Understanding the insanity defense in New York
Movies and television have shaped what most people think the insanity defense looks like. The reality in New York is very different. The defense is rarely raised, hard to prove, and often misunderstood. It does not mean a defendant simply “acts crazy” to avoid punishment. Knowing how the defense actually works can help you separate courtroom fiction from New York law.
What the law actually requires
New York does not use the phrase “not guilty by reason of insanity.” Instead, the law calls it lack of criminal responsibility by reason of mental disease or defect. Under New York Penal Law Section 40.15, a defendant must show that, at the time of the conduct, a mental disease or defect left him without substantial capacity to know or appreciate either the nature and consequences of what he did, or that the conduct was wrong. (N.Y. Penal Law § 40.15 (McKinney))
Two points often surprise people. First, a diagnosis alone is not enough. Many people live with serious mental illness and are still held responsible for their actions. The question is whether the illness stripped away the defendant’s capacity to understand the act or its wrongfulness at that specific moment. (N.Y. Penal Law § 40.15 (McKinney)) Second, the standard asks about understanding, not emotion or self-control. Knowing an act was wrong but feeling unable to stop is generally not a defense.
The burden of proof shifts to the defendant
In most parts of a criminal case, the prosecution must prove guilt beyond a reasonable doubt. The insanity defense works differently. It is an “affirmative defense,” which means the defendant raises it and must prove it by a preponderance of the evidence. (N.Y. Penal Law § 25.00 (McKinney)) That standard means more likely than not, a much lower bar than beyond a reasonable doubt, but the burden still rests on the defense.
This shift does not erase the prosecution’s job. The People must still prove every element of the underlying crime beyond a reasonable doubt, including the defendant’s mental state for that offense. (People v. Kohl, 72 N.Y.2d 191, 527 N.E.2d 1182 (1988)) The insanity defense is a separate question layered on top of the case, not a replacement for it.
You must give advance notice
A defendant cannot spring the defense on the prosecution at trial. New York requires written notice of intent to offer psychiatric evidence, generally within 30 days of the not guilty plea. (N.Y. Crim. Proc. Law § 250.10 (McKinney)) Once that notice is filed, the prosecution may have its own expert examine the defendant. In practice, insanity cases become a battle of experts, with psychiatrists on each side reaching different conclusions about the defendant’s mental state.
It is not the same as being intoxicated
People sometimes assume that being high or drunk can support a “temporary insanity” claim. New York courts have rejected that idea. A New York appeals court held that a psychosis caused only by the voluntary use of an illegal drug is not a mental disease or defect under the statute, even when the defendant genuinely could not appreciate that his conduct was wrong. (People v. Davenport, 215 A.D.3d 576, 187 N.Y.S.3d 226 (2023)) A long-term, settled mental illness that results from prolonged substance use may be treated differently, but a temporary drug-induced state generally will not qualify. (People v. Davenport, 215 A.D.3d 576, 187 N.Y.S.3d 226 (2023))
A “not responsible” verdict does not mean walking free
This may be the biggest misconception of all. A verdict of not responsible by reason of mental disease or defect is not an acquittal that sends the defendant home. By succeeding on the defense, the defendant avoids criminal penalties but instead becomes subject to a separate commitment process under the Criminal Procedure Law. (Jamie R. v. Consilvio, 6 N.Y.3d 138, 844 N.E.2d 285 (2006)) The court holds an initial hearing to decide the level of confinement or supervision needed to treat the person and protect the public. (Jamie R. v. Consilvio, 6 N.Y.3d 138, 844 N.E.2d 285 (2006)) Many people found not responsible are committed to secure psychiatric facilities, sometimes for years, with continued court review before any release. The outcome is treatment and supervision, not freedom.
Why these cases are so difficult
The insanity defense sits at the intersection of law and psychiatry, and juries are often skeptical of it. Success usually depends on detailed medical records, credible expert testimony, and careful attention to strict procedural rules. A missed notice deadline or a weak expert can sink an otherwise legitimate defense. Because the stakes are so high and the standards so specific, this is not an area to navigate alone.
Talk to a defense attorney
If you or a loved one is facing serious charges and mental illness may be involved, the details matter enormously. An experienced New York criminal defense attorney can evaluate whether the defense is realistic, gather the right psychiatric evidence, meet the deadlines, and explain what a not responsible verdict would actually mean for your loved one’s future. Understanding options early is the best way to make informed decisions about a difficult case.