Almost everything the public believes about the insanity defense is wrong, and the errors run in both directions. People think it is common. It is not. People think it is a way out. For most who succeed with it, the confinement lasts longer than the prison sentence would have. People think it turns on whether the defendant knew right from wrong. In Massachusetts, that has not been the whole test since 1844.
Here is what the verdict actually means, what a defendant has to prove, how often it works, and what happens on the day after it does.
What does not guilty by reason of insanity mean?
A verdict of not guilty by reason of insanity means the jury found that the defendant committed the act, and that the defendant is not criminally responsible for it because of mental disease or defect.
That first half gets lost constantly. This is not a finding that the defendant did not do it. Factual guilt is usually conceded outright by the defense, because the entire argument depends on it. The dispute is about culpability, which the criminal law treats as a separate question from causation. We punish people for choosing to do wrong. The insanity defense says that on the evidence, there was no meaningful choice to punish.
The verdict is an acquittal in name, and in almost no other respect. A defendant found not guilty by reason of insanity does not go home from the courthouse.
Why Massachusetts calls it something else
If you search this phrase in Massachusetts you will find lawyers using different words. The phrase used here is not criminally responsible, sometimes shortened to NCR. It means the same thing, and the reason for the different label is that Massachusetts uses a different test than the states where “insanity” became the standard shorthand.
Most people who look this up land on the M’Naghten rule, from the 1843 English case of Daniel M’Naghten, who shot the British prime minister’s secretary while suffering paranoid delusions. M’Naghten asks a single cognitive question: did the defendant know the nature and quality of the act, or know that it was wrong? It is a narrow test. A defendant who knew perfectly well that shooting someone was wrong, but was powerless to stop himself, fails it.
Massachusetts never adopted that as its full rule. The state standard traces to Commonwealth v. Rogers, 7 Met. 500 (1844), where Chief Justice Shaw articulated a test with two branches rather than one, a year after M’Naghten was decided in England.
The McHoul standard
The modern formulation comes from Commonwealth v. McHoul, 352 Mass. 544 (1967), which is why Massachusetts lawyers say “the McHoul standard” the way lawyers elsewhere say M’Naghten. The court adopted the language of the American Law Institute’s Model Penal Code, section 4.01:
“A person is not responsible for criminal conduct if at the time of such conduct as a result of mental disease or defect he lacks substantial capacity either to appreciate the criminality [wrongfulness] of his conduct or to conform his conduct to the requirements of law.”
Read the two branches separately, because the word “either” is doing the work:
- The cognitive branch. Lacking substantial capacity to appreciate that the conduct was wrong. This is roughly the M’Naghten question.
- The volitional branch. Lacking substantial capacity to conform conduct to the law. This is the branch M’Naghten states do not have, and it is where most serious Massachusetts insanity cases are actually fought.
A defendant needs only one. Someone who knew an act was wrong, and could not stop, satisfies the standard here and would fail it in a strict M’Naghten jurisdiction.
Note also the words “substantial capacity.” The test does not require total incapacity, or a defendant who cannot speak or recognize family. It asks whether capacity was substantially lacking, which is a matter of degree and is precisely why these trials turn into duels between expert witnesses.
One historical detail worth keeping straight: the McHoul court described the Model Penal Code definition as “an evolutionary restatement of our rule rather than a substantively new rule.” Massachusetts did not switch tests in 1967. It restated in modern language a dual test it had used since Shaw.
Who has to prove it
This is the most consequential difference between Massachusetts and the federal system, and most coverage gets it backwards.
In Massachusetts, the defendant does not have to prove insanity. Criminal responsibility is an element the Commonwealth must establish. Once evidence sufficient to raise the issue is in the case, from either side, the prosecution must prove beyond a reasonable doubt that the defendant was criminally responsible. That allocation comes from Commonwealth v. Kostka, 370 Mass. 516 (1976).
Think about what that means in a courtroom. The defense does not have to convince twelve people the defendant was insane. The defense has to leave enough doubt that the Commonwealth cannot prove sanity to a certainty. A juror who finds the question genuinely close is instructed to acquit.
In federal court the burden runs the other way. After John Hinckley was acquitted of shooting President Reagan, Congress passed the Insanity Defense Reform Act of 1984, which put the burden on the defendant to prove insanity by clear and convincing evidence and eliminated the volitional branch from the federal test. Several states did the same. Massachusetts did not.
So the identical facts can produce different verdicts depending on the courthouse. That is not a flaw anyone designed. It is what happened when thirty jurisdictions reacted to one 1982 verdict at different speeds.
How often does the insanity defense actually work?
Rarely, and it is raised far less often than people assume.
The standard research is an eight-state study by Lisa Callahan and colleagues, published in 1991 in the Journal of the American Academy of Psychiatry and the Law. The findings have held up as the reference point for thirty years:
- The insanity defense was raised in slightly under one percent of felony cases, 0.93 percent overall, ranging from 0.3 to 5.7 percent by state.
- Where it was raised, it succeeded about 26 percent of the time, with enormous variation between states, from 7.3 percent to 87 percent.
Multiply those together. The defense succeeds in roughly two of every thousand felony cases. And the high success rate within the small pool is misleading if you picture contested trials, because a large share of successful NCR findings are not jury verdicts at all. They are cases where the prosecution’s own expert agreed, and the finding was entered without a fight. Contested insanity trials, the kind that make the news, are won much less often than 26 percent.
There is no reliable published figure for a single lawyer’s insanity defense success rate, in Massachusetts or anywhere else. Nobody keeps that statistic.
