The Contagious Diffusion of Worldwide Terrorism: Is It Less Common Than We Might Think?

Studies of the contagious spread of insurgency and conflict across national boundaries has generated a good deal of empirical research over time. While the contagious spread of terrorism has also been a policy concern, few empirical studies exist on the extent to which terrorism spreads contagiously. This article uses methods developed by criminologists to study the spread of crime to examine the worldwide diffusion of terrorism from 1970 to 2013. We distinguish between contagious increases (based on shared borders) and non-contagious increases (where no borders are shared). We define the “domino effect” as a particular type of contagious diffusion where high levels of terrorism spread to an adjoining country but also remain high in the host country. Our analysis shows that both contagious and non-contagious diffusion has been rare over the past 43 years, non-contagious diffusion is more common than contagious, and when contagious diffusion occurs, it is very likely to occur according to the domino effect.