As US reels from multiple mass shootings in California, can loneliness be a trigger for violence?

There is a loneliness epidemic in the United States -- and experts told ABC News it may be triggering violence.

In California, there have been three shootings in as many days, tied to a perpetrator who may have exhibited signs of social isolation and/or violent behavior, according to authorities.

In Monterey Park, police documents revealed the 72-year-old suspect had been divorced from his wife since 2006, lived alone in Hemet -- about 30 miles Southeast of Riverside -- and was angry and resentful.

A former tenant and longtime acquaintance of the shooter, who wished to remain anonymous, told ABC News that he liked to dance but that he didn't have many friends at either of the dance studios he allegedly targeted.

The suspect "distrusted everyone," the acquaintance said, adding, "I wouldn't say he was aggressive, but he just couldn't get along well with people."

In the Half Moon Bay shooting, the man accused of killing seven farmworkers had a history of making threats after losing his job at a restaurant, according to ABC News-owned KGO.

According to court documents, a former coworker and roommate filed a restraining order against the suspect after he allegedly threatened to kill him. The suspect allegedly tried to suffocate him by putting a pillow over his face if he didn't help the suspect get his job back.

Experts said although there is not a lot of research on isolation, there appears to be a link between loneliness and violence, experts said.

"Clearly isolation and loneliness are at play in a lot of violence," Dr. Edwin Fisher, a psychologist and professor in the department of health behavior at Gillings School of Global Public Health at the University of North Carolina, told ABC News. "They may be important red flags for us to recognize and trying to help people who are prone to violence."

Fisher said there are many types of violence that social isolation and loneliness are related to, including sexually soliciting minors online, intimate partner violence, cyber bullying -- and homicide.

"Grievance and perception of self as a victim, I think both of those are present in the mass murders in California these past few days," he said. "So, in addition to being socially isolated and feeling lonely, feeling grievance, feeling victimized, I'm going to finally pay them back, may be really important in some kinds of violence."

Loneliness epidemic among men

Loneliness perpetrating violence may be affecting American men more than women due to males suffering a "friendship recession."

According to data from the American Enterprise Institute's Survey Center on American Life in 2021, the percentage of men reporting at least six close friends declined by half from 55% in 1990 to 27% in 2021.

The percentage of men reporting no close friends rose five-fold from 3% in 1990 to 15% in 2021. What's more, one in five single men say they don't have any close friends.

On the other hand, women are much more likely to report having close friends and relying on those friends for emotional support, the survey found.

According to the Centers for Disease Control and Prevention, men had a suicide rate four times higher than women in 2020. Men make up 49% of the U.S. population but nearly 80% of suicides, CDC data shows.

"One could speculate that there's something that ties male loneliness to violence," Dr. Nathaniel Glasser, a research fellow and clinical instructor of medicine at the University of Chicago Medicine, told ABC News. "That in situations where boys and men feel loneliness, they -- not always, but occasionally, arguably too frequently -- turn to violent mechanisms to reconcile or cope or otherwise deal with their loneliness."

Glasser pointed to ads from a firearm manufacturer for an assault rifle, one of which read, "consider your man card reissued."

"That's exactly the language that some gun manufacturers use -- or have used -- to speak to men, saying that guns are a way for males trying to reclaim some kind of image of masculinity to do so," he added.

Dr. Elizabeth Tung, a social epidemiologist and assistant professor of medicine at the University of Chicago, agreed.

"There is just so much speculation, but I think there's also like this threat of societal emasculation and men who are lonely and emasculated trying to recapture their masculinity in some ways," she told ABC News.

Experts, such as Dr. Niobe Way, believe the rise of male loneliness may be a factor in the rise of violence.

Way, a professor of developmental psychology at New York University Steinhardt School of Culture, Education, and Human Development, has studied boys through adolescence. She said when they're younger, around age 12 or 13, they talk about wanting close friendships and emotional intimacy.

However, as they get older and start experiencing expectations of traditional masculinity, "they start disconnecting form their desires and feeling much more isolated," she told ABC News.

"The disconnection, the loss that no one seems to care that our boys are feeling isolated," Way, author of "Deep Secrets: Boys' Friendships and the Crisis of Connection," said. "They start to disconnect from their own humanity because they're not able to find the relationships they want, and many are depressed and angry about it.

Violence also leading to loneliness

Tung said the opposite can be true as well, meaning being exposed to violence can also lead to loneliness.

A study she co-authored in 2019 found that adults living in Chicago neighborhoods with high rates of violent crime are more likely to be lonely than the average American.

"We found was that there's a really strong relationship between any kind of exposure to violence, whether it's direct or indirect police violence, community violence, and being more isolated, as well as being lonelier," she told ABC News. 'It's interesting, because the state of isolation and loneliness in the U.S. is already much higher than it was 50 years ago … and so the fact that violence exposure is associated with an even greater augmentation of that statistic is pretty alarming."

In this case, the theory is that community violence increases distrust and suspicion, leading to further isolation and loneliness.

Access to guns

This is all coupled with widespread access to guns in the United States, experts said.

A Pew Research Center survey in 2021 found that four in 10 adults in the U.S. live in a household with a gun, while 30% said they own a gun.

Federal data suggests gun sales have spiked, especially during the COVID-19 pandemic. On top of that, just 21 states and the District of Columbia require background checks on sales of some or all types of firearms, according to Everytown for Gun Safety.

Fisher said he is currently writing a chapter on social isolation and loneliness for the American Psychological Association, and one of the findings show the impotence of gun control.

"One of the findings of the chapter is how global the research is studies from Asia, Africa, Europe, North and South America, all over the world, social isolation and loneliness are problems," he said. "Only in the United States are they leading to many mass murders."

Solving the problem

So how do we combat the problem of loneliness? The experts say there are a few ideas.

"There's a lot of research about really pushing this idea of social prescribing," said Tung. "Helping older people make more social connections, we can prescribe like somebody going to a senior center and joining some kind of community activity."

"But there is a broader cultural thing going on, especially with technology, and if more and more people are spending all of their time connecting through technology rather than in person, I worry that the idea of social prescribing or health care prescriptions for social activities is a little too simplistic."

There's also making sure that people are having robust and varied social interactions that might help pump the brakes on exhibitions of violent behavior, experts said.

"So, when we're talking about people who are isolated, we want to try to be thinking in terms of encouraging varied social connections, and healthy social connections, if you will, as opposed to the community of, you know, child abusers that they're connected with on the web," Fisher said.

He also added that being kind can be just as important as a long talk.

"A really important point is that social connection does not necessarily need to be, deep, intimate conversations over hours and hours," he said. "Casual contacts can be very important in just making us feel connected. So, if you see people who seem to be lonely, who seem perhaps a little bit brittle or disgruntled, kind words can do a lot. It's far from a solution, but it helps."