Friday, October 03, 2014
Every criminal lawyer needs to read Peter Gill’s “Misleading DNA Evidence: Reasons for Miscarriages of Justice” (Academic Press, 2014).
Judges should read it too. It is admirably clear and no expertise is required of the reader. If you have some experience with DNA evidence and Bayesian reasoning you will still find the book useful for its descriptions of how things can go wrong.
Peter Gill is one of the world’s leading experts on forensic DNA. I say ‘one of’ just to be polite to any other contender, for Dr Gill is really the leader.
Things can go wrong (as I so eloquently put it above) because DNA detection is now very sensitive and DNA is everywhere. A typical crime scene will have detectable amounts of DNA from many sources. Partial DNA profiles can be detected too, and this complicates the evaluation of mixed samples. Investigators can contaminate a crime scene, transferring DNA from one thing to another, with even latex gloves being vehicles for transfer. Detection of DNA does not mean that it is necessarily present because it was in a body fluid present at a crime scene. Samples collected for analysis must be sealed, and gloves changed, so that no transfer should occur between them, and in the laboratory the risk of transfer between suspect and scene samples must be minimised. There should be studies of the risks of contamination within a laboratory, and these should be done without the knowledge of those who work in that laboratory.
Courts tend to leap to conclusions that are not scientifically sound. Inferences that could be tested experimentally are often drawn without that evidence. A common example may be the assumption that a defendant’s DNA on a murder weapon found at a crime scene was only on the weapon and was not also on a number of other items which would innocently explain why it was also on the weapon; what the media refers to as the Amanda Knox case is an illustration that Dr Gill uses to make this point: the defendant’s DNA on a knife, allegedly the murder weapon, in a kitchen drawer, was given significance it may not have deserved in the absence of evidence of whether her DNA was also on other items in the drawer (which it surely would have been as she used the kitchen).