Criminologists have long argued that crime is concentrated into “hot spots” at various levels of spatial aggregation and that these hot spots tend to be relatively stable over time. In this paper, we ask if terrorism incidents are also concentrated in a relatively small number of cross-national hot spots and whether that concentration has remained stable over time. Using semi-parametric trajectory analysis, we examine a newly created Global Terrorism Database that includes more than 70,000 domestic and international terrorism incidents for all countries from 1970 to 1997. Based on this trajectory analysis, we divide the nations of the world into four terrorism trajectories. We find considerable evidence for concentration of terrorist events at the national level. For example, the two group trajectories in the data with the fewest cases included 88% of the world's countries but only 25% of all terrorist incidents. By contrast, the fourth trajectory included only 8% of all countries, but 67% of all incidents between 1970 and 1997. There was also considerable, but not total, support for the conclusion that terrorism levels in these countries remained stable over time. In support, three of the four trajectories changed relatively little over the 28 years spanned by the data. However, the fourth trajectory, which included by far the largest number of events, did show a fair amount of convergence with the other three trajectories during the second half of the period. Rapidly rising levels of terrorist strikes in a given country could indicate substantial and prolonged risk of high levels of terrorism in the future.
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