Shankar Vedantam explains that In Crises, People Tend to Live, or Die, Together:
Computer models assume that people will flow out of a building like water, emptying through every possible exit. But the reality is far different. People talk. They confer. They go back to their desk. They change their mind. They try to exit the building the way they came in, rather than through the nearest door.Building engineers at the World Trade Center had estimated that escaping people would move at a rate of more than three feet per second. On Sept. 11, 2001, said Jason Averill, an engineer at the National Institute for Standards and Technology who studies human behavior during evacuations, people escaped at only one-fifth that speed. Although the towers were only one-third to one-half full, the stairwells were at capacity, he said. Had the buildings been full, Averill said, about 14,000 people would probably have died.
One study after the 1993 attack on the World Trade Center found that group size was a significant factor in determining how quickly people exited the building after a van loaded with explosives went off in an underground parking lot: Individuals who were part of larger groups, such as large workplaces, took longer to escape than individuals who were part of smaller groups.
That is because the larger the group, the greater the effort and time needed to build a common understanding of the event and a consensus about a course of action, said sociologist Benigno E. Aguirre of the University of Delaware. If a single person in a group does not want to take an alarm seriously, he or she can impede the escape of the entire group.
The picture of what happened on Sept. 11 is very different from conventional assumptions about crowd behavior, in which it is assumed that people would push each other out of the way to save their own lives. In actuality, human beings in crisis behave more nobly — and this could also be their undoing. People reach out not only to build a shared understanding of the event but also to help one another. In so doing, they may delay their own escape. This may be why groups often perish or survive together — people are unwilling to escape if someone they know and care about is left behind.