Blood Brothers
A Forensic Psychology Perspective of the Princeton Murder Case
On February 22, 2025, the murder of one brother by another shattered the tranquility of Princeton, New Jersey. At 11:15 p.m., police responded to the Michelle Mews Apartments near the university campus, where they discovered the body of 27-year-old Joseph Hertgen, a promising investment analyst. His thirty-one-year-old brother Matthew had beaten and stabbed him to death. This siblicide - the killing of one's sibling - would not only devastate a family and shock a community; it immediately raised questions about the complex dynamics of sibling relationships. As more details emerged, especially about the brutal and bizarre nature of the crime, it also raised questions about the possible role of mental illness and how it relates to criminal responsibility in the American justice system.
twenty-seven year old Joseph Hertgen
A Scene of Unspeakable Violence
It was 11:15 p.m. when Mathew Hertgen called 911 to report a fire and a dead body at the Witherspoon Street apartment he shared with his younger brother, Joseph. First responders arrived eight minutes later. They found no active flames but discovered a deceased Joseph. According to the Mercer County Prosecutor's Office, "When officers responded to the scene, they found Matthew Hertgen, who was determined to be the 911 caller, in the residence along with the body of the victim who exhibited various injuries including signs of blunt force trauma and lacerations."
thirty-one-year-old Mathew Hertgen
Joseph Hertgen's body had severe injuries from both blunt and sharp-force trauma. Investigators soon determined the injuries were created by a golf club and a kitchen knife, with the latter still lodged in his torso. Blood spatter analysis indicated the attack had occurred across multiple rooms of the apartment, and drag marks revealed the body had been moved after death. Nearby lay the burned remains of a family cat, with evidence suggesting it had deliberately been set on fire with the use of an accelerant.
Investigators also discovered on Matthew's open laptop a draft Facebook post dated five months (September 2024) earlier containing violent poetry with phrases like "knives sharpening in the dark" and "blood oozing from eyes." Prosecutors would later cite this as evidence of premeditation.
The Bond of Brothers
Mathew and Joseph Hertgen shared several similarities in their impressive backgrounds. Joseph, a University of Michigan graduate and three-time Academic All-Big Ten soccer player had been working as an investment analyst at Red Bank's Locust Point Capital, where colleagues described him as a rising star recently promoted to oversee energy sector portfolios.
Matthew, the older brother at 31, was voted Best All Around in high school. He was a Wesleyan University alumnus and former collegiate soccer player. Court records revealed a 2017 DUI arrest for Matthew that was resolved through pretrial intervention. This arrest was the only noticeable blip in his otherwise stellar credentials. (He received one speeding ticket in 2023).
However, the two brothers have apparently taken drastically different paths since 2018. While Joseph continued flourishing, Matthew seemed to have lost his way. Not only did he have no recent employment listed on public profiles. The public pictures of him as a student-athlete are so different from those he posted shortly before the murder that it's hard to believe it's the same person. It doesn't take a psychologist to see signs of decompensation.
Still, this doesn't answer one of the most pressing questions raised so far in this investigation: why would he kill his brother? I can't answer that question. But I can talk about what the research says.
Understanding Fratricide: When Brothers Kill Brothers
Siblicide (one sibling killing another) is rare; it makes up only 1.5% to 2% of all family-related murders in the United States. However, when it happens, it most often involves brothers. Bowers and Myers (2021) found that over 70% of siblicide cases involved brothers, with male offenders accounting for nearly 88% of perpetrators.
It also happens most often among adults. Most of us assume that sibling violence is primarily a teen phenomenon; a younger brother, tired of being bullied and teased, snaps. Research tells us something very different; the majority of brother-on-brother homicides involve adults, as seen in this case. Indeed, Bartels and Otis (2022) found in their systematic review that the average age of fratricide offenders typically ranges from 30-35 years, with victims averaging slightly younger. The Hertgen case, involving a 31-year-old perpetrator and 26-year-old victim, falls squarely within this statistical pattern.
The Fatal Impact of Sibling Dynamics
The relationship between age differences and violence risk in siblings reveals complex patterns. Research by Kim and Farber (2021) found siblings with age gaps between 1-5 years face the highest risk of serious conflict, representing over 80% of fratricides globally. The Hertgen brothers' 5-year age gap placed them in this high-risk category.
