Defendants may still testify and call witnesses about observed behavior, but lay evidence alone cannot put either defense before a jury.
MORRISTOWN, NJ – The New Jersey Supreme Court ruled Aug. 3, 2026, that criminal defendants must present expert testimony before asserting insanity or diminished capacity and receiving the corresponding jury instruction.
The unanimous decisions in State v. Arrington and State v. Mustafa establish a statewide evidentiary threshold for two defenses involving a defendant’s mental condition at the time of an alleged offense.
A defendant’s testimony or testimony from relatives and other lay witnesses is not enough by itself. A qualified expert must identify a mental disease or defect and explain how it affected the defendant’s mental state when the conduct occurred.
The decisions leave defendants free to offer otherwise admissible testimony about unusual statements or behavior. Such evidence may complement an expert’s opinion, the court said, but cannot substitute for one.
What an expert must establish
Insanity and diminished capacity serve different purposes under New Jersey law.
Insanity is an affirmative defense. Under state law, a defendant must prove by a preponderance of the evidence that a disease of the mind caused a defect of reason so substantial that the defendant did not understand the nature and quality of the act or did not know it was wrong.
Writing for the court in Arrington, Justice Michael Noriega Hoffman said determining whether a person had a disease of the mind, and what effect it had during the offense, requires knowledge outside an ordinary juror’s experience.
A qualified expert must therefore diagnose or identify the disease and connect it to at least one part of the legal insanity test. Evidence that a defendant behaved irrationally, used drugs or had previously received mental-health treatment does not establish legal insanity without that connection.
Diminished capacity is a failure-of-proof defense. Under N.J.S.A. 2C:4-2, evidence of a mental disease or defect may be used to show that a defendant lacked the state of mind required for a charged offense. The prosecution retains the burden of proving that state of mind beyond a reasonable doubt.
In Mustafa, Justice Fabiana Pierre-Louis wrote that an expert must explain that the identified condition could affect the defendant’s cognitive faculties and did affect the ability to form the required intent at the time of the offense. A diagnosis standing alone is insufficient.
The rulings concern criminal responsibility at the time of an offense. Competency to stand trial, which asks whether a defendant currently understands the proceedings and can assist counsel, is a separate legal question.
Lay testimony remains admissible
The court declined to bar testimony from defendants, relatives or other witnesses who personally observed relevant conduct.
Under New Jersey’s evidence rules, a lay witness may describe what the person saw or heard. A witness may also offer a limited opinion if it is based on personal perception and would help the jury understand the testimony.
That could include descriptions of disorganized speech, apparent paranoia, unusual beliefs or abrupt changes in behavior. The defendant may also testify, subject to the ordinary rules governing relevance and admissibility.
The court held, however, that jurors cannot determine from those observations alone whether a defendant had a legally recognized mental disease or defect and how it affected criminal responsibility or intent.
Arrington sought to testify about his condition after his defense did not produce an expert who would support an insanity claim. Mustafa relied on testimony from a former girlfriend who described conspiratorial beliefs and erratic behavior, but he presented no expert or medical evidence connecting that behavior to his ability to act purposely or knowingly.
The Supreme Court affirmed the rulings that kept the requested defenses and instructions from the juries.
How indigent defendants obtain experts
New Jersey law recognizes that an indigent defendant may need publicly funded services beyond an attorney, including expert witnesses.
The Public Defender Act directs the Office of the Public Defender to provide necessary investigative and preparatory services. Courts have said those services can include experts needed for an adequate defense, although funding is subject to approval.
In State v. Carter, the Appellate Division explained that the Public Defender may consider whether the requested service relates to an issue in dispute, helps answer the prosecution’s evidence and provides sufficient value when weighed against budget limits.
The office’s July 2026 pool-attorney guidelines say mental-health experts are available for cases involving insanity, diminished capacity or competency. Assigned counsel must obtain management approval before an expert begins work, use an approved state vendor and submit a detailed cost estimate.
The new decisions make that process central to whether an indigent defendant can place either defense before a jury. Public funding is available, but it is neither automatic nor unlimited.
The American Civil Liberties Union of New Jersey, which participated as a friend of the court, had argued in Arrington and Mustafa that a categorical expert requirement could prevent defendants without access to an expert from presenting otherwise relevant mental-health evidence. The Supreme Court rejected that position, concluding that the medical determinations required by the statutes cannot reliably be made from lay testimony alone.
Effect on pending cases
Neither opinion establishes a transition period, directs courts to appoint experts or guarantees additional preparation time in pending cases.
A pending case is not automatically dismissed or delayed because the defense has not retained an expert. The practical effect is that a defendant seeking an insanity or diminished-capacity instruction must place admissible expert testimony in the record. Without it, the judge cannot submit that defense to the jury.
Defendants may still offer admissible lay evidence for other relevant purposes. Whether a court permits a late expert, extends discovery or changes a trial date will depend on the posture of the individual case and existing court rules.
The rulings apply statewide and affirm the lower-court decisions denying the requested jury instructions in both cases.