On Thursday, more than 700 people were killed in a stampede outside the holy city of Mecca. The disaster took place during the annual Hajj pilgrimage, which draws about 2 million Muslims to Saudi Arabia each year.
Although this was the deadliest Hajj episode in a quarter-century, it is a story that is sadly familiar. In Mina, where Thursday’s disaster took place, stampedes killed more than 360 people in 2006 and 244 in 2004. In the worst Hajj stampede, 1,426 pilgrims were crushed in a pedestrian tunnel leading to Mecca in 1990.
Worldwide, human stampedes are so common—and so confounding—that they’ve inspired their own body of academic research within the larger field of study on crowd behavior. According to one 2010 study led by Edbert Hsu of the Johns Hopkins School of Medicine, 215 human stampedes took place worldwide between 1980 and 2007, leading to more than 7,000 deaths and 14,000 injuries. Stampedes have been evaluated as a public health issue, and as a sociological phenomenon. Others have asked whether the right algorithm could help identify dangerous crowd surges before they turn deadly.
Large religious gatherings are a particular stampede danger in the developing world. A 2013 paper out of India, for example, found that 79 percent of stampedes in that country have taken place at religious events, as opposed to political or entertainment-related events. In 2014, 178 people were killed in various Indian stampedes, and the country’s annual total death toll from stampedes has topped 300 four times in the past decade. According to Hsu’s research, the deadliest stampedes are concentrated in Southeast Asia and in Africa, and at religious events. But they can and do occur anywhere, as evidenced by the notorious New York stampede at a 2008 Wal-Mart Black Friday sale, which killed a store employee. With a growing global population concentrated in crowded cities, Hsu and his team found in 2010 that stampede incidents were on the rise.