What's particularly notable about adult fratricide is how the power dynamics often persist from childhood. Traditionally, older brothers may maintain psychological dominance even into adulthood, though the physical advantages that define childhood conflicts may diminish.
What Drives a Brother to Kill?
The catalysts for fraternal homicide vary. Research suggests that financial disputes trigger approximately 25% of fratricides. In economically unequal situations - what researchers call "relative deprivation" - one sibling can perceive that he has an unfair disadvantage over the other. When the two live together, particularly when one is financially dependent on the other or when there's a significant discrepancy in success, tensions can escalate rapidly. What begins as resentment can transform into rage, especially when exacerbated by other factors such as mental health issues or substance abuse.
Approximately 30% of fratricide offenders show overt symptoms of a diagnosable mental health condition, with schizophrenia, psychosis, and severe depression being the most prevalent. Padovani et al.'s systematic review found that psychotic disorders, in particular, significantly increased the risk of violence toward family members.
In the Hertgen case, the discovery of disturbing poetry months before the killing containing phrases like "knives sharpening in the dark" raises questions about Matthew's mental state. When he appeared in court wearing an anti-suicide smock, it signaled ongoing concerns about his psychological condition; at least one source has reported that he has attempted suicide since his arrest. Alcohol and substance abuse also play a direct role in over 60% of fratricides, heightening aggression and reducing impulse control, as documented by Messing and Amanor-Boadu (2022).
The Insanity Defense
The marked changes in Mathew Hertgen's appearance over the past five years. The bizarre nature of the crime. The disturbing poems and pictures he posted on social media. All of these have sparked much speculation about the possibility of an insanity defense. Let's see what that road might look like if his defense team travels down it.
The Reality vs. Public Perception of the Insanity Defense
It's an uphill road to travel. Despite common beliefs that it's a popular "get out of jail free" card, the insanity defense is neither common nor particularly successful. It's raised in less than 1% of felony cases nationwide, and according to the Administrative Office of the Courts (2024), this is also true in New Jersey. The dismal success rate is particularly telling. In New Jersey, only about 10% of insanity pleas result in "not guilty by reason of insanity" (NGRI) verdicts, a success rate that is significantly lower than the national average. This low percentage is likely due to New Jersey's stringent legal requirements.
New Jersey's Legal Standard for Insanity
New Jersey's insanity defense is anchored in the M'Naghten Rule, codified under N.J.S.A. 2C:4-1. This standard states that a person is not criminally responsible if, due to a "disease of the mind," they either:
Did not comprehend the nature and quality of their act, or
Knew the act's nature but could not perceive its wrongfulness
This narrowly defined test focuses exclusively on the defendant's understanding and perception at the precise moment of the crime rather than emotional states or impulse control issues. While prosecutors typically have to prove guilt in a criminal case, the burden of proof in an insanity plea rests with the defense, which must demonstrate insanity by a preponderance of evidence (more likely than not).
The key word here is "narrow." The law doesn't consider whether someone had general mental health struggles or even a diagnosed condition. What matters legally is whether, at the precise moment of the crime, their illness prevented them from either understanding what they were doing or knowing it was wrong. Practically speaking, New Jersey courts have consistently interpreted "disease of the mind" to refer only to severe psychotic disorders that fundamentally impair reality testing, explicitly excluding personality disorders, drug-induced conditions, or temporary states of extreme emotion. Evidence of planning, attempts to conceal the crime, or post-offense behavior indicating awareness of wrongdoing typically undermines insanity claims, as these suggest the defendant understood the nature of their actions and recognized their wrongfulness.
Building an Insanity Defense in the Hertgen Case
The second prong of New Jersey's M'Naghten standard—that a mental disease prevented the defendant from knowing what they were doing was wrong—focuses specifically on moral awareness. This prong means that even if Matthew Hertgen fully understood the physical nature of his actions (that he was attacking his brother with lethal weapons), he could still qualify for an insanity defense if his mental illness prevented him from recognizing the wrongfulness of those actions.
