A strange paradox is emerging in America: Overall violent-crime rates are down, but active shooter events — in which a person is trying to kill multiple people in a populated area — appear to be on the rise, according to Federal Bureau of Investigation statistics. Meanwhile, a just-released study finds that although the United States has just about 5 percent of the world's population, the country has 31 percent of the world's mass shooters. The reasons for these numbers are complex, researchers say, but the data suggest that the availability of guns, and perhaps the American obsession with fame, may...