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).
