On September 16th a former navy reservist, Aaron Alexis, shot 12 people dead at a military base in Washington, DC. Though rare, such tragedies are increasingly common in America. The past 30 years have seen 67 mass shootings (in which four or more people were killed by a gunman not involved in a conventional crime), says Mother Jones magazine. There have already been five this year, after seven last year. Massacres grab headlines, and so may explain why many Americans believe, incorrectly, that gun crime is on the rise. In fact, gun murders have fallen by half in the past 20 years. (This is in line with the general decline in crime, but also owes something to modern medicine. Hospitals have become much better at keeping gunshot victims alive, so the kind of shootings that would have been fatal 20 years ago are often not today.) Americans are still much more likely to shoot each other than are people in other rich countries, though.

Clarification: This article has been amended to include the fact that improvements in modern medicine have also helped reduce the number of gunshot deaths.