Q. So, Professor Gill, according to you it is impossible to obtain sure evidence from the examination of the DNA on this bra clasp?
A. Maybe I can take a step back and just explain that when DNA profiling evidence was first introduced, it was only used on visible crime stains and these crime stains were blood stains which one could visualize or semen stains, things like that. Over the last ten years the technique has become much more sensitive, to the extent that we can detect just an handful of cells. Now, the issue with that is that if I touch this surface here, my DNA is now on that surface, and if nobody cleans that chair it will still be there in six months time. Not only that, but if someone comes with plastic gloves and touches that area and then touches another area then it is possible that that DNA can be transferred from one surface to another. And this is something that we verified by experiments in a laboratory. So, specifically with the bra clasp, a forensic scientist's test has to consider all the possibilities. So one possibility, the prosecution's possibility, is that it was touched directly by the defendant, but the other possibility is that some transfer has occurred, either by accident by an investigator, or by some laboratory contamination. Now the problem is that DNA profile … if we visualize a DNA profile, it tells you absolutely nothing about how it got there, it tells you nothing about when it got there either. And this is the issue, really. All we have is a DNA profile. These DNA profiles are very very small, they originate from realistically (?) just a handful of cells. So the issue before the court really is to consider all of the possibilities and thence make a decision based on these possibilities.
Q. Doctor Capra, this story of transfer is upsetting, because this puts into doubt, beyond the Sollecito case, all the use made of DNA.
A. Surely. What the colleague said founds me in agreement. We have to consider DNA analisys not as something in itself, but as something connected to all that has happened before. What I mean is that we perform evidence collection with a certain level of precision, then we pretend to use this material with techniques which have a much higher level of precision, we risk to produce results which are incoherent between these two technical activities. It is a bit like to say … DNA testing is sometimes a bit hard to understand for laymen, but let's make an example [taken from] another field that perhaps is also simpler. If I a person used to handle firearms and today, with little carefulness, I touch an exhibit and then I want to see if on this exhibit there are residues of firearm discharge and I find them, how can I exclude that these residues have been transferred by me, who usually deal [with guns]. So this means that those who collect this evidence have to collect it considering the level of precision (sensitivity) applied in the following analyses.
[...other non forensic interventions]
Q. Professor Gill, so when they tell us there is the DNA, must we say “well, wait just a minute”? Because we laymen think “ok, they have found the Queen of Evidence and so the trial is over”. According to you instead this DNA flies all around.
A. Well we call this the “CSI Effect”. I think the problem is the public are indoctrinated by what they see on TV and unfortunately they think that what they see on TV is true and I think this also extends to lawyers and judges as well. Unfortunately life simply isn't like that. The reality is simply different. And really what we have to do is, as forensic scientists, to consider all the possibilities. And in fact I think in this series of trials forensic scientists on both sides have actually given the various points of view. One of the points of view is that … is the theory of contamination. And clearly that's quite crucial in this case. But, at the end of the day, it's up to the courts to decide, but what is notable in this case is that the conviction was overturned because the court said that it was the onus of the prosecution to prove that contamination had not occurred. The latest judgement, Nencini judgement [the Italian translator incorrectly translates Nencini with Cassation, but indeed the argument is valid for both] reversed that ruling by saying that it was the responsibility of the defense to prove that contamination had occurred. And this absolutely crucial in this case. Where does the responsibility lie to show whether or not contamination has occurred? What I must say is that I think normally it would be for the prosecution to prove that contamination has not occurred and it is extremely unusual, I think, to put the responsibility and in effect this is known as reverse burden of proof, which is [incomprehensible word] unusual, I think.