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Crime linkage analysis (CLA) can be applied in the police investigation-phase to sift through a database to find behaviorally similar cases to the one under investigation and in the trial-phase to try to prove that the perpetrator of two or more offences is the same, by showing similarity and distinctiveness in the offences. Lately, research has moved toward more naturalistic settings, analyzing data sets that are as similar to actual crime databases as possible. One such step has been to include one-off offences in the data sets, but this has not yet been done with homicide. The purpose of this paper is to investigate how linking accuracy of serial homicide is affected as a function of added hard-to-solve one-off offences.

A sample (N = 117–1160) of Italian serial homicides (n = 116) and hard-to-solve one-off homicides (n = 1–1044, simulated from 45 cases) was analyzed using a Bayesian approach to identify series membership, and a case by case comparison of similarity using Jaccard’s coefficient. Linking accuracy was evaluated using receiver operating characteristics and by examining the sensitivity and specificity of the model.

After an initial dip in linking accuracy (as measured by the AUC), the accuracy increased as more one-offs were added to the data. While adding one-offs made it easier to identify correct series (increased sensitivity), there was an increase in false positives (decreased specificity) in the linkage decisions. When rank ordering cases according to similarity, linkage accuracy was affected negatively as a function of added non-serial cases.

While using a more natural data set, in terms of adding a significant portion of non-serial homicides into the mix, does introduce error into the linkage decision, the authors conclude that taken overall, the findings still support the validity of CLA in practice.

This is the first crime linkage study on homicide to investigate how linking accuracy is affected as a function of non-serial cases being introduced into the data.

The purpose of this study was to investigate cognitive behavior therapists and trainees’s beliefs about various aspects of traumatic memory and to investigate cognitive behavior therapists’ practices in relation to alleged traumatic experiences and whether they are linked with their beliefs about various aspects of traumatic memory.

In the current study, the authors surveyed, by a questionnaire (in Italian), certified Italian cognitive behavioral (CB) therapists and trainees with respect to their beliefs in traumatic memories and whether they discussed about the possibility of repressed memory with their patients.

The majority of participants held strong beliefs about many controversial aspects related to traumatic memory, such as the mind being able to block out of consciousness memories of traumatic experiences. Also, more than half of CB therapists stated that they sometimes discussed about the importance of traumatic events in the genesis of their patient’s disorder and half of them sometimes talked with patients about memories for traumatic events of which they may be unaware. Such practices could lead to false memories of abuse

One particularly novel finding relates to the evidence that therapists reported that they had discussed with patients the importance of traumatic events in the genesis of their illness and frequently noted that they talked about the possibility of repressed memories with them. In turn, patients may be induced to recall traumatic experiences from their lives, thereby producing false memories which may tear families apart and could even lead to wrongful convictions.

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.

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.

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.

The findings should be replicated using local homicide data to maximise the validity of the model in countries outside of Italy.

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.

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.

Homicidal behaviour is influenced by a complex interaction of behavioural, situational and environmental factors that raise many challenging psychological questions. A large and continually growing body of research has explored the crime of homicide, its epidemiology, victims and perpetrators. The area is developing rapidly, opening up new avenues of study. The paper aims to discuss these issues.

This special issue of the Journal of Criminal Psychology brings together an exciting array of papers on homicidal behaviour, examining a wide range of issues including juvenile homicide perpetrators, school shootings, child homicide, homicide-suicide and differences in offence behaviours and victim characteristics between hard-to-solve one-off homicides and serial homicides.

The range of papers included in this special edition cover a wide range of aspects of homicidal behaviour, reflecting the importance of – and the need for – applied research moving away from examining general homicide to specialised research focusing on subtypes of homicide and subgroups of homicide offenders. A research agenda is proposed.

This editorial gives an introduction to the themes explored in this special issue and provides an overview of the selected papers.