People opposed to gun control often have argued that they need firepower to protect themselves against criminals. Take this example from January 2013 when a Georgia woman shot a crowbar-wielding intruder who broke into her home and confronted her and her two young children [source: CBS News]. Gun control opponents say that a vast number of crimes are prevented by armed citizens, who either shoot an assailant -- an event that happened 326 times in 2010, according to a 2012 Wall Street Journal state-by-state analysis of crime statistics -- or more often, chase the would-be criminal away by brandishing a weapon [source: Palazzolo and Barry].
There is some social science to back up that thesis. Perhaps the most often-cited evidence is a 1995 study by Northwestern University School of Law researchers Gary Kleck and Marc Gertz. Based upon a random telephone survey of 5,000 Americans, they concluded that there were between 2.1 and 2.5 million defensive gun uses each year This works out to about 1 percent use of a gun for defensive purposes [source: Kleck and Gertz].
But critics questioned whether Kleck's and Gertz's findings were reliable. Harvard public health researcher David Hemenway published a paper refuting this and pointing out that "since only 42 percent of U.S. households own firearms and victims in two-thirds of the occupied households were asleep, the 2.5 million figure requires us to believe burglary victims use their guns in self-defense more than 100 percent of the time" [source: Hemenway]. Another mid-1990s study, based upon a Justice Department survey of nearly 60,000 households, came up with a much smaller estimate of about 21,500 defensive gun uses annually [source: Committee on Law and Justice].
Even if the low-end estimates are closer to the truth, this still could mean that tens of thousands of crimes are prevented by gun owners annually. But a 2009 University of Pennsylvania School of Medicine study found that people with a gun were 4.5 times more likely to be shot in an assault than those who were unarmed [source: Science Daily].