Six of the nine deadliest mass shootings in the United States since 2018 were by people who were 21 or younger, representing a shift for mass casualty shootings, which before 2000 were most often initiated by men in their mid-20s, 30s and 40s.

“We see two clusters when it comes to mass shooters, people in their 40s who commit workplace type shootings, and a very big cluster of young people — 18, 19, 20, 21 — who seem to get caught up in the social contagion of killing,” said Jillian Peterson, a criminal justice professor who helped found the Violence Project, which maintains a comprehensive national database of mass shootings.

There is no single, easy explanation for why young men are more likely to engage in mass shootings. (Girls and women make up a small percentage of all perpetrators.) But many of the causes cited most often by law enforcement officials and academics seem intuitive — online bullying, the increasingly aggressive marketing of guns to boys, lax state gun laws and federal statutes that make it legal to buy a semiautomatic “long gun” at 18.