Democrats and Republicans will forever argue about the effectiveness of gun laws to prevent mass shootings. But what does the latest academic research show?

The short answer is that many proposed laws probably would not have much impact on curbing the mass shootings that dominate the news. But they could lessen their severity, and might also bring down overall gun violence.

Despite their notoriety, mass shootings — as defined by criminologists — generally do not happen often enough for detailed data analysis. Moreover, there are at least eight databases of mass shootings, including one maintained by The Washington Post, with different definitions and parameters. An upcoming paper for the Justice Department, written by a team led by James Alan Fox of Northwestern University, Grant Duwe of the Minnesota Department of Corrections and Michael Rocque of Bates College, attempts to craft a common definition: A mass public shooting is any event in which four or more individuals, not including the assailant(s), were killed by gunfire in a public setting within a 24-hour period. Mass shootings associated with criminal activity are excluded.

Under this definition, there were three or four mass shootings a year through most of the 2010s, but then the number spiked to seven in 2017, 10 in 2018 and eight in 2019, according to the database, provided to the Fact Checker by Duwe.

The team, drawing on the existing databases and supplemental research, found that “the number of mass public shootings has indeed increased over the past four and one-half years, particularly over the past decade. However, even at its peak in 2018, the number of such incidents has not surpassed ten in any year, and often has been much lower.” Moreover, some of the increase can be linked to growth in population. The incident count tripled since the mid-1970s but the rate per 100 million of population increased by a factor of two.

Fox told the Fact Checker that most mass shooters are very determined individuals and that even with an average of seven or eight mass shootings a year, new laws might only reduce the number by one a year. But he said stricter gun control laws would be “the right thing to do for a different reason” — they might help reduce overall gun violence.

While it is generally correct that states with tougher gun laws tend to have lower gun fatality rates, those rankings change when suicides — which make up about 60% of gun deaths — are excluded. Rural areas, which may have less restrictive gun laws, have a lot of suicides of older single men who become lonely. Access to guns is believed to triple the risk of suicide, according to a 2014 study. But Fox said he would exclude suicides from such calculations. “There is a big difference between homicide and suicide,” he said. “The victim of a homicide does not choose to be killed.”

Here’s a summary of key research on the effectiveness of various laws, either at the federal or state level.

ASSAULT WEAPONS BAN

In 1994, President Bill Clinton signed into law a ban on assault weapons and large-capacity magazines (LCMs), defined as those that could hold more than 10 rounds. The law — which grandfathered in an estimated 1.5 million assault weapons and 25 million LCMs already owned by Americans — was in place for 10 years until Congress let it lapse.

Even supporters of the law have acknowledged that it was riddled with loopholes, such as allowing copycat weapons to be sold, that limited its effectiveness. Some research, however, suggests the ban became more effective toward the end of the 10-year period because it helped cap and then reduce the supply of assault weapons and LCMs.

Biden claimed that mass shooting deaths tripled after the law expired. He appears to be relying on a study of mass shooting data from 1981 to 2017, published in 2019 in the Journal of Trauma and Acute Care Surgery by a team led by Charles DiMaggio, a professor of surgery at New York University’s Langone Medical Center. That group found that an assault weapons ban would have prevented 314 out of 448, or 70%, of the mass shooting deaths during the years when the ban was not in effect. But the data used in that study has come under attack by some analysts.

Meanwhile, Louis Klarevas, a research professor at Teachers College at Columbia University, studied high-fatality mass shootings (involving six or more people) for his 2016 book “Rampage Nation.” He said that compared with the 10-year period before the ban, the number of gun massacres during the ban period fell by 37% and that the number of people dying because of mass shootings fell by 43%. But after the ban lapsed in 2004, the numbers in the next 10-year period rose sharply — a 183% increase in mass shootings and a 239% increase in deaths.

Correlation does not necessarily equal causation, however. Fox, in a 2016 study co-written with Emma Fridel of Northeastern University, noted that “rather than assault weapons, semiautomatic handguns are the weapons of choice for most mass shooters.” (About 70% of mass public shootings after 1992 relied exclusively or primarily on semiautomatic handguns.) They wrote that “the frequency of incidents was virtually unchanged during the decade when the ban was in effect” and that “not only were there countless assault weapons already on the street, but also assailants had a variety of other powerful firearms at their disposal.”

The new mass-shooting database shows that there were 31 mass shootings in the decade before the 1994 law, 31 in the 10 years the law was in force (Sept. 13, 1994 to Sept. 12, 2004) and 47 in the 10 years after it expired. As noted, some of that increase stems from population growth.

