On Oct. 1, 2017, the Las Vegas Strip became the site of the deadliest mass shooting in American history. A shooter killed 59 people and injured 515, bringing the victim total to 574.

Mass shootings in the United States are getting worse, statistics show. Here's a look at the history of mass shootings to explain the historical and contemporary trends.

Definition

A mass shooting is defined by the FBI as a public attack, distinct from gun crimes that happen within private homes, even when those crimes involve multiple victims, and from drug- or gang-related shootings.

Historically, through 2012, a mass shooting was considered a shooting in which four or more people (excluding the shooter or shooters) were shot. In 2013, a new federal law reduced the figure to three or more.

Frequency Increasing

Every time a mass shooting occurs, a debate is spurred in the media about whether such shootings are happening more often. The debate is fueled by a misunderstanding of what mass shootings are.

Some criminologists argue that they are not on the rise because they count them among all gun crime, a relatively stable figure year-over-year. However, considering mass shootings as defined by the FBI, the disturbing truth is that they are rising and have increased sharply since 2011.

Analyzing data compiled by the Stanford Geospatial Center, sociologists Tristan Bridges and Tara Leigh Tober found that mass shootings have progressively become more common since the 1960s.

Through the late 1980s, there were no more than five mass shootings per year. Through the 1990s and 2000s, the rate fluctuated and occasionally climbed as high as 10 per year.

Since 2011, the rate has skyrocketed, climbing first into the teens then peaking at 473 in 2016, with the year 2018 ending at a total of 323 mass shootings in the United States.

Number of Victims Rising

Data from the Stanford Geospatial Center, analyzed by Bridges and Tober, shows that the number of victims is rising along with the frequency of mass shootings.

The figures for deaths and injuries climbed from below 20 in the early 1980s to spike sporadically through the 1990s to 40 and 50-plus and reach regular shootings of more than 40 victims through the late 2000s and 2010s.

Since the late 2000s, there have been 80-plus to 100 deaths and injuries in some mass shootings.

Most Weapons Legally Obtained

Mother Jones reports that of the mass shootings committed since 1982, 75 percent of the weapons used were obtained legally.

Among those used, assault weapons and semi-automatic handguns with high-capacity magazines were common. Half of the weapons used in these crimes were semi-automatic handguns, while the rest were rifles, revolvers, and shotguns.

Data on weapons used, compiled by the FBI, show that if the failed Assault Weapons Ban of 2013 had been passed, the sale of 48 of these guns for civilian purposes would have been illegal.

Uniquely American Problem

Another debate that crops up following a mass shooting is whether the United States is exceptional for the frequency at which mass shootings occur within its borders.

Those who claim that it does not often point to Organization for Economic Co-operation and Development (OECD) data which measure mass shootings per capita based on a country's total population. Looked at this way, the data indicate that the U.S. ranks behind nations including Finland, Norway, and Switzerland.

But these data are based on populations so small and events so infrequent that they are statistically invalid. Mathematician Charles Petzold explains on his blog why this is so, from a statistical standpoint, and further explains how the data can be useful.

Instead of comparing the United States to other OECD nations, which have much smaller populations and most of which have had just one to three mass shootings in recent history, compare the U.S. to all other OECD nations combined. Doing so equalizes the scale of the population and allows for a statistically valid comparison.

This comparison indicates that the United States has a mass shooting rate of 0.121 per million people, while all other OECD countries combined have a rate of just 0.025 per million people (with a combined population three times that of the United States.)

This means that the rate of mass shootings per capita in the U.S. is nearly five times that in all other OECD nations. This disparity is not surprising given that Americans own nearly half of all civilian guns in the world.

Shooters Nearly Always Men

Bridges and Tober found that of the mass shootings that have occurred since 1966, nearly all were committed by men.

Just five of those incidents—2.3 percent—involved a lone woman shooter. That means men were the perpetrators in nearly 98 percent of mass shootings.

Domestic Violence Connection

Between 2009 and 2015, 57 percent of mass shootings overlapped with domestic violence, in that the victims included a spouse, former spouse, or another family member of the perpetrator, according to an analysis of FBI data conducted by Everytown for Gun Safety. Additionally, nearly 20 percent of attackers had been charged with domestic violence.

Assault Weapons Ban

The Federal Assault Weapons Ban that was in effect between 1994 and 2004 outlawed the manufacture for civilian use of some semi-automatic firearms and large-capacity magazines.

It was prompted into action after 34 children and a teacher were shot in a schoolyard in Stockton, California, with a semi-automatic AK-47 rifle in 1989 and by the shooting of 14 people in 1993 in a San Francisco office building, in which the shooter used semi-automatic handguns equipped with a "hellfire trigger," which makes a semi-automatic firearm fire at a rate approaching that of a fully automatic firearm.

A study by The Brady Center to Prevent Gun Violence published in 2004 found that in the five years before the ban's implementation, assault weapons it outlawed accounted for nearly 5 percent of gun crime. During its period of enactment, that figure fell to 1.6 percent.

Data compiled by the Harvard School of Public Health and presented as a timeline of mass shootings show that mass shootings have occurred with much greater frequency since the ban was lifted in 2004, and the victim count has risen sharply.

Semi-automatic and high-capacity firearms are the weapons of choice for those who perpetrate mass shootings. As Mother Jones reports, "more than half of all mass shooters possessed high-capacity magazines, assault weapons, or both."

According to this data, a third of the weapons used in mass shootings since 1982 would have been outlawed by the failed Assault Weapons Ban of 2013.