The Behaviour Consistency of Serial Murder Offenders in South Africa

Abstract

In serial murder investigations lacking forensic evidence, investigators need valid and reliable ways of linking crimes committed by the same offender. The underlying principles of crime linkage are based on the observed consistent, yet unique behaviours, demonstrated by offenders across crimes. The purpose of crime linkage is to assist in investigations by identifying whether two or more crimes are likely to have been committed by the same offender, and provide similar-fact evidence in legal proceedings, connecting crimes at trial even when forensic or other evidence only partially links an offender to a series of crimes. This study examined behaviour consistency and distinctiveness among serial murder offenders in South Africa using solved and unsolved (yet forensically linked) murder series. The study aimed to empirically test the underlying principles of crime linkage research using an ecologically valid sample. The study’s novel approach of including unsolved murder series addresses limitations and criticisms regarding the ecological validity and reliability of previous crime linkage research. Data were collected from original case files of the South African Police Service’s (SAPS) Investigative Psychology Section (IPS), and a sample of 21 murder series with a total of 126 victims was identified. Crime scene behaviours were coded using a binary system to indicate presence (1) or absence/missing (0) of a behaviour. Jaccard’s coefficient was used to measure similarity of crime pairs quantitatively. Results indicated that linked crime pairs were significantly larger than unlinked crime pairs, across all datasets. Binary logistic regression analysis with resulting Receiver Operating Characteristic (ROC) and Area Under the Curve (AUC) indicated effective discrimination accuracy between linked and unlinked crime pairs when all crime scene behaviours were considered (AUC = 0.87). While the study found support for the underlying principles of crime linkage, it also revealed variability among offenders’ behaviour. Behavioural domains influenced by victim-offender interaction proved less effective at differentiating between linked and unlinked crime pairs. Future research should focus on refining the operationalisation of crime scene behaviours to improve linkage accuracy. By using original police case files and incorporating unsolved series, this research strengthens the ecological validity and reliability of crime linkage research, providing valuable insights for investigative and legal contexts.