In the absence of a police presence, how victims responded often made the difference between life and death, Dr. Blair said.

In 16 of the attacks studied by the researchers, civilians were able to stop the perpetrator, subduing him in 13 cases and shooting him in 3 cases. In other attacks, civilians have obstructed or delayed the gunman until the police arrived.

As part of the research, Dr. Blair and his colleagues looked at survival rates and the actions taken by people in classrooms under attack during the Virginia Tech massacre, in which Seung-Hui Cho killed 32 students and teachers before killing himself.

In two classrooms, the students and instructors tried to hide or play dead after Mr. Cho entered. Nearly all were shot, and most died. In a third classroom, Prof. Liviu Librescu, a Holocaust survivor, told his students to jump out the second-story window while he tried to hold the classroom door shut, delaying Mr. Cho from coming in. Professor Librescu was killed, but many of the students survived, and only three were injured by gunfire. In another classroom, where the students and teacher blocked the door with a heavy desk and held it in place, Mr. Cho could not get in, and everyone lived.