Is transnational terrorism becoming more threatening? A time-series investigation

This study applies time-series techniques to investigate the current threat posed by transnational terrorist incidents. Although the number of incidents has dropped dramatically during the post-cold war period, transnational terrorism still presents a significant threat. In recent years, each incident is almost 17 percentage points more likely to result in death or injuries. Three alternative casualties series (incidents with injuries and/or deaths, the proportion of incidents with casualties, and incidents with deaths) are investigated. These series increased in November 1979 with the takeover of the U.S. embassy in Tehran and again after the fourth quarter of 1991. The growth of religious terrorism appears to account for the increased severity of terrorist attacks since the last quarter of 1991. All three casualties series displayed more deterministic factors than the noncasualties series, which is largely random after detrending. Cycles in the aggregate incident series are solely attributable to the underlying casualties series.