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Clauset and another computer-science student, Maxwell Young, started looking for that data. They found it in a database of global terrorist attacks maintained by the Memorial Institute for the Prevention of Terrorism in Oklahoma City, a homeland-security education and training institute founded in the wake of the 1995 Oklahoma City bombing. Once they'd built a computer program that could extract and parse the database's records on the nearly 30,000 terrorist attacks in 187 countries worldwide since 1968, they started looking for patterns.
Clauset's goal wasn't to solve the world's problems; it was just to answer the latest interesting question that had caught his fancy. "I'm fairly opportunistic, not motivated by the moral imperative," he explains. "If something seems interesting or weird, I might dive right in, looking for data, building models and teaching myself what's known and not known."
Clauset and Young were far from the first scholars to dive into the subject of human conflict. Since before the ancient Greek historian Thucydides penned The History of the Peloponnesian War in the fifth century B.C., people have been trying to figure out why and how humans kill one another. Not surprisingly, over the past decade, much of that scholarship has focused on terrorism. "Before 9/11, terrorism was not a hot research topic," says Victor Assal, political-science professor at the University at Albany-SUNY and director of the university's homeland-security concentration. "But since then, there has been an explosion of research. The government has poured a lot of money into the public and private sectors to study it."
The vast majority of scholarship on violence, terrorist-related or otherwise, has focused on the cause and effect of violent incidents: the motives and ideologies involved, the social and political repercussions that followed. But Clauset wasn't interested in digging into details — he wanted to look at the big picture.
And when he and Young looked at the big picture of terrorism, comparing the severity of attacks with their frequency, they noticed something interesting. "This looks like the same pattern as earthquakes," Clauset remembers thinking. The size and occurrence of earthquakes follows what's called a power-law distribution. There are usually lots of small earthquakes, a few mid-sized earthquakes and one or two powerful quakes. It's more common for statistical information to follow a normal or bell-curve distribution, in which most data points fall near the average. Human running speed, for example, has a bell-curve distribution, with most people running at about the average speed. If running instead followed a power-law distribution, Clauset explains, "there would be a small number of people who could run so fast they could launch themselves into orbit."
But terrorist attacks seemed to follow a power-law distribution: There had been lots of small attacks, killing one or two people; a few mid-sized attacks, like the 1995 Oklahoma City bombing, which killed 168 people, and the 1988 Lockerbie plane bombing, which killed 254; and one really large attack — 9/11, which killed nearly 3,000. Before the two could announce their discovery, though, they had to be sure they were right. "We were just grad students," says Clauset. "This is about terrorism, and we don't want to stick our necks out there and be wrong."
As it turned out, there was no standard statistical measure to determine whether a given mathematical pattern really followed a power-law distribution. So Clauset set about creating one, and sure enough, the terrorism data passed the power-law test.
Another concern remained: The data was too clear, the pattern too obvious. "Surely, someone has thought of this before," Clauset recalls thinking. But the only example they could find of similar research was the work of Lewis Fry Richardson, an English mathematician and Quaker pacifist who drove an ambulance in World War I. Along with doing major work in the fields of weather forecasting and fractals, Richardson would go on to analyze all the wars from 1815 to 1945 and conclude that they followed a power-law distribution; he found the same pattern in gang killings in Chicago and Shanghai.
Clauset and Young's results appeared to fit perfectly with Richardson's work; in the scale of human conflict, terrorism usually falls between murders and wars. So when the two, joined by University of Essex political-science professor Kristian Gleditsch, published their terrorism findings in the Journal of Conflict Resolution in 2007, they proposed what they called Richardson's Law, the conjecture that all deadly human conflicts, from random murders up to world wars, follow a simple power-law distribution.
Clauset still isn't sure why violent incidents such as terrorism follow this pattern, with lots of small attacks and a few very large ones. One explanation is that the power-law distribution arises from terrorists preferring to target locations with higher population densities, like markets or coffee shops, rather than choosing targets at random. Another possibility is that big terrorist attacks are just a lot harder to pull off. Small, violent strikes involve minimal planning and manpower, so it's fairly easy for terrorist organizations to accomplish a lot of them. Major attacks like 9/11, however, require lots of personnel and years of planning, so only a small number make it to deployment without falling apart or getting exposed.
Clauset and Young did more than just find interesting patterns within the data, however; their work could help change the way people think about future terrorist attacks. The common view among policy planners is that there is a difference between "big terrorism" and "little terrorism." Little terrorism, like abortion-clinic bombings or shootings in the Middle East, is often written off as unsurprising, meaningless violence, perhaps because there are so many incidents. Big terrorism like airplane hijackings, on the other hand, is deemed much more important, but also inherently unpredictable. "The implicit assumption is that big events are beyond understanding and could strike anywhere, at any time," says Clauset.