“Dangerous speculation that is another example of confirmation bias”

Peter Gill coauthored the 1985 article in Nature that proposed using DNA “fingerprinting” in forensic science. Later on, Dr. Gill developed a “super-sensitive” method of DNA typing known as “low copy number” (LCN) or “low-template” profiling.

John M. Butler, the author of the best-known textbook on forensic DNA typing, wrote in 2014:

In my opinion. over the past three decades no one has done more to advance forensic DNA analysis and interpretation than Peter Gill.

However the use of LCN DNA as trial evidence has been criticized by forensic scientists on both sides of the Atlantic. Its validity was a central issue at the so-called Omagh bombing trial in Northern Ireland in 2007, which ended in an acquittal as the judge was convinced that the LCN DNA evidence was unreliable. The US forensic biologist Dan Krane testified for the defense. Although Peter Gill testified for the prosecution, judge Weir commended his integrity:

…Dr Gill… was willing to carefully consider the propositions put to him by [defense attorney] Mr Pownall QC and, where appropriate, to disagree with his colleague [another prosecution expert] on important issues both general and specific to the case. In my view it was extremely fortunate that the prosecution decided late in the day to call Dr Gill as his evidence greatly helped to inform and bring some objectivity to the debate.

The Omagh trial fiasco prompted a suspension of the use of LCN DNA in UK courts followed by a reinstatement with stricter conditions attached. Prominent US scientists such as the FBI veteran Bruce Budowle and Dan Krane, as well as their UK colleague Allan Jamieson, have continued to criticize the use of LCN DNA for purposes other than providing investigative leads.

Some time in 2009 it became clear that key DNA evidence in the Knox-Sollecito case was highly suspicious, and that it involved LCN DNA – on the knife and possibly on the bra clasp. Greg Hampikian, a forensic geneticist and director of the Idaho Innocent Project, and Elizabeth Johnson, an independent DNA consultant, voiced these concerns in a letter to the Italian court co-signed by seven other forensic professionals, including Dan Krane.

Bruce Budowle also sent a letter of his own, criticizing the knife DNA evidence as improperly obtained.

Finally, the appeal court ordered an independent examination of the DNA evidence collection and testing, which led to the damning 2011 report by Prof. Conti and Prof. Vecchiotti of La Sapienza.

Peter Gill turned to this case relatively recently, apparently while working on his new book, Misleading DNA Evidence. The essence of his findings can be found in this interview with Italian reporters. He was even interviewed by The Daily Mail a few days ago but as I avoid linking to tabloids, it can be found by Googling (“Peter Gill” “Daily Mail”).

I would like to provide short excerpts from the book itself, as quoted by Dr. Zupancic on a support site. It’s remarkable that Prof. Gill has joined his opponents on the issue of LCN DNA usability, Bruce Budowle and Dan Krane, in criticizing the collection and testing of evidence in this nightmarish case. On the knife, Gill remarked:

…this is the kind of profile I would expect to observe, if it had originated from a contamination event.

On the bra clasp, Gill admits that

Y-chromosome testing suggested the presence of three or more males, and the DNA mixture was from three or more people,

which is exactly what the defense has been saying in support of the contamination hypothesis. Gill agrees that this hypothesis makes sense:

There is strong evidence to show that the failure of investigators to change their gloves in between handling items and potentially touching door handles is high risk, giving credibility to the defense proposition that Sollecito’s DNA was transferred as a result of cross contamination.

More generally, Peter Gill is concerned with the reasoning of the convicting judge Massei (referring to the 2009-10 trial; the logic of the supreme court and judge Nencini is similar):

…the problem is that the court debate has been based on speculation rather than experimentation – the latter is needed for any scientific debate.

In particular, Gill criticizes Massei for theorizing that Knox’s DNA on the handle of the knife comes from stabbing rather than merely cooking as

dangerous speculation that is another example of confirmation bias – there is nothing in the scientific literature that remotely supports such an inference.

Forensic science, Gills reminds us, “should not be based upon ‘armchair arguments’ about possibilities and theories.” And forensic scientists must be able to support their testimony with experimental data, not learned hand-waving:

If a scientist expresses an opinion, then this opinion must be qualified by experimental evidence. If an opinion is expressed that appears to have no supporting evidence (in terms of peer review or data analysis) so that it cannot be tested objectively, then it has no scientific basis.

Many thanks to Prof. Gill for defending good science and reason. I must add that by going through the incomplete information the police lab turned over to the defense in 2009, innocence supporters have detected evidence not merely of negligence but of massive data suppression and contamination.

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