In the six weeks after the Newtown shootings – when a young man fatally gunned down 20 children and six adults – handgun acquisitions in California rose by 53 percent over expected levels, the study said.

In the six weeks after a couple armed with semiautomatic weapons killed 14 people at an event held by the San Bernardino County Department of Public Health in 2015, handgun purchase rates were 85 percent higher than expected among residents of the city of San Bernardino and adjacent neighborhoods, and 35 percent higher elsewhere in California, the study found.

The study was published May 1 in the Annals of Internal Medicine journal. The lead author is David Studdert, a professor of medicine and of law at Stanford. The senior author is Garen Wintemute, MD, MPH, professor of emergency medicine at UC–Davis.

The researchers write that their findings have implications for public health, given that firearm ownership is a risk factor for firearm–related suicide and homicide.

“For some, a gruesome mass shooting may induce repulsion at the idea of owning a weapon,” the authors wrote. “For others, it may motivate acquisition. Mass shootings are likely to boost sales if they heighten concerns over personal security, because self–protection is the most commonly cited reason for owning a firearm.”

More than 32,000 people die of gunshot wounds in the United States each year, according to the Centers for Disease Control and Prevention. While mass shootings account for less than 1 percent of those deaths, they are the most visible form of firearm violence because of the extensive broadcast and social media coverage that surround them.Using detailed, individual–level information on firearm transactions in California between 2007 and 2016, the researchers analyzed acquisition patterns after two of the highest–profile mass shootings in U.S. history. They found large and significant spikes occurred among whites and Hispanics, and among individuals who had no record of having previously acquired a handgun.

Although these spikes in handgun purchases after both mass shootings were large, they were also short–lived and accounted for less than 10 percent of annual handgun purchases statewide.

“Concerns about firearm violence and the public health risks of firearm ownership should stay focused on the much larger volume of weapons that routinely changes hands, and the immense stock that already sits in households,” the researchers wrote.

“On the other hand, the cumulative effect of such ‘shocks’ as Newtown and San Bernardino shootings on firearm prevalence may be substantial,” they wrote. “Moreover, firearm acquisitions seem to be sensitive to a range of other events that are also common, such as federal elections, new firearm safety laws and terrorist attacks.” Taken as a whole, these events may drive significant increases in overall firearm prevalence, which may in turn increase the risk for firearm–related morbidity and mortality in the long run, they wrote. The authors urged further research to explore the cumulative effects and temporary shifts in acquisition patterns, their causes, and their implications for public health, crime and social cohesion.

Other Stanford co–authors are researcher Yifan Zhang, PhD, and Jonathan Rodden, PhD, professor of political science.

A researcher at Monash University in Australia also co–authored the study.