On August 3, 2026, a unanimous New Jersey Supreme Court decided the Monmouth County case of State v. Kader Mustafa. The principal issue under N.J.S.A. 2C:4-2 concerned whether expert testimony was required for the defendant to use a diminished capacity defense.
Justice Pierre-Louis, my Rutgers Law classmate, wrote for the Court in relevant part: Applying those principles, we now hold that expert testimony is required for defendants to invoke the diminished capacity defense and have the jury instructed on that defense. The plain language of N.J.S.A. 2C:4-2 dictates that “evidence that the defendant suffered from a mental disease or defect is admissible whenever it is relevant to prove that the defendant did not have a state of mind which is an element of the offense.” (emphasis added). Understanding what is or is not a mental “disease or defect” — and further determining whether that ailment altered defendant’s state of mind at the time of the offense — requires specialized knowledge beyond the ken of average jurors. Accordingly, under our Rule 702 jurisprudence, expert testimony must be presented to the jury to assist with the diminished capacity determination. Jurors should not be left to speculate in the highly specialized area of mental illness in the high-stakes setting of a criminal trial.
N.J.S.A. 2C:4-2 is silent as to the type and caliber of evidence required before the trial court charges a jury with instructions regarding diminished capacity. However, the statute’s plain language explains that in order for evidence of diminished capacity to be admitted, the evidence must establish the following facts that undoubtedly require specialized knowledge in the field of mental health: that defendant (1) “suffered from a mental disease or defect,” and (2) as a result of that mental disease or defect, “defendant did not have a state of mind which is an element of the offense.”
The assessments required by N.J.S.A. 2C:4-2 ask jurors to reach beyond their common knowledge and understanding. The determination of whether someone is suffering from a mental condition requires specialized knowledge in mental health fields. Indeed, this Court’s jurisprudence has held that a diminished capacity instruction is warranted when the record shows that “experts in the psychological field believe that the kind of mental deficiency at issue can affect a person’s cognitive faculties.” Lay people, including average jurors, are not skilled in diagnosing mental diseases. Without psychological or psychiatric expert testimony, jurors may resort to relying on stereotypes and assumptions about mental illnesses instead of reliable evidence.
Additionally, lay people are not equipped with the specialized knowledge needed to distinguish between someone who is actually suffering from mental illness and someone who is pretending to suffer from mental illness to escape culpability for alleged criminal acts. Even mental health professionals have identified the difficulties they face in deciphering whether individuals are truly suffering from mental illness or are malingering — feigning of a psychological or physical ailment for gain. See Jeffrey J. Walczyk et al., A Review of Approaches to Detecting Malingering in Forensic Contexts and Promising Cognitive Load-Inducing Lie Detection Techniques, 9 Frontiers in Psychiatry 1, 2 (2018) (“Despite ongoing advances in malingering detection, many individuals successfully malinger mental and cognitive disorders in order to avoid prison.”); Barry Rosenfeld et al., Litigator’s Handbook of Forensic Medicine, Psychiatry, and Psychology § 15:1 (2024) (noting that “observable signs of mental illness can often be identified, such as slowed speech, hyperventilating, and bizarre clothing or movements, but these too can be easily feigned, provided the individual gives a convincing presentation”).
Average jurors also cannot be expected to understand and distinguish between someone suffering from mental illness and someone under the influence of certain narcotics or hallucinogens that cause behavior that might mimic symptoms associated with certain mental diseases or defects. Here, defendant’s ex-girlfriend, Fiore, noted that defendant took multiple doses of Adderall prior to the shooting. Given the complexity of diagnosing mental health illnesses and, in some cases, the added variable of controlled substance use, jurors should not be left to make assumptions about a defendant’s mental status and the impact that such mental status had on the defendant’s ability to form the requisite mens rea without testimony from an expert witness to assist the jury in understanding any diminished capacity evidence before them. See Biunno, Weissbard & Zegas, cmt. 2.3 on N.J.R.E. 702 (“A jury should not be allowed to speculate without the aid of expert testimony in any area where laypersons could not be expected to have sufficient knowledge or experience.”).
