Insanity is an excuse. It concedes that the defendant committed the act with the required mental state and argues that a severe mental disease or defect makes it unjust to hold him responsible. It is raised far less often than people assume and succeeds less often still. The tests differ by jurisdiction, and identifying which one applies is the first move in any answer, because they ask genuinely different questions.
Rule
- M'Naghten. The most common test. A defect of reason from disease of the mind meant the defendant did not know the nature and quality of the act, or did not know that it was wrong.
- Irresistible impulse. Adds a volitional branch, excusing a defendant who knew the act was wrong but could not control his conduct.
- The Durham product test. Asked only whether the act was the product of mental disease or defect. It proved unworkable and has been abandoned in the jurisdiction that created it.
- Model Penal Code. Lacking substantial capacity, as a result of mental disease or defect, either to appreciate the criminality of the conduct or to conform conduct to the law.
- Burden and effect. Many jurisdictions place the burden on the defendant. A successful defense usually leads to commitment for treatment, not release.
- Not the same as competency. Insanity looks at the defendant's state at the time of the offense. Competency looks at the ability to understand and assist in the proceedings now.
Leading cases
- M'Naghten's Case: the English decision that produced the knowledge of nature and wrongfulness formula still used in most American jurisdictions.
- Durham v. United States: adopted the product test in the District of Columbia, which later replaced it because the standard gave juries too little to work with.
- Clark v. Arizona: states have substantial latitude to define the insanity defense and to limit how mental illness evidence may be used on the question of mens rea.
Where students go wrong
The first mistake is confusing insanity with incompetency. One is about criminal responsibility at the time of the act; the other is about the ability to stand trial today, and they are decided at different points by different standards.
The second is treating any mental illness as sufficient. The illness must produce the specific cognitive or volitional failing the test names, and expert evidence has to tie the two together.
The third is forgetting diminished capacity. It is a separate argument that mental illness prevented the defendant from forming a specific intent, and unlike insanity it can result in conviction of a lesser offense rather than acquittal.
FAQ
What is the M'Naghten test?
A defendant is not responsible where a defect of reason from a mental disease meant he did not know the nature and quality of his act, or did not know it was wrong.
Is a successful insanity defense a release?
Rarely. A defendant found not guilty by reason of insanity is usually committed for treatment, often for a period that can exceed the sentence for the offense.
How does the Model Penal Code test differ?
It uses substantial capacity rather than absolute knowledge, and it includes a volitional branch, so it is broader than M'Naghten on both fronts.
Related terms
Working through this in your own casebook takes longer than reading it here. Syllume starts from your syllabus and turns each assigned case into a brief you can study from.