Misleading DNA Evidence: Reasons for Miscarriages of Justice

This article is a brief review of my book “Misleading DNA Evidence: Reasons for Miscarriages of Justice”, published by Academic Press, Elsevier (Gill, 2014). It is nearly 30 years since the first demonstration of DNA profiling in forensic science. Since then, the technique has evolved remarkably. In the early days, only large “visible” crime-stains (e.g. blood, semen) were analysed. This was imposed by the relatively poor sensitivity relative to the today’s standards. There is an inherent advantage to the interpretation of macro-DNA samples, in that it is much easier to deduce the relevance of a supposed crime-stain to the crime-event itself. From the perspective of a court, the fact that a DNA profile may match a defendant is of secondary interest to the questions: “how” and “when” did the DNA-transfer take place? For a defendant to be found guilty, a court must be convinced that the DNA profile is associated with the crime-event itself. The forensic scientist attempts to apply “deductive logic” in order to associate the DNA profile with some other aspect of the case – it is not the “fact” of a matching DNA profile that is of primary interest, rather it is the “context” or the “relevance” of the DNA profile to the crime-event itself. To make deductive inferences, a chain of associations is implied. For example, the DNA profile may be found along with a positive test for blood. The scientist may deduce that the origin of the DNA is from blood – an association is made. The confirmation of a body fluid “source” is often sufficient to imply an “activity” – if blood is present then the prosecution may imply that the defendant or victim bled at the crime-scene; alternatively, the confirmation of semen may imply sexual assault. Once these associations have been accepted, the court’s task to decide the ultimate issue of guilt/innocence is a short step to take.