Can hard-to-solve one-off homicides be distinguished from serial homicides? Differences in offence behaviours and victim characteristics

Abstract

Purpose – The purpose of this paper is to explore the differences (if any) between serial and hard-to-solve one-off homicides, and to determine if it is possible to distinguish the two types of homicides based on offence behaviours and victim characteristics. Design/methodology/approach – A sample of 116 Italian serial homicides was compared to 45 hard-to-solve one-off homicides. Hard-to-solve one-off homicides were defined as having at least 72 hours pass between when the offence came to the knowledge of the police and when the offender was caught. Logistic regression was used to predict whether a killing was part of a series or a one-off offence. Findings – The serial killers targeted more strangers and prostitutes, displayed a higher level of forensic awareness both before and after the killing, and had more often an apparent sexual element in their offence. Conversely, the one-off homicides were found to include more traits indicative of impulsive and expressive behaviour. The model demonstrated a good ability (AUC=0.88) to predict whether a homicide belonged to the serial or one-off category. Research limitations/implications – The findings should be replicated using local homicide data to maximise the validity of the model in countries outside of Italy. Practical implications – Being able to distinguish between serial and one-off homicides based on information available at a new crime scene could be practically useful for homicide investigators managing finite resources. Originality/value – Studies comparing serial homicides to one-off homicides are scarce, and there are no studies explicitly trying to predict whether a homicide is an isolated case or part of a series.

Comparing serial homicides to single homicides: A study of prevalence, offender, and offence characteristics in Sweden

“…It has previously been recognised that not all serial‐homicide offenders are sexually motivated (FBI, ). The findings from this study, in which sexual motives were identified in 32% of cases, are comparable to figures from Italy (Pakkanen et al, ) and Germany (Harbort & Mokros, )—where, respectively, 30% and 41% of serial‐homicides offences were reported to be sexually motivated—yet lower than the figure of 55% reported from the United States (Kraemer et al, ). In the present study, sexual serial offenders had sometimes been convicted of other, nonlethal offences, including rape and attempted homicide; and in a few cases, offenders were tried but not convicted of additional crimes.…”

“…Four of the seven factors (female victim, victim prostitute, wounds to the hands, and forensic awareness) from the model developed by Pakkanen et al () were significantly associated with serial cases, and a model incorporating the four factors was able to differentiate serial cases from single cases significantly better than chance (AUC = 0.76; SE = 0.35; 95% CI [0.70, 0.83]; n = 254), as could the full model incorporating all seven factors (AUC = 0.69; SE = 0.35; 95% CI [0.62, 0.76]; n = 254).…”

“…The second finding that has implications for police investigations is that the model by Pakkanen et al () was able to differentiate serial homicide from single‐homicide offences in Sweden as well, although the predictive validity was lower than in the original cohort (AUC = 0.69 vs. AUC = 0.88). This discrepancy is probably due to the reduced effect size that is often observed in replication studies when a model is tested in a sample other than the sample in which it was originally developed.…”

“…Other scholars have also emphasised that more research from non‐American settings is needed (Harbort & Mokros, ; Jenkins, ; Salfati, Horning, Sorochinski, & Labuschagne, ; Sturup & Lindqvist, ). Research has shown that offenders who kill two or more victims are different from offenders who kill only one victim (Fox & Levin, ; Harbort & Mokros, ; Kraemer, Lord, & Heilbrun, ; Pakkanen, Zappalà, Bosco, Berti, & Santtila, ; Trojan & Salfati, ). There has been a lively debate concerning the classification and definition of multiple‐homicide offenders (DeLisi & Scherer, ; Reid, ), especially with regard to serial killers (Adjorlolo & Chan, ; Fridel & Fox, ).…”

“…There has been a lively debate concerning the classification and definition of multiple-homicide offenders (DeLisi & Scherer, 2006;Reid, 2017a), especially with regard to serial killers (Adjorlolo & Chan, 2014;Fridel & Fox, 2017). A serial-homicide offender will, in this study, be operationalised as an individual who commits two or more homicides during a period of more than 24 hr (in accordance with Adjorlolo & Chan, 2014;FBI, 2008;Pakkanen et al, 2015;Santtila et al, 2008;Yaksic, 2015). Research has reported increasing trends of serial-homicide offenders both in the United States and internationally until the 1980s and 1990s, respectively, and after that decreasing trends (Aamodt, 2015;Fox & Levin, 2014;Hickey, 2013;Quinet, 2011;Schlesinger, 2001).…”