Testing the assumptions of crime linkage with stranger sex offenses: a more ecologically-valid study

An increasing amount of research has been conducted on crime linkage, a practice that has already been presented as expert evidence in some countries; however it is questionable whether standards of admissibility, applied in some jurisdictions, have been achieved (e.g., the Daubert criteria). Much research has assessed the two basic assumptions underpinning this practice: that offenders are consistent in the way they commit their crimes and that offenders commit their crimes in a relatively distinctive manner. While studies of these assumptions with stranger sex offenses exist, they are problematic for two reasons: (1) small samples (usually < 30 series), and (2) samples consisting solely of serial offenses. The current study improved on past research through the use of a much larger dataset (N=50 series, 194 offenses; and N= 50 one-off offenses) and by sampling the offenses of both serial and one-off sex offenders, thereby representing a more ecologically valid test of the assumptions. The two assumptions were tested simultaneously by assessing how accurately 365 linked crime pairs could be differentiated from 29,281 unlinked crime pairs through the use of Leave-One-Out Cross-Validation logistic regression followed by Receiver Operating Characteristic analysis. An excellent level of predictive accuracy was achieved providing support for the assumptions underpinning crime linkage.