Supreme Court on the Insanity Defense

Although the most important case decided by the Supreme Court today had nothing in particular to do with disability law, the Court's other case, Clark v. Arizona, involved an important disability-related question. The case involves Arizona's extremely narrow insanity defense. The traditional M'Naghten rule, in effect in many jurisdictions, provides that an individual will be found not guilty by reason of insanity if s/he either didn't know what s/he was doing or didn't know that what s/he was doing was wrong. Arizona uses a truncated version of the M'Naghten test, under which the defendant can be found not guilty by reason of insanity only if s/he didn't know that what s/he was doing was wrong. The first M'Naghten prong -- that the defendant didn't know what s/he was doing -- is irrelevant. Moreover, the state makes that truncated insanity defense the only vehicle for presenting expert evidence of the defendant's mental disease; accordingly, such evidence cannot be used to negate the mens rea element of the offense.

In today's decision, the Court upheld the Arizona scheme by (what was basically) a 6-3 vote against a due process challenge. Justice Souter wrote the opinion for the Court. He relied on the states' broad constitutional latitude to define the insanity defense and on the possibility of juror confusion if mental disease evidence were relevant to mens rea in a way that it could not be relevant to the insanity defense. Chief Justice Roberts, Justice Scalia, Justice Thomas, and Justice Scalia joined the Court's opinion in full, and Justice Breyer joined the Court's opinion in substantial part. Justice Kennedy dissented, joined by Justices Stevens and Ginsburg.