New Scientist Puts Another Chink in DNA Forensics’ Armor

Posted by Osagie K. Obasogie on October 14th, 2010

Despite its presumed infallibility, several reports have repeatedly demonstrated that DNA forensics may not be the gold standard for determining guilt that it is often held out to be. For example, I wrote on the phenomenon of coincidental matches in DNA databases earlier this year in Science Progress:

a recent examination of Arizona’s 65,493 database profiles led to a surprising result: 122 pairs matched at 9 loci, 20 pairs matched at 10, and two pairs of siblings matched at 11 and 12. There are also reports that other state  databases are experiencing similar oddities. Illinois’ state databases   reportedly showed that out of 220,000 profiles, 903 matched at 9 or more loci. And it has also been reported that Maryland’s database had 32 pairs of profiles matching at 9 loci and 3 matched at 13. These figures call into  question the motivating claim behind DNA database searches—that profiles are unique and coincidental matches are extremely rare—which opens up   the possibility for false convictions.
In a two-part special report, the New Scientist’s Linda Geddes draws attention to the role of subjectivity in determining the significance of DNA evidence. Geddes writes:
Interpreting alleles in a mixed or partial sample is where the subjective opinion of an analyst could play a part. To test this, New Scientist teamed up with Itiel Dror, a neuroscientist at University College London and head of Cognitive Consultants International, and Greg Hampikian of Boise State University in Idaho.

We took a mixed sample of DNA evidence from an actual crime scene – a gang rape committed in Georgia, US – which helped to convict a man called Kerry Robinson, who is currently in prison. We presented it, and Robinson’s DNA profile, to 17 experienced analysts working in the same accredited government lab in the US, without any contextual information that might bias their judgment.

In the original case, two analysts from the Georgia Bureau of Investigation concluded that Robinson “could not be excluded” from the crime scene sample, based on his DNA profile. (A second man convicted of the same
crime also testified that Robinson was an assailant, in return for a lesser jail term.) Each of our 17 analysts independently examined the profiles from the     DNA mixture, the victim’s profile and those of two other suspects and was asked to judge whether the suspects’ profiles could be “excluded”, “cannot be excluded” or whether the results were “inconclusive”.

If DNA analysis were totally objective, then all 17 analysts should reach the same conclusion. However, we found that just one agreed with the original judgment that Robinson “cannot be excluded”. Four analysts said the evidence was inconclusive and 12 said he could be excluded.
In the second part of the special report, Geddes discusses how these different interpretations are due, in large part, to a lack of standards for determining match probabilities.  This can lead the statistical weight given to any one piece of DNA evidence to vary significantly. And, these various approaches can often mean the difference between being free and going to prison for a very long time. 

Posted in Civil Society, DNA Forensics, Media Coverage, Osagie Obasogie's Blog Posts


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