In the Spring 2008 Bulletin of the Santa Fe Institute (pdf, p. 18).
Aaron Clause has spent nearly three years modelling the statistics of terrorism, but holds little hope that a mathematical model can predict whether a given man will walk a bomb into a given cafe on a given afternoon. He does believe that in large enough social systems, the capricious behaviors of individuals seem to fade in the face of collective patterns. “A classic question that many historians have asked over the years is, ‘Where does individual control end and statistical behavior take over?’” Clauset says. A physicist and computer scientist by training, he is pursuing that question.
His work to date has led him to conclude that terrorist attacks conform to patterns, at least on a global scale. In February 2007, Clauset, a Santa fe Institute postdoc, and his partners maxwell young (now a graduate student at the university of Waterloo) and Skrede Gleditsch (a reader at the university of essex) published a study in the Journal of Conflict Resolution that made a novel claim: the frequency of severe terrorist attacks, when taken worldwide, seems to follow a remarkably simple equation. the statistical distribution fits severe events like 9/11 to the same curve as more common but less severe ones that kill a dozen or so people. the pattern suggests that such rare and large events are not outliers, as was previously thought, but are somehow interconnected with the smaller attacks. the authors claim that if an underlying connection exists, then taking measures to discourage small-scale attacks might also prevent severe ones.