The effect of the ageing of crime scene objects on the results of scent identification line-ups using trained dogs

In a scent identification line-up, a trained dog matches the scent trace left by a perpetrator at the crime scene to the odour of a suspect in a line-up of different odours. The procedures are strictly defined and the results are routinely used by the police and as evidence in court in a number of European countries. This paper describes the effect of ageing of the odour trace collected at the crime scene on the performance of the dogs in recognising the perpetrator in a line-up. The results show that whilst the dogs perform faultlessly in matching odours collected on the same day, the results drop to a lower level and become more variable in the period studied (2 weeks to 6 months). However, the results do not show a systematic decrease in performance. A possible explanation is the development of a steady state in the glass jars containing the perpetrator odour trace after initial differential evaporation of components of the residue or break down of unsaturated components into saturated ones. Prevention of this initial change may prevent the drop in performance observed in this study, thus increasing the reliability of these scent identifications.