In this week’s magazine, I have a piece about crowd disasters and what causes them. (Subscribers can read the story in full; everyone can watch the accompanying (http://www.newyorker.com/online/blogs/newsdesk/2011/01/seabrook-crowd-dynamics.html).) In the developed world, deadly crowd disasters tend to occur at big sales such as the one at a Wal-Mart in Valley Stream, on Long Island, on Black Friday, 2008; at concerts such as the Who show in Cincinnati, in 1979, at which eleven people died; at sporting events; and at nightclubs. In the developing world, meanwhile, you find them at religious festivals and pilgrimage sites. The crowds in Cairo’s Tahrir Square have been growing, but political protests or uprisings almost never result in crowd disasters. Which seems, at first glance, counterintuitive.

Reports on the deaths of protesters in Egypt have focussed on those caused by security forces. The crowds themselves have not turned deadly. Perhaps that’s because there is more of a common spirit at political gatherings, especially those in which the crowd itself represents a democratic alternative to the regime in power. Most of the deadly crowds you see in the West are manufactured by commercial interests. They aren’t spontaneous gatherings of the people. There is no popular will being expressed when fans line up for a Justin Bieber appearance at the mall. People don’t gather outside a Wal-Mart on a freezing cold night after Thanksgiving to bring about social change. They are there for more personal, arguably more selfish reasons: Wii bundles, or the touch of a celebrity’s hand.

There’s a professor at M.I.T. named Richard Larson who has done some fascinating studies of the psychology of people waiting in lines and what makes certain people tolerate long waits better than others. His main conclusion is that people will wait peacefully and patiently when they perceive that the queue is just and fair, but they will become impatient or unruly if they think the line isn’t just. In other words, the crowd expects to be treated with dignity. Maybe protesting a corrupt regime, and feeling that you are on the side of justice, supplies that feeling of dignity to the crowd, and that’s why political crowds rarely turn violent when they are unprovoked. There is little dignity in waiting for most of the night outside a Wal-Mart to get a deal on a big-screen TV.

Scientists who study crowd disasters often explain them as failures of collective intelligence. Unlike ants, for example, we are unable to communicate across the swarm: the people in the back, pushing forward, don’t realize they are crushing the people in the front. But if we lack a strictly biological capacity for collective behavior, perhaps we do possess a collective political intelligence that in certain situations, such as the demonstrations in Egypt, amounts to the same thing.

Join a live chat with Seabrook about crowds, today at 3 P.M. E.T. And read more from our coverage of the protests in Egypt and beyond.

Photograph: Lewis Whyld/PA Wire