Research Summary: Public mass shootings in the United States have become substantially more deadly over time. We document this increase, offer a model to explain it, review supporting evidence for the model, and present new findings on offenders from 1966‐2019. It appears that societal changes have led to more public mass shooters who are motivated to kill large numbers of victims for fame or attention, and more shooters who have been directly influenced by previous attackers. They often spend extended time planning their attacks and appear increasingly likely to acquire powerful weapons and develop specific strategies to enhance their lethality. Policy Implications: New policies should address the aforementioned factors. For instance, the deadliest public mass shooters’ desires for fame and attention might be countered by a change in media coverage policies. Additionally, the deadliest perpetrators’ lengthy planning periods are associated with more warning signs being reported to police, so that information could justify denying many potential attackers access to firearms through extreme risk protection orders and red flag laws.