DNA Identification: Contamination Happens, But Don’t Paint All DNA Matches as “High Risk”

Posted: July 29, 2013 in Failed Forensics
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In today’s world, we think of DNA matching as the gold standard of identification: a scientifically precise method of matching human tissue or fluids left at a crime scene to a particular individual.  But in an op-ed article last week, High-Tech, High-Risk Forensics, published in the New York Times, Cal Hastings professor of law Osagie Obasogie reminds us that even DNA identification isn’t foolproof.  It’s an excellent point and one worth remembering.  But then he stumbles over the details of DNA identification, labeling it high risk, because many people might have the same DNA profiles.  Risk is always present, but is the risk of getting DNA identifications high because many people might have the same DNA profile?  No.

Professor Obasogie tells us the story of a man arrested for a murder near San Jose, California, when his DNA was found on the victim’s body.  Only after the man was arrested and jailed did his alibi surface: at the time of the murder, he was a patient at a hospital, suffering from severe intoxication, and there were voluminous records to prove it.  He was freed after five months, and prosecutors now think the most likely explanation is that paramedics who transported the man to the hospital were the same ones called to the crime scene later that night.  The DNA was likely transferred to the victim from the paramedics’ clothing or equipment.  Professor Obasogie decries “the certainty with which prosecutors charged Mr. Anderson with murder,” because it  “highlights the very real injustices that can occur when we place too much faith in DNA forensic technologies.” This is hard to argue with; for those who can remember this far back, it recalls lawyer Barry Scheck cross-examining Dennis Fung in the O.J. Simpson murder trial over the sloppy collection of DNA and other evidence.  Scheck and other lawyers argued that the DNA that seemed to implicate Simpson just couldn’t be trusted; “garbage in, garbage out.” Professor Obasogie is not saying that the paramedics did anything wrong or were sloppy in any conventional sense; rather he’s arguing (correctly) that contamination can happen even when we’re not aware of the possibility.  Thus caution is always advisable, even with DNA.

But then Professor Obasogie begins to argue that there are deeper problems with DNA identification than just contamination.

Consider the frequent claim that it is highly unlikely, if not impossible, for two DNA profiles to match by coincidence. A 2005 audit of Arizona’s DNA database showed that, out of some 65,000 profiles, nearly 150 pairs matched at a level typically considered high enough to identify and prosecute suspects. Yet these profiles were clearly from different people.

Professor David H. Kaye of Penn State Dickinson School of Law has pointed out the problem with this argument on his blog Forensic Science, Statistics & the Law.  In a blog post on July 26, Professor Kaye explains why Professor Obasogie is wrong on this point.  In his words:

The 150 or so matches were, in fact, mismatches. That is, they were partial matches that actually excluded every “matching” pair. Only if an analyst improperly ignored the nonmatching parts of the profiles or if these did not appear in a crime-scene sample could they be reported to match.

There is much more to Professor Kaye’s thorough and lucid explanation.  If you are interested in a real understanding of how DNA actually works, I strongly recommend Professor Kaye’s post.


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