Geolocalized mafia homicides in Palermo (1950–2020): insights into the evolution of Cosa Nostra

Abstract We investigate the statistical properties of mafia-related homicidal events occurred in the city of Palermo (Italy) from 1950 to 2020. Such data are extracted from a unique electronic archive containing microdata on mafia-related homicides, attempted homicides and disappearances committed in Sicily. This electronic archive is maintained by the Palermo Prosecutor’s Office (Direzione Distrettuale Antimafia) and is regularly consulted by magistrates investigating crimes associated with Cosa Nostra. Specifically, we aim to understand how geolocalizing homicides within the greater Palermo area can assist researchers and law enforcement authorities (Judiciary and Law Enforcement Agencies) in better characterizing and potentially predicting the temporal (micro-)patterns of crimes perpetrated by Cosa Nostra syndicates. To this end, we combine homicidal microdata with population density data at the submunicipal level of neighbourhoods (quartieri). This integration allows us to identify areas where the incidence of homicides is significantly higher or lower than expected under a null hypothesis of random distribution, adjusted for the actual population density in Palermo. In this way, we highlight neighbourhoods disproportionately affected by homicidal events, not in absolute terms, but relative to local population figures during a certain time period. The time periods we are considering are obtained by using an unsupervised statistical procedure that is able to highlight the temporal points when the statistical properties of the homicidal time series changes its statistical properties. The underlying idea is that such changes are associated to a regime shift in the Cosa Nostra criminal activities. The analyses presented in this paper may be usefully employed, at the very least, to reconstruct the broader context in which homicides occur.