It is not possible to discuss Diminished Capacity (Diminished Responsibility) without first understanding the legal concept of insanity since both are joined at their ideological hip by mens rea. Diminished Capacity, like insanity, is a legal concept not a medical diagnosis.

The overarching principle of diminished capacity is that an accused’s level of responsibility for committing an illegal act is reduced because a mental disease or defect diminished his ability to form the intent to commit the act. The act itself must be in the “specific intent” category of crimes, such as murder.

Some law-and-order social philosophers and moralists have objected to the concept of Diminished Capacity because it creates a nuanced grey area of relative wrongdoing. They prefer an “all or nothing” approach to responsibility: if you do the act, you do the time. But what if there is impaired understanding? “Diminished capacity” emerged out of complex cases with just such nuances.

In California, the legal concept of Diminished Capacity was introduced by Bernard Diamond, M.D., my teacher and mentor, who was then Professor and Chair of Criminology at U.C. Berkeley, Professor of Law at Boalt Hall at U.C. Berkeley, and Professor of Psychiatry at U.C. San Francisco. He reasoned that was not all or nothing and limited to insanity but that a transition area existed between insanity and full competence to form intent. The transition between the two constituted a diminished capacity to form intent which could lead to less responsibility. Following the introduction of the concept of Diminished Capacity or Diminished Responsibility (depending on the jurisdiction) in an article in the Stanford Law Review, case law began to be published in the federal, military and in various states’ appellate courts citing this article and others defining what was meant by capacity being “diminished.”

In my practice, I have examined many defendants about their level of mens rea.

The mental illnesses which could lead to diminished capacity are a heterogeneous group. The expected conditions are there: Schizophrenia, Manic-Depressive illness (Bipolar Disorder), Post-Traumatic Stress Disorder (PTSD), Mental Retardation, Dementia, Dissociative states, intoxication, obsessions and compulsions (for irresistible impulse). Certain medical conditions also affect the ability to form intent such as medical shock due to blood loss or drug reactions.

Over time, a perception arose with the public that a creative defense attorney could “diminish” the prosecution’s prima facie case and reduce punishment with a clever psychiatric hypothesis which was expressed as an opinion. In California, the legal “last straw” was the successful defense of Dan White’s revenge murder of the Mayor of San Francisco, George Moscone, and gay Supervisor Harvey Milk, using the (misnomer) “Twinkie Defense.” The popular perception was that after Supervisor Dan White, who was hypoglycemic, ingested sugary Twinkies, he could not fully form the intent to sneak into City Hall, search out the Mayor against whom he held a grudge (because the Mayor refused to rescind White’s recent protest resignation from the City Council), shoot him, and then proceed to the other end of the building to hunt down and kill Harvey Milk, his repeated antagonist on the City Council. In fact, testimony of psychiatrists and psychologists was that Mr. White had been suffering bouts of depression and bipolar mood swings, aggravated by a number of factors, and, more as an after thought, a junk food diet which caused hypoglycemia. Jurors believed his capacity was diminished and found him guilty of a reduced charge. (Dan White was convicted of voluntary manslaughter, served approximately five years, and about two years after his release, committed suicide).

White’s perceived acquittal raised a public hue and cry and mobilized State legislators who, I think, identified with the slain public officials, and enacted legislation to do away with the Diminished Capacity defense in California. An “actually formed” mens rea defense became necessary. The new statute states at Sec. 28(b): “There shall be no defense of diminished capacity, diminished responsibility or irresistible impulse.” Sec. 29 states that experts testifying about the defendant’s mental condition “shall not testify as to whether the defendant had or did not have the required mental states” because that “shall be decided by the trier of fact.” Nevertheless, experts are permitted to testify about the “actually formed” mental state, which has led to some confusion as to what the expert is permitted to testify about. Generally, the expert is permitted to testify about the seriousness of the mental illness or mental state at the time of the commission of the act, but cannot testify that the condition diminished the person’s capacity to form intent as related to the facts. The jury is required to bridge the gap between the psychiatric information (”actually formed” mental state) and the jury instructions related to diminished capacity without testimonial opinions by experts whose expertise could assist the trier of fact about information a layman is not expected to know.

The California Supreme Court discusses this legislation in People v. Saille 820 P.2d 786 (1991).

I have testified in many insanity and diminished capacity criminal trials over the years both for prosecution and defense, at state, federal, and military levels.