Abstract

The “cooling-off period”, or inter-murder interval, has been used to differentiate between serial, spree, and mass murderers for decades. The present research examines the utility of this concept by studying the distribution of 2837 inter-murder intervals for 1012 American serial homicide offenders using data from the Consolidated Serial Homicide Offender Database. The distribution is smooth, following a power law in the region of 10–10,000 days. The power law is cut off in the region when inter-murder intervals become comparable with the length of human life. Otherwise there is no characteristic scale in the distribution. The decades long inter-murder intervals are not anomalies, but rare events described by the power-law distribution and therefore should not be looked upon with suspicion. This study found there to be no characteristic spree or serial homicide offender interval, only a monotonous smooth distribution lacking any features. This suggests that there is only a quantitative difference between serial and spree killers which represent merely different aspects of the same phenomenon.