Case linkage in Australian serial stranger rape

Abstract

Purpose Case linkage theory and practice have received growing empirical support; however, they have yet to be examined fully within Australia. For sexual assault case linkage to be successful, it is assumed that a serial rapist will behave relatively consistently across offences yet distinctively compared to other offenders. The purpose of this paper is to test the underlying principles of case linkage, behavioural consistency and distinctiveness, as well as distinguishing accuracy. Design/methodology/approach In total, 250 solved stranger rapes by 171 offenders (46 serial rapists, 125 one-off rapists) were taken from Queensland Police Service (QPS) crime records. All possible crime pairings were created and cross-crime similarity was assessed using Jaccard’s coefficient. Receiver operating characteristics (ROC) analysis was used to examine the ability to distinguish between linked and unlinked offence pairs. Findings Serial linked pairs had the highest Jaccard’s coefficient (0.456), followed by non-serial unlinked (0.253) and finally, serial unlinked pairs (0.247). Within the ROC analysis, an area under the curve value was found of 0.913, indicating excellent distinguishing accuracy. Both the underlying principles of behavioural consistency and distinctiveness were supported through theoretical and practical methods. This paper provides the first analysis of serial rape case linkage in Australia, adding validity to this practice. Research limitations/implications The authors wish to acknowledge the support and assistance from the QPS in undertaking this research. The views expressed in this publication are not necessarily those of the QPS and any errors of omission or commission are the responsibility of the authors. Practical implications This paper provides validity to the practice of case linkage using a database within Australia. The results of this paper can be used to inform investigators of serial offender behaviours. The theories of offender consistency and distinctiveness are supported, highlighting the importance of behavioural evidence for practitioners. This paper provided a practical increase of the quantity and quality of offences uploaded on the Australian violent and sexual crimes database, which will assist further linkage efforts. Originality/value This paper is the first in Australia to examine consistency, distinctiveness and case linkage of serial stranger rape. Thus is contributes significantly not only to an increased understanding of serial rape and case linkage in Australia but also brings Australia closer to modern research practices in this field.

Receiver operating characteristic curves in the crime linkage context: Benefits, limitations, and recommendations

“…A second approach to studying crime linkage, and the one that will be focused on in this paper, has conceptualized the linkage task as one that requires a decision to be made about whether pairs of crimes are the work of the same offender or not (e.g., Bennell & Canter, 2002; Davidson & Petherick, 2020; Tonkin et al, 2011). We will call this approach the “pairwise linking task”.…”

“…In recent years, there has been an increased focus on studying the processes underlying the crime linkage task, and in evaluating the degree to which it is possible to accurately link crimes using behavioral, temporal, and geographic information (Bennell et al, 2014). One of the most popular methods for assessing the degree to which this task can be accomplished accurately involves the use of receiver operating characteristic (ROC) analysis (e.g., Bennell & Canter, 2002; Davidson & Petherick, 2020; Tonkin et al, 2011). In fact, since it was first used for this purpose in 2002 (Bennell, 2002), this method of analysis has been used in at least 32 other published studies.…”