Three men who lived in their family homes well into adulthood.

And three knives allegedly used in brutal killings.

There are stark parallels among three Bergen County killings, each of which occurred in the last 14 months. But the deeper question of what drove Jesus Lopez of Paramus, Eric Kaplan of Fair Lawn and Pawel Boduch of Englewood to allegedly kill those closest to them remains difficult to answer.

There are a few theories, including one that examines the men’s youth, including how their families treated them, or if their childhood friends teased them for being different. And a suspicion that such prolonged cohabitation between parents and children can sometimes lead to tragedy.

"Very often they get into arguments," said Louis Schlesinger, a professor of forensic psychology at John Jay College of Criminal Justice in Manhattan. "It's anger. Uncontrolled anger. And if they have some sort of very severe mental disorder, the controls are weakened. They're prone to outbursts, they're irritable ... and they take it out on the people that are right there."

Domestic violence is hardly rare.

The National Domestic Violence Hotline, an organization dedicated to helping survivors, estimates that 12 million American women and men are victims of rape, physical violence or stalking by an intimate partner every year. When Paterson's homicide rate spiked in 2017, officials blamed five lethal domestic violence stabbings that happened while police were focused on fighting street crime.

Less common is this particular brand of domestic violence, in which a mentally disturbed adult assaults or kills a parent.

So what drove Boduch, 43, to allegedly murder his parents, Edward and Miroslawa, last week? What prompted Lopez to slit the throat of his mother, Susana Lopez, as police charged? What caused Eric Kaplan to allegedly push a knife through the neck of Ann Kaplan, his 64-year-old mother, as she called 911?

Giuseppe Fazari, a criminal justice professor at Seton Hall University, said adults who express themselves through violence often suffered similar trauma when they were young and impressionable. The process, which Fazari called "brutalization," can start at the hands of parents or peers and lead to ferocious outbursts decades down the line.

"That becomes the method through which they resolve conflict — aggression and violence," Fazari said. "In some instances it's a protracted process. In other instances, it's a more abbreviated process. But it's a process nonetheless."

And for the mentally ill who are non-communicative, violence may be the only way they know how to interact, said Anthony Gangi, the host of "Tier Talk," a podcast about the corrections field.

"It's like a child," Gangi said. "When you have a child that's developing, they don't know how to reason. So they get frustrated. They cry. They get angry. Because they're not being heard.”

Attorneys for the accused would not say this week if any of those theories applied to their specific clients.

But John Bruno, the Rutherford attorney who represents Lopez, said he planned to argue in court that Lopez is insane. A state Superior Court judge could decide this summer whether that defense has merit or if the case should move to trial, Bruno said. If the judge deems Lopez insane at the time of the crime, he'll be indefinitely committed to a psychiatric facility.

"They're all very sad and tragic cases, every one of them," Bruno said of cases like his client’s.

The Bergen County Prosecutor's Office declined to comment, as did Paul Brickfield, the River Edge attorney representing Kaplan.

Ilene McFarland, the public defender representing Boduch, who pleaded not guilty on Friday, did not respond to a request for comment.

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Details are still scarce about why Edward Boduch, 71, and Miroslawa Potocka, 72, were killed in their Lafayette Place home last week. Authorities say they found a bloody knife wrapped in bloody clothing in Boduch's bedroom when they found the bodies Feb. 16. But a search of Boduch’s history did not reveal any major run-ins with the law.

Lopez and Kaplan, however, walked a troubled path that was documented extensively by local police.

Lopez was 30 years old when he is alleged to have killed his mother with a boxcutter and then hidden her body in a detached garage in October 2017. Her remains were found two weeks later by Paramus police during a welfare check.

But Lopez's legal problems began much earlier — his past was littered with arrests, and included charges of threatening a police officer, marijuana possession and driving without a license. Police visited his Spring Valley Road home often, and neighbors said he and his mother fought savagely. Lopez was committed to a county psychiatric hospital at least five times.

Kaplan, now 32, was similarly troubled. But he appeared more violent — in 2014, he stabbed his brother in the chest with a Swiss Army knife, then told police he had fallen on a piece of glass, according to police records.

Authorities charged Kaplan with aggravated assault, among other things. But the Prosecutor's Office let him enroll in a pretrial intervention program that lets first-time offenders avoid their charges if they don't get arrested again for a certain length of time.

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Four years later, authorities said he stabbed Ann Kaplan in the neck, killing her. Fair Lawn police said they found him locked in the bathroom, splattered with blood.

In another time, each of these men might have been locked away in an asylum, experts say.

But the decades-long movement toward deinstitutionalization, which shifts the care of the mentally ill from public hospitals to community-based services, means there are fewer available public hospital beds. The number has dropped by more than 96 percent since its peak in the 1950s, according a 2016 report from the Arlington-based Treatment Advocacy Center.

Schlesinger, the John Jay professor, said he was not surprised knives were used in all three instances. Every kitchen has one, and homes in which mentally ill people live rarely have guns, he said. But he was careful to point out that living with a person stricken by mental illness does not automatically put family members' lives at risk.

"Because a person has a mental illness does not mean they're going to be homicidal," Schlesinger said. "The majority of people who kill are not mentally ill ... These are rare events."

Email: janoski@northjersey.com