Cycles and Substitutions in Terrorist Activities: A Spectral Approach

SUMMARY We use spectral analysis to provide evidence that cycles have characterized the following time series: all terrorist events, skyjackings, kidnappings, barricade and hostage‐taking, and all events not involving hostages. The series for all events had a periodicity of 28 months, while skyjackings had two significant periodicities ‐ 4.1 months and 28 months. Only a single significant periodicity was associated with barricade and hostage events and kidnappings ‐72 months and 48 months, respectively. We also found that nonlinear trends best represented four of the five series. Cross‐spectral analysis was then applied to the six possible pairs of series; each pair displays from three to nine statistically significant coherencies. Moreover, we discovered evidence of orthogonality for five of the six pairs studied. This evidence suggests that terrorists substitute between related events; this substitution primarily showed up in the short run.