Carrying a gun increases the risk of getting shot and killed.

From New Scientist:

Packing heat may backfire. People who carry guns are far likelier to get shot – and killed – than those who are unarmed, a study of shooting victims in Philadelphia, Pennsylvania, has found.

It would be impractical – not to say unethical – to randomly assign volunteers to carry a gun or not and see what happens. So Charles Branas's team at the University of Pennsylvania analysed 677 shootings over two-and-a-half years to discover whether victims were carrying at the time, and compared them to other Philly residents of similar age, sex and ethnicity. The team also accounted for other potentially confounding differences, such as the socioeconomic status of their neighbourhood.

Despite the US having the highest rate of firearms-related homicide in the industrialised world, the relationship between gun culture and violence is poorly understood. A recent study found that treating violence like an infectious disease led to a dramatic fall in shootings and killings.

Overall, Branas's study found that people who carried guns were 4.5 times as likely to be shot and 4.2 times as likely to get killed compared with unarmed citizens. When the team looked at shootings in which victims had a chance to defend themselves, their odds of getting shot were even higher.

While it may be that the type of people who carry firearms are simply more likely to get shot, it may be that guns give a sense of empowerment that causes carriers to overreact in tense situations, or encourages them to visit neighbourhoods they probably shouldn't, Branas speculates. Supporters of the Second Amendment shouldn't worry that the right to bear arms is under threat, however. "We don't have an answer as to whether guns are protective or perilous," Branas says. "This study is a beginning."