Gun violence also affects more than its victims. In areas where it is prevalent, just the threat of violence makes neighborhoods poorer. It's very difficult to quantify the total harm caused by gun violence, but by asking many people how much they would pay to avoid this threat -- a technique called contingent valuation -- researchers have estimated a cost to American society of $100 billion dollars.

Guns are also involved in suicides and accidents. 19,392 of 38,264 suicides in 2010 involved a gun (50%), according to the CDC. There were 606 firearm-related accidents in the same year -- about 5% of the number of intentional gun deaths.

How many guns are there in the U.S.?

There are about 310 million guns in the country. About 40% of households have them, a fraction that has been slowly declining over the last few decades, down from about 50% in the 1960s. Meanwhile, the overall number of guns has increased to about one gun per person, up from one gun for every two persons in the 1960s. This means that gun ownership has gotten much more concentrated among fewer households: if you own one gun, you probably own several. America has the highest rate of gun ownership of any country in the world, by a wide margin (see: international comparison).

(More: A long running poll by Gallup; the wide-ranging General Social Survey; aNew York Times demographic breakdownby Nate Silver)



How do mass shootings differ from other types of gun violence?

The FBI defines a "mass murder" as four or more murders during the same incident. This is an arbitrary number, but a dividing line is useful when asking whether there are differences between mass shootings and other kinds of gun violence. The most comprehensive public list of U.S. mass shootings is the spreadsheet of 62 incidents from 1982-2012, compiled by Mother Jones. Their list shows:

Mass shootings happen all over the country.

Killers used a semi-automatic handgun in 75% of incidents, which is about the same percentage as the 72% in overall gun violence.

Killers used an assault weapon in 40% of incidents. This is much higher than overall assault weapon use in crimes, estimated at less than 2%.

The guns were obtained legally in 79% of mass shootings.

Many of the shooters showed signs of mental illness, but in only two cases was there a prior diagnosis.

There were no cases where an armed civilian fired back.

2012 was the worst year in American history, in terms of total victims. A graph of yearly victims shows a slight upward trend. But the pattern is a lot less clear without the 2012 peak, and because yearly numbers vary so widely, it's likely that there will be many fewer victims next year.

Several criminologists deny that mass shootings are increasing. Although these incidents dominate headlines and conversation, it's important to remember that they account for only a small fraction of gun violence in the United States. For example, the spike of 72 deaths in 2012 includes only 0.8% of all firearm-related homicides in 2011 (the last year for which statistics are available.) Many gun deaths, especially in large cities, never make the news. This means that the most effective gun violence reduction strategies -- in terms of lives saved -- might not target mass shootings at all.