LARGE-CAPACITY MAGAZINES

While the assault weapons ban may not have reduced the number of mass shootings, there is some evidence that the 1994 law’s restrictions on LCMs may have been effective in reducing the death toll.

Christopher S. Koper, an associate professor of criminology at George Mason University, said in a 2020 study that LCMs enable rapid spray fire that gives shooters the ability to wound higher numbers of victims in public settings. So restrictions on LCMs can have an effect.

“Data on mass shooting incidents suggest these magazine restrictions can potentially reduce mass shooting deaths by 11% to 15% and total victims shot in these incidents by one quarter, likely as upper bounds,” Koper wrote, adding, “It is reasonable to argue that the federal ban could have prevented some of the recent increase in persons killed and injured in mass shootings had it remained in place.”

Moreover, a number of studies of state-level bans on LCMs, such as by Mark Gius of Quinnipiac University and by Klarevas, indicate that such laws are associated with a significantly lower number of fatalities in mass shootings. Fox co-wrote a 2020 study of state gun laws that concluded that bans on LCMs are associated with 38% fewer fatalities and 77% fewer nonfatal injuries when a mass shooting occurred.

But even states such as California, which outlaws LCMs that hold more than 10 bullets, have suffered from mass shootings that involved LCMs. When Syed Farook and Tashfeen Malik killed 14 people in San Bernardino, Calif., in 2015 with legally purchased guns and rifles, four high-capacity magazines were found, perhaps holding as many as 30 rounds. Many mass shooters also acquire a large inventory of weapons, making reloading less necessary.

UNIVERSAL BACKGROUND CHECKS

There is evidence that universal background checks — including between private parties — could have an impact on mass shootings. State laws requiring a permit to purchase a firearm, which includes a background check on all purchases, are associated with 60% lower odds of a mass public shooting occurring, Fox’s 2020 study found.

But most mass murderers legally purchase the firearms they use in their killing sprees. Salvador Ramos, identified by police as the gunman who killed 19 children and two teachers in Uvalde, purchased two AR-15 semiautomatic rifles and ammunition as soon as he turned 18. He had never been convicted of a felony or had a history of criminal violence, so there was no prohibition against him buying the weapons.

The current system also fails. In 2015, Dylann Roof killed nine people with a .45-caliber Glock pistol that held 13 rounds at a historic African American church in Charleston, S.C. Roof legally purchased his gun from a store, but the FBI said he should have failed the background check because he had been charged with possessing Suboxone without a prescription. However, because of clerical mistakes, the FBI said the examiner did not get hold of the report before the three-day waiting period ended, and so the store went through with the purchase.

This three-day period has become known as the “Charleston loophole” that some lawmakers have sought to close. But it’s possible Roof might have passed the background check if it had been done correctly. The FBI statement incorrectly referred to a felony drug charge, but it was a misdemeanor for possession; he did not admit to being an addict. The FBI later said Roof would have been denied a gun based on an “inference of current use.”

FIREARMS PROHIBITIONS BASED ON MENTAL HEALTH

Anyone who slaughters innocent people with firearms in theory would be expected to have mental health issues. But most people who have mental health issues are not killers; in fact, they are more likely to be victims of gun violence. Nearly one in five adults in the United States live with a mental illness, according to the National Institute of Mental Health, while epidemiological research suggests that nearly half the U.S. population may experience some symptoms of mental illness in their lifetime.

That makes it difficult to know when to draw the line, especially because mental illness is not a predictor of violence. “Databases that track gun homicides, such as the National Center for Health Statistics, similarly show that fewer than 5% of the 120,000 gun-related killings in the United States between 2001 and 2010 were perpetrated by people diagnosed with mental illness,” noted Jonathan Metzl and Kenneth MacLeish of Vanderbilt University in a 2016 study. They said that other factors, such as alcohol and drug use, may increase the risk of turning toward violent crime even more. A history of childhood abuse is also considered a predictive risk factor.

Red-flag (“extreme risk”) laws — which generally allow police to take firearms away from people who exhibit concerning behavior — have been passed in 19 states and the District of Columbia, according to Everytown for Gun Safety, which advocates for gun-control laws. Between 1999 and 2021, at least 16,857 extreme risk petitions were filed, the group says. Florida, which passed such a law after the Marjory Stoneman Douglas High School shooting in 2018, has used it 6,000 times since then.

Yet New York’s red-flag law was not invoked against Payton Gendron, the suspect in the racist attack in Buffalo this month that left 10 people dead. He had said in school he planned to commit a murder-suicide and was taken to a hospital for a mental health evaluation. Police chose not to seek a red-flag order, apparently because he did not name a specific target. New York’s governor has since signed an executive order seeking to strengthen the law.

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