On August 3, 2026, the New Jersey Supreme Court decided the Essex County case of State v. Jeremy Arrington. The principal issue under N.J.S.A. 2C:4-1 was whether a defendant could lay a foundation for an insanity defense without expert testimony.
Justice Hoffman wrote for a unanimous Court in relevant part: Ultimately, the insanity defense implicates complex and esoteric concepts that fall outside the common experience of jurors. Requiring an expert to translate such concepts enables the jury to draw “meaningful inferences” about a defendant’s “disease of the mind” and how such a disease affected defendant’s reasoning. We recognize — as Arrington has argued and as our above overview of the history of the defense has shown — that expert testimony was not always required to assert the insanity defense. However, from the time the M’Naghten Rule was established, such testimony was contemplated, and it has gained increasing traction and importance along with our improved understanding of the complexities of mental disorders. See Ake, 470 U.S. at 80. We now hold that it has become indispensable.
With respect to Arrington’s case, we conclude that even if we did not require expert testimony to support the insanity defense as a bright line rule, the limited evidence Arrington intended to proffer for the insanity defense was insufficient to warrant a jury instruction on the defense. Therefore, the trial court did not violate Arrington’s constitutional rights in precluding him from asserting such a defense absent support from an expert.
The record is devoid of evidence indicating how Arrington intended to prove that he was suffering from a “disease of the mind” that materially and contemporaneously impacted his reasoning. Arrington suggested that he would rely upon his own testimony and the State’s evidence to assert that he “would not have been able to engage in this type of conduct” — namely, the crimes committed at the apartment — “unless he . . . did not know the difference between right and wrong” at the time of the alleged attack. However, the mere fact of a gruesome crime has no determinative bearing on whether Arrington was suffering from a “disease of the mind,” or on Arrington’s ability to appreciate the nature and quality of his actions or to distinguish right from wrong at the operative time. The evidence of the brutal murders perpetrated on the victims . . . would not seem to raise any question of sanity unless one were to adopt the dubious doctrine that no one in his right mind would commit a murder. Accepting such reasoning would set a dangerous precedent, where a jury might determine a defendant satisfies the statutory requirements of insanity merely from the recitation of a crime’s heinous nature.
Rather, the record reflects that Arrington might have been faking symptoms of mental illness — making expert testimony even more critical. In a note authored by an advanced practice nurse who evaluated Arrington, the nurse described how Arrington informed a mental health counselor that he was, in fact, faking mental illness “so it would be in his records for when he goes to court.” Considering that lay people lack the specialized knowledge required to identify malingering, only an expert opinion could have confirmed or negated the possibility of such a diagnosis in the context of insanity.
In requiring an expert opinion to support the insanity defense, we do not negate the well-established principle that a lay witness with personal knowledge may also testify with respect to the insanity defense. Although Arrington argues, based on Risden, that lay testimony is independently sufficient, in that case an expert also testified in support of defendant’s insanity defense. The Appellate Division recognized the significance of lay testimony but did not permit the jury to rely exclusively on lay opinion when determining whether defendant suffered from a “disease of the mind” at the relevant time. Although lay testimony as to insanity might be admissible, it is unlikely in the extreme that exclusive reliance would ever be placed on it.
Ake also affirmed that lay witnesses may “describe symptoms they believe might be relevant to the defendant’s mental state.” And many M’Naghten jurisdictions similarly allow lay witnesses to make observations about a defendant’s conduct but reserve the diagnosis of a mental illness for medical experts. See State v. Davis (N.C. 1998) (allowing a nurse to report “general observation[s]” about the defendant but not make a “specific psychiatric diagnosis”); State v. Raine (Mo. Ct. App. 1992) (permitting the defendant’s family members to describe defendant’s behavior but not give an opinion concerning defendant’s “mental disease or defect”); Doyle v. State (Okla. Crim. App. 1989) (permitting the defendant’s sister to testify to defendant’s conduct but not his “psychological problems”); Fortune, 302 A.3d at 787 (allowing lay witnesses to “state their observations of [the defendant’s] behavior and mental state before and after” the crime but not to testify as to defendant’s state of mind at the time of the crime); White v. Commonwealth (Va. Ct. App. 2005) (observing that lay witnesses may recite defendant’s “observed behavior” but may not “express an opinion as to the existence of a particular mental disease or condition” (quoting Mullis v. Commonwealth, (Va. Ct. App. 1987))) Thus, if Arrington had presented acceptable expert testimony on his alleged insanity, lay witnesses or Arrington himself may have been able to provide evidence complementary to the requisite expert testimony, as long as such evidence met all relevant evidentiary rules.
Arrington further contends that, unlike the competency statute, the insanity defense statute does not expressly contemplate the use of an expert. However, this argument overlooks the development of the insanity defense statute as distinct from the competency statute. As we explained in State v. Gorthy, “the court decides whether a competency hearing is required.” (2016). Thus, the competency statute authorizes the court to appoint experts “who report to the court regarding the defendant’s condition and . . . ability to understand and participate in the legal process” in order to assist the court in making an informed decision. Under the previous statutory framework, the court was also empowered to conduct a hearing to determine the defendant’s sanity. The Legislature, in distinguishing the competency statute from the insanity statute, withdrew the court’s authority to investigate a defendant’s sanity prior to trial, thereby placing the responsibility for raising such an issue solely within the defendant’s “autonomy.” Although the insanity defense statute thus recognizes “the defendant’s independent choice” regarding “trial strategy,” it does not obviate the need for expert testimony. Ibid. The court’s role as gatekeeper persists, requiring judges to evaluate the reliability of evidence before it is presented to the jury.
Insanity is an affirmative defense in New Jersey and 45 other states. That means that the defense has the burden to prove it by a preponderance of the evidence. Affirmative defenses are exceptions to the general rule that the prosecution bears the burden of proof beyond a reasonable doubt and the defense is not required to prove anything. The role of the defense is to raise reasonable doubt. Four states do not allow an insanity defense, although they allow evidence of mental disease or defect in various forms to negate the required criminal mind state of an offense. Those four states are: Kansas, Montana, Utah, and Idaho.
Here, where Arrington planned to offer no evidence — beyond his own self-serving testimony — the trial court properly exercised its gatekeeping function in finding Arrington’s lack of evidence insufficient to raise the insanity defense, while maintaining his right to testify as to his state of mind or version of events. Though Arrington contends that the trial court’s decision to prevent him from advancing an insanity defense improperly infringed upon his constitutional rights, it is well settled that a defendant’s right to the defense of his choice is not absolute.
A defendant’s own testimony, or that of a lay witness alone, is insufficient to advance the insanity defense. A qualified expert must credibly diagnose a “disease of the mind” and elucidate its impact on the defendant’s behavior at the time of the offense. Such a requirement aligns with the language of the insanity defense statute; follows precedent interpreting the applicable evidentiary rule in New Jersey as well as precedent from other jurisdictions, including the United States Supreme Court; assists the jury in resolving a complex and esoteric issue that exceeds the common understanding of a lay person; and ultimately safeguards the reliability and the legitimacy of the insanity defense. Here, defendant’s proffered testimony failed to satisfy either that standard or the rule set forth over 140 years ago in Graves, 45 N.J.L. at 350, 359-60, that testimony of a defendant alone cannot suffice to support the insanity defense. Accordingly, we affirm the decision of the Appellate Division.
