New Jersey Supreme Court Says Expert Mental Health Testimony Is Required for Insanity and Diminished Capacity Defenses

The New Jersey Supreme Court has issued a major decision that will shape how mental health defences are handled in future criminal cases across the state. In two unanimous rulings, the court made it clear that defendants cannot rely on an insanity defence or a diminished-capacity defence unless they present testimony from a qualified mental health expert.

The decision reinforces the importance of expert medical evidence in criminal trials where a defendant argues that a mental illness affected their actions. The court said that without professional testimony, juries could be left to make decisions based on assumptions instead of reliable medical evidence.

The rulings came from two separate murder cases involving William Arrington and Kader Mustafa, both of whom are serving lengthy prison sentences after being convicted of murder.

In the first case, William Arrington challenged his conviction after he was prevented from presenting an insanity defence during his trial. Prosecutors said Arrington carried out a deadly attack inside a Newark apartment in November 2016 after becoming angry over a Facebook comment made by the daughter of his former girlfriend.

According to court records, Arrington forced his way into the apartment, restrained several people, and attacked them with a knife. Two children were fatally stabbed, while a 23-year-old woman died after being shot in the head. The violent incident led to multiple murder convictions, and Arrington ultimately received a sentence totalling 375 years in prison.

Before trial, Arrington’s legal team hired a psychologist who diagnosed him with several mental health conditions, including bipolar disorder and schizophrenia. However, a psychiatrist working for the state reached a different conclusion after evaluating him.

The state’s psychiatrist testified that Arrington appeared to be pretending to have mental illness. That opinion was supported in part by a jail record indicating that Arrington had allegedly admitted to a mental health counsellor that he was exaggerating his symptoms for use in court proceedings.

After reviewing the evidence during a competency hearing, the trial judge ruled that Arrington was mentally competent to stand trial. Because his attorneys could not secure an expert willing to support an insanity defence, the judge also barred that defence from being presented before the jury.

Arrington later argued that he should have been allowed to personally testify that he did not understand his actions were wrong because of his mental condition. He claimed preventing him from making that argument violated his constitutional right to present a complete defence.

Both the Appellate Division and the New Jersey Supreme Court rejected that argument.

The state’s highest court explained that determining whether someone suffered from a qualifying mental disease at the time of a crime is a medical question that requires specialised knowledge beyond the understanding of ordinary witnesses. The justices concluded that expert testimony is necessary to establish the legal requirements for an insanity defence.

The second case involved Kader Mustafa, who was convicted of first-degree murder following a fatal road rage shooting in Monmouth County in May 2018.

Authorities said the incident began after another driver, Sciasia Calhoun, used her vehicle’s high-beam headlights. Investigators said Mustafa allowed her vehicle to pass before accelerating, driving aggressively, flashing his own high beams, and eventually firing multiple shots through his vehicle’s window. Calhoun was struck in the head and later died from her injuries.

Mustafa was convicted of murder, weapons offences, and additional criminal charges. Although his attorneys initially considered using an insanity defence, Mustafa later withdrew that strategy and instead argued that he lacked the mental capacity required to commit the crime because of a mental disorder.

During the trial, family members and other witnesses described unusual behaviour and concerns about Mustafa’s mental health before the shooting. His former girlfriend testified that he often believed people were following him and compared his behaviour to that of a relative diagnosed with schizophrenia. She also testified that Mustafa had taken multiple doses of Adderall within a short period on the night of the shooting.

His attorneys argued that testimony from people who observed his behaviour should have been enough to support a diminished-capacity defence, even without a formal medical diagnosis from an expert.

The New Jersey Supreme Court disagreed.

The justices ruled that diminished-capacity claims, like insanity defences, must also be supported by qualified expert testimony. The court said mental health conditions that may affect criminal responsibility involve complex medical issues that require evaluation by trained professionals.

According to the court, allowing these defences without expert evidence could encourage verdicts based on speculation instead of scientifically supported findings. Requiring expert testimony helps ensure that juries receive reliable information when deciding whether a defendant’s mental condition affected legal responsibility for a crime.

Legal experts say the ruling provides clearer guidance for prosecutors, defence attorneys, and trial judges throughout New Jersey. Defendants who wish to argue that mental illness prevented them from understanding their actions or forming criminal intent will now need credible testimony from licensed mental health professionals to support those claims.

The decision does not change New Jersey’s insanity law or eliminate mental health defences. Instead, it establishes that these defences must be backed by qualified expert evidence before they can be presented to a jury.

Because the ruling came from the state’s highest court, it will serve as binding precedent for lower courts handling future criminal cases involving insanity or diminished-capacity claims. Defence attorneys will need to secure appropriate expert evaluations early in criminal proceedings if they intend to raise either defence.

The decision is expected to have a lasting impact on criminal trials across New Jersey by reinforcing the role of medical expertise in cases involving questions of mental illness and criminal responsibility.

Sources

• New Jersey Supreme Court
• New Jersey Judiciary (Official Court Opinions