of course be a very difficult condition to identify. Similar to insanity, it is nevertheless meant to indicate a mental condition that is less profoundly severed from reason than full blown insanity, yet severe enough to mitigate mens rea in a criminal act. In Byrne (1960), it was ruled that abnormality of the mind signifies a mental condition that is so deviant from the social norm that a healthy and reasonable person would judge it abnormal. This not only includes abnormal thoughts and sensory perceptions, but also and especially circumstances that abnormally affect the defendant's ability to exercise his will.
Famous abuses of the diminished capacity defense have caused it to lose favor in recent decades. Take, for example, the 1979 case of Dan White, who assassinated San Francisco Mayor George Moscone and Supervisor Harvey Milk. White's defense team argued that he had been severely depressed after losing his job as city Supervisor and, suffering from an abnormality of mind brought on by no exercise and copious consumption of Coca Cola and Twinkie products, committed killings that he would not have committed in a normal state of mind. White was ruled to have suffered from diminished capacity and, being incapable of the criminal intention required for murder, his liability was reduced from murder to voluntary manslaughter. In 1982, public outrage over the ruling prompted California to abolish the diminished capacity defense altogether.