Saperstein (2010) discusses recent work on the striking empirical observation that the frequency and severity of terrorist attacks follows a power-law relationship — a pattern noted for wars by Richardson (1948, 1960) more than 50 years ago. We are pleased with Saperstein's interest in our...

In the spirit of Lewis Richardson’s original 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,†that is, the...