Insanity Defense
A defendant is not criminally responsible for an act if, at the time of the act, they were suffering from a mental disease or defect that caused them to lack criminal responsibility — either the ability to understand what they were doing or to conform their conduct to law.
Elements / Test
M’Naghten Test (1843) — majority of states; cognitive test:
- At time of act, defendant did NOT know: (1) the nature and quality of the act OR (2) that the act was wrong (legally or morally, jurisdiction-dependent)
Irresistible Impulse Test (supplement to M’Naghten):
- Defendant knew the act was wrong but was unable to control their conduct due to mental disease
Durham “Product” Test (D.C. — largely abandoned):
- Criminal act was the “product” of mental disease or defect; criticized as too broad
MPC § 4.01 “Substantial Capacity” Test:
- As a result of mental disease or defect, lacked substantial capacity to: (1) appreciate the criminality of the conduct, OR (2) conform conduct to the requirements of law
- “Appreciate” (not just “know”) = deeper understanding; “substantial” (not total) capacity required
- Used in federal courts pre-Hinckley
Federal (post-Hinckley) — 18 U.S.C. § 17:
- Defendant, as a result of severe mental disease or defect, was unable to appreciate the nature and quality or wrongfulness of the acts
- Only cognitive prong; no volitional/control prong; harder standard than MPC
Exceptions and Edge Cases
- Guilty but mentally ill (GBMI): Verdict available in ~20 states; defendant is convicted but receives mental health treatment — does not preclude full punishment
- Diminished capacity: (Distinct from insanity) Defendant’s mental illness negates specific intent required for crime — reduces to lesser offense
- Competency to stand trial: Separate from insanity — must understand proceedings and assist counsel (Dusky v. United States); incompetent = proceedings stayed, not acquitted
- Burden of proof: Federal and most states — defendant bears burden (usually by preponderance) to prove insanity; some states retain prosecution’s burden to disprove
- Abolition: Idaho, Kansas, Montana, Utah have abolished the insanity defense; constitutionality contested
Policy Rationale
Criminal liability requires moral culpability; severely mentally ill persons who lack understanding or control are not culpable in the same way. Concerns: inconsistent application; difficulty of expert testimony; rare use despite perception.
Key Cases
| Case | Rule |
|---|---|
| M’Naghten’s Case (H.L. 1843) | Cognitive test: knew nature of act or knew it was wrong |
| United States v. Brawner (D.C. Cir. 1972) | Adopted MPC substantial capacity test for federal courts (pre-Hinckley) |
| Clark v. Arizona (2006) | State may limit insanity defense to M’Naghten without violating due process |
| Jones v. United States (1983) | Insanity acquittee may be confined longer than criminal sentence |