This decline in gun violence is part of an overall decline in violent crime. According to the FBI's data, the national rate of violent crime has decreased 49 percent since its apex in 1991. Even as a certain type of mass shooting is apparently becoming more frequent, America has become a much less violent place.

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Much of the decline in violence is still unexplained, but researchers have identified several reasons for the shift. Here are five.

1. More police officers on the beat

Additional manpower helps police departments respond to and prevent violence. In 1994, President Bill Clinton signed a major crime bill that set aside enough federal funding for law enforcement agencies nationally to add 100,000 officers, though the ranks of the country's police forces had already been expanding as local governments dedicated more resources to their departments to control increasing rates of crime.

In New York City, the recruitment of more officers was a crucial reason that the decline in crime was larger and more sustained than in other cities, according to Franklin Zimring, a criminologist at the University of California at Berkeley. The economist Steven Levitt estimates that larger police forces reduced crime by 5 percent to 6 percent. Gun violence, presumably, declined along with crime in general.

2. Police using computers

It wasn't just that police departments hired more people. They also started using computers to collect data on crime and to direct their officers' efforts more efficiently. When they have accurate, current information on where crime is happening, they can identify the neighborhoods where they are needed most.

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As Zimring notes, police in New York were among the first to realize the potential for computers to aid in fighting crime. Departments nationwide are now using versions of New York's CompStat system, short for Comparative Statistics. A recent study by the Brennan Center for Justice suggests that similar systems across the country reduced homicide by about 11 percent.

3. Less booze

A gradual decline in the amount of alcohol that Americans drink is another explanation for the decline in violence. About four in 10 prisoners convicted of murder were using alcohol at the time of the offense, according to a federal report. Americans drank 21 percent less alcohol in 2000 than in 1980, though consumption has increased since then, data from the National Institutes of Health shows. The authors of the report from the Brennan Center believe that this decline can account for 5 percent to 10 percent of the overall decline in crime.

4. Less lead

Besides alcohol, lead is another substance that has been shown to make humans more aggressive. Lead is toxic, and it can affect the behavior of children who are exposed to the metal while their brains are still developing. After the passage of the Clean Air Act in 1970, refiners were required to sell unleaded gasoline. Jessica Reyes, an economist at Amherst College, has argued that the children born after that law took effect breathed in less lead from car exhaust and that their brains were healthier as a result. She has estimated that the removal of lead reduced violent crime by no less than 56 percent. Other researchers are skeptical that lead could have caused such a large decline in U.S. violence, but many agree that the Clean Air Act had some effect on crime.

5. A better economy

Unemployment declined sharply from the recession under the Reagan administration through the boom under the Clinton administration, and income for the typical household increased. In better economic circumstances, communities and families might have more resources to dedicate to protecting themselves from crime — for example, by installing alarms in their homes. People also have more opportunities to earn money legally, removing one reason that some break the law.

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The authors of the Brennan Center report conclude that the increase in household income can probably explain about 5 percent to 10 percent of the decline in crime, similar to their estimate for alcohol. Yet economic factors seem more likely to affect rates of property crime than violent crime, and the relationship between the economy and the rate of gun violence in particular isn't clear.

Correction: An earlier version of this post inaccurately described a statistic from the Pew Research Center. That version stated that there were 725 victims of crimes who survived gunshot wounds per 100,000 people in the population in 1993, and that the statistic declined to 175 over the next 20 years. Those figures refer to the number of victims of crimes involving guns that did not result in death, such as robberies, including those in which no one was shot. This version has been corrected. We regret the error.