The shooting at an Aurora, Colo., movie theater screening of “The Dark Knight Rises” Friday underscored a chilling trend: While gun violence has plummeted over the decades, mass murder has increased slightly.

Shooting massacres do not mirror the more volatile statistical curve of individual gun violence. Recent incidents — such as the shooting at a Toronto shopping mall in June — make it seem as if rampages are on the rise, but they have been a fact of life for a long time and will remain that way, said Jack Levin, a criminology professor at Northeastern University and author of “Extreme Killing.”