Hudson County Criminal Lawyer Blog
The New Jersey Supreme Court has answered a question that has lingered in our criminal courts for decades: can a defendant obtain a diminished capacity jury instruction on lay testimony alone? In State v. Mustafa, a unanimous decision issued on August 3, 2026, the Court held that expert testimony is required both for a defendant to invoke the diminished capacity defense and for a trial court to instruct the jury on it. The Court affirmed the Appellate Division and confirmed that an expert is necessary to support an insanity defense. For anyone who handles serious violent crime cases, this opinion changes how mental health evidence must be developed, and it changes it well before trial begins.
The case arose from a late night shooting on a Monmouth County highway in May 2018. The defendant was driving with his then girlfriend when the victim, traveling on the same roadway, turned on her high beams because one of her headlights was out. The defendant became upset, pulled over to let her pass, then accelerated until he nearly struck her bumper while flashing his own high beams. As her car began to exit, he put his vehicle in neutral, rolled down his window, drew a handgun, and fired. She died of a gunshot wound to the head. A grand jury returned a six count indictment charging first degree murder, weapons offenses, and endangering. Defense counsel initially served notice of an insanity defense, but the defendant later instructed his attorney to withdraw it, and a pretrial memorandum recorded that decision in plain terms, noting that the insanity defense was withdrawn and that there would be no psychiatric expert testimony. The trial court found the waiver knowing and voluntary.
Diminished capacity, however, was raised explicitly at trial, and a fair amount of lay evidence about the defendant’s mental health reached the jury. His then girlfriend testified that he had begun seeing her psychiatrist to obtain Adderall because he liked how it made him feel, that he smoked marijuana daily, and that he had immersed himself in conspiracy research. She described his belief that he was the target of gang stalking, that people were shooting laser beams, radiation, and concentrated microwaves at him, and that he wore tin foil under a baseball cap and helmet to deflect the rays. She said his statements reminded her of a relative who has schizophrenia. On the night of the shooting, she testified, he took Adderall three times within several hours, yelled about people trying to hit him with radiation and follow him, and screamed that he could not take it anymore and had to fight back. The defendant did not testify and called no witnesses. When counsel requested a diminished capacity charge at the close of evidence, arguing that a medical diagnosis was not required, the trial court denied it because there was no medical or expert testimony in the record. The jury convicted on all counts and the defendant received an aggregate life sentence.
The Court grounded its holding in familiar evidence principles rather than in anything unique to the criminal code. Under N.J.R.E. 702, expert testimony is not appropriate to explain what a jury can understand on its own, but our courts have consistently required it for matters that fall beyond the ken of the ordinary juror. The test is whether the subject is so esoteric that jurors of common judgment and experience cannot form a valid judgment without assistance, and when the answer is yes, the alternative is speculation. The Court drew on its recent cell site location decision, where it held that leaving jurors to draw inferences from technical data without expert explanation risked confusing and misleading them, and it observed that commentators on our evidence rules have long said that any claim of mental illness affecting behavior is probably esoteric enough to require an expert.
Turning to the statute, the Court explained that N.J.S.A. 2C:4-2 makes evidence of a mental disease or defect admissible whenever it is relevant to prove the defendant lacked a state of mind that is an element of the offense. Neither the statute nor any other provision of the code defines mental disease or defect, and Galloway made clear that the category is broad and not confined to clinically defined conditions. But Galloway also framed the inquiry in expressly clinical terms, asking whether psychological experts believe that the kind of deficiency alleged can affect cognitive faculties and did affect this defendant’s ability to form the requisite mens rea. Under the two prong test restated in Baum, the defendant must present evidence of a mental disease or defect that interferes with cognitive ability sufficient to prevent the formation of the required intent, and the record must contain evidence that the claimed deficiency actually affected his cognitive capacity at the time of the crime. The trial court must also determine, as a threshold matter, that the evidence of the condition is relevant and sufficiently accepted within the psychiatric community to be reliable in a courtroom. As the Court pointed out, those determinations are impossible to make on lay observation alone.
The reasoning behind the holding is practical and worth understanding, because it tells you what the record has to show. Without psychological or psychiatric testimony, the Court warned, jurors are left to rely on stereotypes and assumptions about mental illness rather than reliable evidence. Lay people are not equipped to distinguish genuine mental illness from malingering, a difficulty that even trained clinicians acknowledge. Nor can average jurors be expected to separate the effects of mental illness from the effects of narcotics or stimulants that produce similar behavior, a concern the Court illustrated with the evidence of repeated Adderall dosing on the night of the shooting. A jury may certainly consider a defendant’s behavior, but it cannot conclude on its own that the behavior is a symptom of illness without an expert to interpret it. The Court applied that framework directly to this record and found nothing identifying a mental disease or defect at all. Lay testimony describing unusual conduct remains admissible under Risden, and testimony that someone appeared wild or irrational requires no expertise, but description is not diagnosis. A witness comparing the defendant’s statements to those of a relative with schizophrenia does not supply the missing link between a recognized condition and the defendant’s capacity to act purposely or knowingly.
For those of us practicing in Hudson, Essex, Union, Passaic, and throughout New Jersey, the lesson is about timing and preparation rather than doctrine. If there is any realistic prospect of a mental health defense, retain and consult an expert early, and treat the decision to forgo psychiatric testimony as a decision that closes two doors rather than one. Withdrawing an insanity defense no longer leaves diminished capacity available as a lesser mental health argument built on lay witnesses, and a pretrial memorandum stating that there will be no psychiatric expert now forecloses the charge outright. It is also worth noting that the defendant here introduced hospital records at sentencing but not at trial, and the Court declined to expand the record on appeal to include them. Evidence that lives only in your file, or only in a sentencing submission, will not support an instruction. None of this prevents you from arguing intoxication where the facts support it, or from arguing straightforwardly that the State has failed to prove a purposeful or knowing state of mind, and those arguments remain available without an expert. For prosecutors, the decision confirms that evidence of strange conduct, however vivid, does not by itself entitle a defendant to a diminished capacity charge or create a basis for reversal.