A Dartmouth College study is the first to map the interplay of personal choice and social networks that has led to the United States being one of the world's most heavily armed countries, with 120 firearms for every 100 people. The researchers describe in Science Advances how individual incentives to buy firearms can lead to a phenomenon they call "overarming." In an overarmed society, the collective cost of firearm ownership outweighs the individual benefits of possessing a gun.