3 mass shootings strike California in 48 hours: Here's why high-profile gun violence spreads like contagion

• 29 people were killed in two California mass shootings over 48 hours.
• Research shows that high-profile mass shootings spread like contagion.
• While motives haven't been named, one mass shooting increases the likelihood of another.

It's happened again.

A news cycle focusing on the loss of life in one high-profile mass shooting was interrupted with the news of another.

On January 21, a 72-year-old man stormed into a Lunar New Year celebration at the Star Dance Studio in Monterey Park, opening fire. Eleven people were killed as a result of the shooting, and nine were injured. The suspected shooter then ended his own life, also with a gun. 

On Monday, another seven people were killed in two shootings in Half Moon Bay, California — about 380 miles from Saturday's rampage. 

California Gov. Gavin Newsom was in the hospital meeting victims from Monterey Park when he was pulled aside and briefed on the next attack. 

"It is a disease, violence, and it is contagious," Dr. Gary Slutkin, former head of the World Health Organization's Intervention Development Unit, told Insider after a previous spate of mass shootings in 2021. 

Law enforcement have not identified motives in either Monterey Park or Half Moon Bay attacks, but people who frequented the dance studio where the first attack took place told the LA Times that the shooter was disturbed and may have lashed out over jealousy.

While the attacks were not connected — nor were they connected to a third mass shooting in Oakland on Monday night in which one person was killed and seven were injured — research shows that back-to-back rage killings are not uncommon.

"One event is a risk factor for another event, just like one event of COVID is a risk factor for another event of COVID," Slutkin told Insider at the time.

Slutkin told Insider that the more people see violence and take it in as "normal" the more likely they are to commit violence themselves.

Dr. Jonathan M. Metzl, the Director of the Center for Medicine, Health, and Society at Vanderbilt University, Nashville, echoed Slutkin.

"We know historically there is a strong copycat phenomenon with high-profile mass shootings," he said in 2021. "And so when there's one in the news it tends to spur a number of copycat events, so people feel a contagion effect. One leads to another, leads to another." 

Both experts were speaking to Insider when the country was reeling from a month of particularly high-profile mass shootings.

On March 16, 2021, eight people, including six Asian women, were killed at several spas in metro Atlanta by a man armed with an AR-15. Two weeks later, a gunman with the same weapon killed 10 people at a Colorado grocery store.

On on April 15, another man carrying an rifle killed eight people and injured several others at a FedEx facility near Indianapolis International Airport. 

At the time, Slutkin said the back-to-back shootings that spring may have appeared particularly jarring because COVID-19 stay-at-home orders had resulted in fewer mass killings the year before. 

Last year, however, seeing high-profile mass shootings on the news became a regular occurence once again, with the November 19 shooting at Club Q in Colorado Springs, the May 24, 2022 at Robb Elementary School in Uvalde, Texas, and the May 14, 2022 shooting at a Buffalo grocery store, to name a few. 

And far more mass shootings never make the national news, with many taking place outside the public view.

In 2022 alone, the Gun Violence Archive totaled 647 mass shootings, defined as an incident in which four or more people are shot, not including the shooter.

Since January 1, 2023 — 24 days ago — there have been 39 mass shootings, according to the data.

Meztl said that when creating policies on how to interrupt gun deaths, people should consider all gun violence and not just high-profile mass shootings. 

In 2022, at least 44,290 people died from gun violence in the US, and a majority died by suicide. 

Metzl told Insider he believed that in addition to thinking beyond mass shootings, passing gun laws including background checks and bans on high-capacity magazines can help prevent gun violence deaths. 

"So there are plenty of policies we can announce that will lower rates of everyday gun death," Metzl told Insider. "It's not so much, 'Oh, this one policy is going to stop a mass shooting,' but we can save more lives by enacting common sense policies."