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

Published date03 August 2015
Date03 August 2015
AuthorTom Pakkanen,Angelo Zappalà,Dario Bosco,Andrea Berti,Pekka Santtila
Can hard-to-solve one-off homicides be
distinguished from serial homicides?
Differences in offence behaviours and
victim characteristics
Tom Pakkanen, Angelo Zappalà, Dario Bosco, Andrea Berti and Pekka Santtila
Tom Pakkanen is a Forensic
Psychologist and Phd-Student
at Åbo Akademi University,
Åbo, Finland.
Angelo Zappalà is based at
Åbo Akademi University, Åbo,
Finland and Centre of Forensic
Science, Turin, Italy.
Dario Bosco is based at Centre
of Forensic Science, Turin, Italy.
Andrea Berti is based at Arma
dei Carabinieri, Reparto
Investigazioni Scientifiche,
Rome, Italy.
Professor Pekka Santtila is
based at Åbo Akademi
University, Åbo, Finland.
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 ¼0.88) 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.
Keywords Homicide, Offender profiling, Behavioural crime linking, Hard-to-solve, One-off, Serial
Paper type Research paper
Behavioural crime linking has been a fast growing topic of interest in the field of
forensic psychology research (Bennell et al., 2014). However, a vast majority of the studies
conducted have looked at serial offences only. This paradigm has not taken into account that in
the field, forensic investigators in fact deal with a mix of serial and one-off offences. Therefore,
to further the usefulness of behavioural crime linking research for practitioners, it is essential to
understand whether serial offences differ from one-off offences, and if they can be distinguished
from one another. This could help investigators determine whether they should be looking
for other similar offences, or focus solely on the homicide at hand. This study was designed to
Received 31 January 2015
Revised 4 June 2015
15 June 2015
Accepted 16 June 2015
The authors wouldlike to
acknowledgeReparto Investigazioni
Scientifiche di Rom a for providing
the data for theone-off cases, and
Dr Amy Burrell forher comments
on the manuscript.
VOL. 5 NO. 3 2015, pp. 216-232, © Emerald Group Publishing Limited, ISSN 2009-3829 DOI 10.1108/JCP-01-2015-0005
reflect the practicalities that the police are faced with in a homicide investigation, and thus
produce useful information to aid investigators manage their finite resources.
Behavioural crime linking
Behavioural crime linkage refers to the practice of attempting to determine whether two
or more crimes were committed by the sam e offender by means o f analysing simila ritiesand
differences in cr ime scene behaviour (e.g. Woodhams et al., 2007). There are several
potential benefits for crime investigators in linking crimes based on behaviour observable
at the crime scene (e.g. Rainbow, 2014). For example, a successfully linked crime series
can be investigated (and prosecuted) as a whole, compiling the evidence from the separate
crimes against the offender (Grubin et al., 2001). It is also resource efficient: linking crimes by
means of DNA, for example, is both costly and time consuming (Craik and Patrick, 1994),
in addition to the fact that DNA evidence is not always readily available at the crime scene
(Grubin et al., 1997).
Behavioural crime linkage is based on the assumptions that offenders behave consistently
from one situation to another (behavioural consistency) (e.g. Canter, 1995), and that there is
enough variability between offenders to allow them to be distinguished from one another
(behavioural distinctiveness) (e.g. Woodhams et al., 2007). A number of studies have found
evidence for the assumptions of crime linkage in serial crime (for a review of the research
to date on behavioural crime linkage, see Bennell et al., 2014). While serial homicide has
been studied less than, for example, rape, a few studies have found serial homicide offenders
to be somewhat consistent in their behaviour over the course of their series (Bateman and
Salfati, 2007; Salfati and Bateman, 2005; Salfati et al., 2015; Santtila et al., 2008; Sorochinski
and Salfati, 2010).
A problem concerning the ecological validity in crime linkage research is that, until recently,
studies have been conducted using only serial offences rather than samples that include both
serial and one-off offences. In reality the police deal with a pool of crimes, where only a portion
(varying depending on the type of crime) of them have been committed by serial offenders
(Pakkanen et al., 2014). Thus, when conducting linkage analysis and looking only at serial
offences, the current research might overestimate linkage efficiency. The question research
thus far has largely overlooked is Can crime linkage correctly discover linked offences even if
the sample includes both serial and one-off offences?A first step to answering this question is
to determine whether serial and one-off offences can be distinguished from each other. If there
are no significant differences between serial and one-off offences, and they cannot be
distinguished from one another (null hypothesis), the identification of offence series in a mixed,
ecologically more valid, sample of offences would be harder than previously thought based on
earlier behavioural crime linking studies.
Another potential benefit in being able to distinguishing one-off homicides from serial homicides is
to inform and develop inclusion criteria for cases to be added to crime linking databases, such as
ViCLAS[1] (Pakkanen et al., 2014). From a practical point of view, it is unrealistic to include all
homicides in such databases, as inputting cases is a resource intensive task. It would, therefore,
make sense to include cases into a crime linking database that are most likely to be part of a
series. To date, however, no studies have been conducted which distinguish between serial and
one-off homicides.
Differences between one-off and serial homicide
Serial murder is commonly defined as the unlawful killing of two or more victims by the same
offender(s),in separate events (Adjorloloand Chan, 2014; Mortonand Hilts, 2005). While thereis an
abundance of researchdescribing serial killings and serial killers (e.g. Godwin, 2007; Miller, 2014;
Salfati et al., 2014),studies comparing serial homicides to one-off homicides are scarce. The few
studies thathave done this (Fox and Levin, 1998;Harbort and Mokros, 2001; Krameret al., 2004),
have found that serial offenders target women and strangers more often than one-off killers do,
while one-off offenders tend to kill their familyand friends more often than strangers. Analysing the
killerscrime scene behaviour, Harbort and Mokros (2001) found that serial offendershomicides
VOL. 5 NO. 3 2015

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