Criminal judges are often tasked with assessing whether a defendant is under the influence of drugs or suffering from a mental illness during a routine court appearance in which the defendant is required to communicate basic information for scheduling purposes. A judge should not ignore evidence of intoxication since an inability to maintain sobriety during a brief morning court appearance could indicate that defendants are dangers to themselves or others. Judges are also tasked with maintaining respect for courtroom decorum by not permitting participants in court proceedings to show up intoxicated. At the same time, a judge has to be carful not to accuse someone of being intoxicated if they are actually suffering from a mental illness. The same goes for someone who is having an adverse reaction to lawfully prescribed and consumed medications. This task becomes particularly difficult when defendants might be both mentally ill and intoxicated. Bipolar disorder, severe depression, and anxiety disorders are common examples of a mental illnesses that can present as if someone is under the influence of drugs.
The Court continued in relevant part: During oral arguments, defense counsel asserted that, in the context of defendant’s strange behaviors and beliefs, a “reasonable person could certainly conclude that that person might be suffering from a mental disease or defect.” We disagree. While a jury can consider defendant’s behaviors, it cannot conclude that those actions are signs of mental illness without the aid of an expert interpreting them. Expert testimony is required for the jury to possess the “tools necessary to understand” and “interpret” relevant evidence.
Expert testimony is also an essential consideration for the trial court when it is determining whether to instruct the jury on diminished capacity. As noted, this Court has held that a trial court must give a diminished capacity charge to the jury when (1) the defendant “has presented evidence of a mental disease or defect that interferes with cognitive ability sufficient to prevent or interfere with the formation of the requisite intent or mens rea,” and (2) “the record contains evidence that the claimed deficiency did affect the defendant’s cognitive capacity to form the mental state necessary for the commission of the crime.”
Additionally, before charging the jury with diminished capacity, the trial court must “determine that the evidence of the condition in question is relevant and sufficiently accepted within the psychiatric community to be found reliable for courtroom use.” A trial court, therefore, is required in the first instance to make determinations about whether a reasonable jury could find that a defendant suffers from a mental disease or defect that impaired their ability to act culpably. The trial court must further discern, all based on the evidence presented at trial, whether the evidence in question of the alleged mental condition is “relevant and sufficiently accepted within the psychiatric community to be found reliable for courtroom use.” Those determinations require expert evidence.
Here, there was no evidence before the jury that identified a mental disease or defect from which defendant suffered. Aside from Fiore’s theory that she believed defendant’s behavior was similar to the behavior of her uncle who has schizophrenia, the jury heard no testimony and saw no medical records or other documentary evidence showing defendant suffered from a mental disease or defect. Even if defendant did suffer from a mental disease or defect, no such evidence was presented to the jury and, as this Court stated in Breakiron, “not every mental disease or defect has relevance to the mental states prescribed by the Criminal Code.” Having a mental disease or defect is only half the analysis under N.J.S.A. 2C:4-2 regarding admissibility of evidence of mental illness. Defendants must also “link” their state of mind during the commission of the crime “to an underlying mental disease or defect.” Defendants who do not present such evidence at trial are not entitled to a diminished capacity charge.
As the United States Supreme Court noted in Clark, states can constitutionally prohibit a diminished capacity defense altogether and in doing so may exclude or limit evidence helpful to the defense if the exclusion serves interests like fairness and avoiding misleading the jury. Introducing the diminished capacity charge to a jury without expert testimony on mental diseases and defects, and the ability of those ailments to impact the required mens rea for the offense, would risk confusing the jury.
In this case, defendant exhibited behaviors many might deem unusual, including wearing tin foil on his head and expressing fears that people are out to get him and shooting laser beams at him. Lay testimony from witnesses like Fiore is certainly admissible to describe observations of defendant’s behavior. As we held in Risden, such testimony “requires no expertise” as it comes from “the common understanding and experience of mankind” and “is helpful to an understanding of the witness’s testimony and an appreciation of the mental or emotional state of the person described.” But only an expert can explain whether defendant was suffering from a mental disease or defect and whether that disease or defect affected defendant’s capacity to form the requisite mens rea to commit the charged offense. In order for the jury to be instructed on diminished capacity, expert testimony must be presented. The Appellate Division’s judgment is affirmed.