The Effects of Coding Bias on Estimates of Behavioural Similarity in Crime Linking Research of Homicides

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 (c) 2012 John Wiley & Sons, Ltd.