After the 2012 mass shooting of children and teachers at Sandy Hook Elementary School in Connecticut, a leader of the National Rifle Association proclaimed: “The only thing that stops a bad guy with a gun is a good guy with a gun.”

It would seem that many Californians agreed, according to new research by Stanford Health Policy’s David Studdert and other researchers at academic institutions.

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 among first-time gun owners over expected levels.

When a couple armed with semi-automatic weapons targeted a San Bernardino County public health event in December 2015, killing 14 people in 2015, handgun purchase rates were 85 percent higher than expected among residents of the city of San Bernardino and adjacent neighborhoods, compared with 35 percent higher elsewhere in California.

In a new study in the Annals of Internal Medicine, lead author Studdert, a professor of medicine at Stanford Medicine and professor of law at Stanford Law School, writes that their findings have implications for public health as firearm ownership is a risk factor for firearm-related suicide and homicide.

“There is strong evidence linking gun ownership to risks of gunshot injuries, so any sudden boost in firearm ownership could have public health implications,” Studdert said. On their own, these two mass shootings are unlikely to have caused enough of a change in ownership patterns to have significant public health effects.

“But over time, purchasing responses to a succession of unnerving events like this — from mass shootings to terrorist attacks, to elections — could change levels of gun ownership enough to increase overall rates of gun injury and death.”

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The authors write that for some, mass shootings may induce repulsion at the idea of owning a weapon. But for others, they note, 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,” they said.

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,” write Studdert and his colleagues, Stanford Health Policy researcher Yifan Zhang, PhD; Jonathan Rodden, PhD, a professor of political science at Stanford; Rob J. Hyndman, PhD, a professor of statistics at Monash University in Australia; and Garen J. Wintemute, MD, MPH, an expert on gun violence at the University of California, Davis.

“On the other hand, the cumulative effect of such ‘shocks’ as Newtown and San Bernardino shootings on firearm prevalence may be substantial,” they write. “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, they said, 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. The authors urge further research should explore the cumulative effects and temporary shifts in acquisition patterns, their causes, and their implications for public health, crime and social cohesion.