In Missouri, there are two defenses available to the accused that are often confused. They are Insanity and Diminished Capacity. The first is formally called “Not guilty by reason of mental disease or defect excluding responsibility” (NGRI). The second is the “Diminished Capacity” defense.
These are two very different doctrines. The Insanity defense, if accepted, would absolve the defendant of all responsibility of the circumstances surrounding the alleged incident. Further, because it is an affirmative defense, there are certain time frames in which the defendant must notify the prosecution they intend to claim the defense.
On the other hand, the “Diminished Capacity” defense would merely negate a necessary mental element of the crime that is charged. For example, if the defense is accepted, the defendant could be convicted of 2nd Degree murder (or manslaughter and so forth) instead of 1st degree murder. The state also maintains the burden of proving the mental state of the crime charged beyond a reasonable doubt. The defense merely provides evidence that could impact their ability to reach that burden.
This is how the defense was described in the Missouri case State v. Anderson: “Proof of mental derangement short of insanity as evidence of lack of deliberate or premeditated design. In other words, it contemplates full responsibility, not partial, but only for the crime actually committed.”
This area of law is extremely tricky. Also, medical experts are absolutely necessary to evaluate the defendant and testify according to their findings.