Frequency of Severe Terrorist Events

In the spirit of Richardson’s original (1948) study of the statistics of deadly conflicts,we study the frequency and severity of terrorist attacks worldwide since 1968.

We show that these events are uniformly characterized by the phenomenon of scale invariance, i.e., the frequency scales as an inverse power of the severity.

We find that this property is a robust feature of terrorism, persisting when we control for economic development of the target country, the type of weapon used, and even for short time-scales. Further, we show that the center of the distribution oscillates slightly with a period of roughly ≈ 13 years, that there exist significant temporal correlations in the frequency of severe events, and that current models of event incidence cannot account for these variations or the scale invariance property of global terrorism.

It has been suggested that the ≈ 13 value may be related to the modal life-expectancy of the average terrorist group. However, we caution against such conclusions for now, as these aforementioned variations on our analysis can shift the peak by several years.


Our analysis suggests that the changes in event frequencies have not been evenly distributed with respect to their severity, but rather that less severe
attacks are now relatively more frequent, while the frequency of “major” or tail events has remained unchanged.

Finally, we describe a simple toy model for the generation of these statistics, and briefly discuss its implications.

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