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Purpose: To conduct a test of the principles underpinning crime linkage (behavioural consistency and distinctiveness) with a sample more closely reflecting the volume and nature of sexual crimes with which practitioners work, and to assess whether solved series are characterized by greater behavioural similarity than unsolved series. Method: A sample of 3,364 sexual crimes (including 668 series) was collated from five countries. For the first time, the sample included solved and unsolved but linked-by-DNA sexual offence series, as well as solved one-off offences. All possible crime pairings in the data set were created, and the degree of similarity in crime scene behaviour shared by the crimes in each pair was quantified using Jaccard's coefficient. The ability to distinguish same-offender and different-offender pairs using similarity in crime scene behaviour was assessed using Receiver Operating Characteristic analysis. The relative amount of behavioural similarity and distinctiveness seen in solved and unsolved crime pairs was assessed. Results: An Area Under the Curve of.86 was found, which represents an excellent level of discrimination accuracy. This decreased to.85 when using a data set that contained one-off offences, and both one-off offences and unsolved crime series. Discrimination accuracy also decreased when using a sample composed solely of unsolved but linked-by-DNA series (AUC =.79). Conclusions: Crime linkage is practised by police forces globally, and its use in legal proceedings requires demonstration that its underlying principles are reliable. Support was found for its two underpinning principles with a more ecologically valid sample.

This study explored whether a coding bias due to knowledge of which crimes have been committed by the same offender exists when behavioural variables are coded in serial murder cases. The study used an experimental approach where the information given to the participants (N = 60) concerning correct linkages between a number of murder series was manipulated. The participants were divided into three different groups (n = 20 in each). These three groups received correct, incorrect, or no information about the linked series prior to the coding. The results showed that there is no clear evidence to support the hypothesis of a bias in the coding. The risk of expectancy effects and suggestions on how to minimise them in behavioural crime linking research were discussed, and suggestions on how to improve the validity of possible future replications of the experiment were given. The practical implications of expectancy effects on behavioural crime linking decisions for the justice system were also discussed. Copyright © 2012 John Wiley & Sons, Ltd.