DNA as failed evidence: When is a match not a match?

Posted: July 7, 2012 in Failed Forensics, Resisting Science, Wrongful Convictions
Tags: , , , , , , , , , ,

DNA evidence has become a forensic tool of precision and power in the criminal justice system.  Nothing can touch its scientific pedigree, the strength of its ability to make an identification, or its rigorous protocols for collection, analysis, and presentation of probability-based evidence.

But something new is on law enforcement’s radar: using DNA in a way that brings us back to the bad old days of using the blunt instrument of serology to “match” a perpetrator to the crime.

In July 5’s Chicago Tribune, reporter Steve Mills’s article, “Weak DNA Evidence Could Undermine Justice, Experts Say,”, tells the story of Cleveland Barrett, who was arrested for a predatory criminal sexual assault on a 9-year-old girl.   Part of the evidence against Barrett: according to prosecutors, DNA showed Barrett committed the crime.

But this wasn’t the kind of millions- or billions-to-one DNA identification we’ve grown accustomed to seeing, because prosecutors were using tiny amounts of DNA in an unusual way.  According to Mills’ story, the DNA used in Barrett’s case

was different. It was not from semen, as is often the case in rapes; instead it came from male cells found on the girl’s lips. What’s more, the uniqueness of the genetic link between the DNA and Barrett was not of the 1-in-several-billion sort that crime lab analysts often testify to in trials with DNA evidence. Instead…the genetic profile from the cells matched 1 in 4 African-American males, 1 in 8 Hispanic males and 1 in 9 Caucasian males.  Fact is, the DNA profile from the cells on the victim’s lips could have matched hundreds of thousands of men in the Chicago region.

And yet, the prosecutor’s closing argument used the word that juries know signifies an almost irrefutable, scientifically solid identification: “match.”

The use of weak DNA evidence like this is “preposterous” and has “almost no value at all as evidence,” according to Edward Blake, a leading DNA authority quoted by Mills.

Just what the criminal justice system needs: a way to make some of the most powerful scientific evidence ever devised into misleading junk science that claims to “match” a defendant to a crime.  Law enforcement resists existing science that can make its basic procedures like eyewitness identification, interrogation, and forensics better; yet it seems determined to push one of its best tools away from scientific precision, and toward inaccuracy.

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