Some states allow the defense of diminished capacity, which is short of insanity, to prove that as a result of a mental defect the defendant did or did not have the required state of mind (mens rea), to commit an offense. In other words, diminished capacity if used as a defense to a crime negates the specific mental state required for a particular crime.
California abolished the defense of diminished capacity after it was used in the trial of Dan White, who was accused of and convicted for killing Harvey Milk and George Moscone. Dan White's diminished capacity defense became notorious after it was labeled “The Twinkie Defense,” due to public perception that White's lawyers had argued that he had a diminished mental capacity because he had ingested a lot of sugar and junk food before the killing. Diminished capacity is also known as “diminished responsibility.”
California Penal Code Section 25 states that the defense of diminished capacity has been abolished. However, California Penal Code Section 28(a) provides that evidence that a defendant suffers from a mental disease, mental defect, or mental disorder is admissible “solely on the issue whether or not the accused actually formed a required specific intent, premeditated, deliberated, or harbored malice aforethought, when a specific intent crime is charged.” In other words, psychiatric evidence is still admissible to completely negate the mental element required for proof of the crime.