By Michael J. Klein, Clinical Assistant Professor of Surgery, New York University

The shooting deaths of three children and three adults inside a Nashville school has put further pressure on Congress to look at imposing a ban on so-called assault weapons. Such a prohibition would be designed cover the types of guns that the suspect legally purchased and used during the March 27 attack.

Speaking after the incident, President Joe Biden issued his latest plea to lawmakers to act. “Why in God’s name do we allow these weapons of war on our streets and at our schools?” he asked.

A prohibition has been in place before. As Biden has previously noted, bipartisan support in Congress helped push through a federal assault weapons ban in 1994 as part of the Violent Crime Control and Law Enforcement Act.

That ban was limited – it covered only certain categories of semiautomatic weapons such as AR-15s and applied to a ban on sales only after the act was signed into law, allowing people to keep hold of weapons purchased before that date. And it also had in it a so-called “sunset provision” that allowed the ban to expire in 2004.

Nonetheless, the 10-year life span of that ban – with a clear beginning and end date – gives researchers the opportunity to compare what happened with mass shooting deaths before, during and after the prohibition was in place. Our group of injury epidemiologists and trauma surgeons did just that. In 2019, we published a population-based study analyzing the data in a bid to evaluate the effect that the federal ban on assault weapons had on mass shootings, defined by the FBI as a shooting with four or more fatalities, not including the shooter. Here’s what the data shows:

Before the 1994 ban:

From 1981 – the earliest year in our analysis – to the rollout of the assault weapons ban in 1994, the proportion of deaths in mass shootings in which an assault rifle was used was lower than it is today.

Yet in this earlier period, mass shooting deaths were steadily rising. Indeed, high-profile mass shootings involving assault rifles – such as the killing of five children in Stockton, California, in 1989 and a 1993 San Francisco office attack that left eight victims dead – provided the impetus behind a push for a prohibition on some types of gun.

During the 1994-2004 ban:

In the years after the assault weapons ban went into effect, the number of deaths from mass shootings fell, and the increase in the annual number of incidents slowed down. Even including 1999′s Columbine High School massacre – the deadliest mass shooting during the period of the ban – the 1994-2004 period saw lower average annual rates of both mass shootings and deaths resulting from such incidents than before the ban’s inception.

From 2004 onward:

The data shows an almost immediate – and steep – rise in mass shooting deaths in the years after the assault weapons ban expired in 2004.

Breaking the data into absolute numbers, from 2004 to 2017 – the last year of our analysis – the average number of yearly deaths attributed to mass shootings was 25, compared with 5.3 during the 10-year tenure of the ban and 7.2 in the years leading up to the prohibition on assault weapons.

We calculated that the risk of a person in the U.S. dying in a mass shooting was 70% lower during the period in which the assault weapons ban was active. The proportion of overall gun homicides resulting from mass shootings was also down, with nine fewer mass-shooting-related fatalities per 10,000 shooting deaths.

Taking population trends into account, a model we created based on this data suggests that had the federal assault weapons ban been in place throughout the whole period of our study – that is, from 1981 through 2017 – it may have prevented 314 of the 448 mass shooting deaths that occurred during the years in which there was no ban.

And this almost certainly underestimates the total number of lives that could be saved. For our study, we chose only to include mass shooting incidents that were reported and agreed upon by all three of our selected data sources: the Los Angeles Times, Stanford University and Mother Jones magazine.

Furthermore, for uniformity, we also chose to use the strict federal definition of an assault weapon – which may not include the entire spectrum of what many people may now consider to be assault weapons.

Cause or correlation?

It is also important to note that our analysis cannot definitively say that the assault weapons ban of 1994 caused a decrease in mass shootings, nor that its expiration in 2004 resulted in the growth of deadly incidents in the years since.

Many additional factors may contribute to the shifting frequency of these shootings, such as changes in domestic violence rates, political extremism, psychiatric illness, firearm availability and a surge in sales, and the recent rise in hate groups.

Nonetheless, according to our study, Biden’s claim that the rate of mass shootings during the period of the assault weapons ban “went down” only for it to rise again after the law was allowed to expire in 2004 holds true.

As the U.S. looks toward a solution to the country’s epidemic of mass shootings, it is difficult to say conclusively that reinstating the assault weapons ban would have a profound impact, especially given the growth in sales in the 18 years in which Americans have been allowed to purchase and stockpile such weapons. But given that many of the high-profile mass shooters in recent years purchased their weapons less than one year before committing their acts, the evidence suggests that it might.

Editor’s note: This is an updated version of an article originally published on June 8, 2022.