This prong is particularly relevant in cases involving psychotic delusions. For instance, if Matthew suffered from paranoid delusions causing him to genuinely believe that his brother posed an imminent, existential threat (perhaps believing Joseph was plotting to kill him or had been replaced by an impostor), he might have understood he was inflicting harm but believed his actions were morally justified or necessary for self-protection. The violent poetry found on Matthew's laptop could be interpreted in two opposing ways: prosecutors might present it as evidence of premeditation and awareness of wrongdoing, while the defense could argue it demonstrates a deteriorating mental state with increasingly paranoid or delusional thinking.
For a successful insanity defense under this prong, Matthew's attorneys would need to demonstrate through psychiatric testimony and supporting evidence that, at the precise moment of the killing, his perception of right and wrong was so profoundly distorted by mental illness that he could not comprehend the moral wrongfulness of killing his brother. Looking at cases where NGRI pleas have been successful, this would likely involve:
1. Establishing Pre-existing Mental Illness
The foundation of any insanity claim is documenting a legitimate, severe mental illness that predates the crime. Heilbrun et al. (2021) emphasize that courts give substantially more weight to mental health conditions documented before the offense occurred. Matthew's disturbing poetry might suggest psychological disturbance, but successful insanity defenses typically require more substantial evidence:
Prior psychiatric hospitalizations
History of prescribed antipsychotic medications
Consistent therapy records documenting symptoms of severe mental illness
Contemporary witness accounts of bizarre behavior or delusional thinking
Without such history, courts often view insanity claims skeptically, particularly in cases involving extreme violence.
2. Connecting Mental Illness to the Legal Standards
Even if a defense team can establish serious mental illness, they would face the steeper challenge of proving his condition met the specific M'Naghten criteria at the time of the killing. This point is where many insanity defenses falter. It's not enough to have a diagnosis. The defense must demonstrate that the specific symptoms of that illness directly caused the defendant to either not understand they were killing someone or not recognize that killing is wrong.
Here's a hypothetical (and obvious) example of how this might work:
Consider a case where a defendant diagnosed with paranoid schizophrenia develops a specific delusion that his brother has been replaced by an alien impostor who is poisoning his food to harvest his organs. The delusion manifests in several documentable ways: the defendant begins refusing to eat food prepared by his brother, installs hidden cameras in the kitchen, and keeps detailed journals documenting the "evidence" of his brother's "impostor status" (misinterpreting normal behaviors as sinister). The defendant might report to his therapist that he hears voices warning him, "he'll kill you if you don't act first," and show increasing agitation and fear around his brother.
If this defendant then attacked his brother during an acute psychotic episode, believing with absolute conviction that he was defending himself against an imminent threat, he might meet the second prong of the M'Naghten standard. Even though he knew he was physically harming another person (understanding the nature of his act), his paranoid delusion prevented him from knowing his action was morally wrong because his distorted reality convinced him he was engaging in necessary self-preservation against an alien threat planning to kill him.
For this defense to succeed, psychiatric experts would need to establish several key elements: that the delusion existed before the violence (documented in therapy notes, psychiatric hospitalizations, or witness accounts), that it directly motivated the violent act (not just a general condition but the specific reason for the attack), and that the delusion was so firmly fixed that it completely overrode the defendant's ability to distinguish right from wrong in that specific context. Medical records showing consistent treatment for paranoid schizophrenia, toxicology reports ruling out substance-induced psychosis, and contemporaneous evidence of delusional beliefs would all strengthen this defense by demonstrating that the defendant was genuinely unable to recognize the wrongfulness of his actions due to a severe mental disease.
The Consequences of an NGRI Verdict
Even if Matthew Hertgen were to be found not guilty by reason of insanity, he would not be free. Individuals found NGRI often spend more time institutionalized than they would have spent behind bars and under conditions that can be equally restrictive.
The Bottom Line
The investigation into the murder of Joseph Hertgen has just begun. Facts may change. More information will come to light. I wrote this article not to speculate about motives, diagnoses, or outcomes but to apply a forensic psychology lens to some of the issues currently being bantered about.
But there are some things forensic psychology can't do. Describe what this family is going through. Take their pain away. Undo what's already been done.
I wish we could.
References
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