America's killing contagion

America leads the world in mass shootings. Why? Here's everything you need to know: Are mass shootings becoming more common? Without a doubt, although their prevalence depends on how mass shootings are defined. A recent Harvard University analysis that combined FBI data with other statistics concluded that since 2011, the frequency of mass attacks has tripled. But the commonly used definition -- at least four people shot dead in a public place -- would exclude last month's shooting on live TV in...