What happens after a not guilty by reason of insanity verdict
The defendant is taken into custody at the courthouse. This is the part the public consistently does not know.
In Massachusetts the mechanism is G.L. c. 123, section 16:
- Section 16(a): the 40-day evaluation. The court orders hospitalization at a designated facility for observation and examination for up to forty days.
- Section 16(b): the initial commitment. The district attorney or the facility’s medical director petitions for commitment. If the statutory criteria are met, the court commits the person, and that order runs for six months.
- Section 16(c): renewal. After the first six months, commitment is renewed in additional one-year periods, and there is no cap on the number of renewals.
- Section 16(e): the district attorney stays involved. The DA gets notice and can be heard. If the superintendent wants to loosen movement restrictions or discharge the person, the court and the district attorney handling the criminal case must be notified.
Read that structure and the practical reality follows. Release requires an affirmative finding, at a hearing, that the person no longer meets the commitment standard, over the objection of a district attorney with an institutional interest in opposing it, in a case that was serious enough to reach trial. Renewal requires only that the standard still be met.
Empirical work on this is consistent and counterintuitive: defendants found not guilty by reason of insanity for serious violent offenses frequently spend as long or longer in secure psychiatric confinement than comparable defendants who were convicted spend in prison. A prison sentence has an end date that is knowable at sentencing. A civil commitment does not.
We walk through the Massachusetts process in more detail in what actually happens after a not criminally responsible finding.
Why “he got off on an insanity plea” is wrong twice
The phrase contains two errors.
First, it is not a plea. You cannot plead not guilty by reason of insanity the way you plead guilty. It is a defense, presented at trial, decided by the fact finder. There is no version where a defendant checks a box and avoids the proceeding.
Second, “got off” describes an outcome that does not usually occur. The realistic range of outcomes for a successful NCR defendant in a homicide case is years to decades in a secure facility, with periodic hearings, an opposing district attorney, and no guaranteed release date.
There is a fair criticism available, and it is a different one: that the outcome is decided by psychiatrists and judges at commitment hearings rather than by the jury that heard the case, and that it is far less visible to the public than a sentencing.
How these cases are actually tried
The evidence that decides an insanity case is rarely the testimony people expect.
Expert psychiatric opinion matters, but jurors tend to discount it, reasoning that each side hired its own. What tends to move a verdict is contemporaneous, non-expert evidence about the defendant’s state before the offense: medical records, prescriptions and dosage changes, text messages, search history, statements to family, the behavior of the defendant in the hours afterward.
Prosecutors, for their part, look for evidence of planning and of concealment, because both suggest a mind that appreciated wrongfulness. A defendant who hid evidence understood there was something to hide. That inference is often the heart of the Commonwealth’s case.
We took this apart in practical terms in how an insanity defense is actually fought.
Common questions
What is the difference between not guilty by reason of insanity and not criminally responsible?
Nothing substantive. They describe the same verdict. “Not criminally responsible” is the phrase used in Massachusetts practice; “not guilty by reason of insanity” is the general American term.
Does Massachusetts use the M’Naghten rule?
No. Massachusetts uses the McHoul standard, adopted from Model Penal Code section 4.01, which includes both a cognitive branch and a volitional branch. M’Naghten has only the cognitive one. Massachusetts has used a two-branch test since Commonwealth v. Rogers in 1844.
Who has to prove insanity in Massachusetts?
The Commonwealth has to disprove it. Once the issue is properly raised, the prosecution must prove criminal responsibility beyond a reasonable doubt, under Commonwealth v. Kostka. In federal court the opposite is true: the defendant must prove insanity by clear and convincing evidence.
How often does the insanity defense succeed?
It is raised in under one percent of felony cases and succeeds in about 26 percent of those, so roughly two in a thousand felony cases overall. Many successes are uncontested rather than won at trial.
Do you go free if found not guilty by reason of insanity?
No. In Massachusetts you are hospitalized for evaluation for up to forty days under G.L. c. 123, section 16, then committed for six months, then renewed in one-year increments with no limit. Confinement often exceeds the equivalent prison term.
Is it a plea?
No. It is a defense decided at trial, not something a defendant can enter the way a guilty plea is entered.
What is the difference between insanity and competency to stand trial?
Timing. Insanity looks at the defendant’s mental state at the moment of the offense. Competency looks at the present: whether the defendant can understand the proceedings and assist counsel now. A defendant can be competent to stand trial and still not criminally responsible for the act, which is the ordinary posture of these cases.
Can a mental illness diagnosis alone win the defense?
No. A diagnosis is necessary but nowhere near sufficient. The question is whether, at the time of the conduct, the illness deprived the defendant of substantial capacity to appreciate wrongfulness or to conform conduct to the law. Most defendants with serious mental illness do not meet that standard.
Case text quoted from the opinions in Commonwealth v. McHoul, 352 Mass. 544 (1967) and Commonwealth v. Kostka, 370 Mass. 516 (1976), verified against the CourtListener database. Commonwealth v. Rogers is quoted as cited within McHoul. Statutory text from G.L. c. 123, section 16 as published by the Massachusetts Legislature. Prevalence and success figures from Callahan et al., “The Volume and Characteristics of Insanity Defense Pleas: An Eight-State Study,” Journal of the American Academy of Psychiatry and the Law 19(4):331 (1991). General information about Massachusetts law, not legal advice. Anyone facing criminal charges should consult a lawyer about their own case.