Weak DNA Evidence Poses Problems in Court


Prosecutors implicated Cleveland Barrett, of Chicago, in the sexual assault of a nine-year-old girl by telling the jury that Barrett’s DNA was found on the victim, but what investigators had found was actually much less powerful than what the public has come to expect from DNA evidence. The DNA was not from semen—common in rape cases—rather from cells taken from the girl’s lips. The DNA profile that the analyst was able to extract from the evidence was of little probative value, matching 1 in 4 African Americans, 1 in 8 Hispanics, and 1 in 9 Caucasians—hundreds of thousands of men in the Chicago area alone. 


Barrett was acquitted (after spending a year in prison), but experts say that there are numerous other cases across the country in which similarly weak DNA evidence is being presented in court, posing grave risks to the defendants in these cases and to the future of DNA evidence.


As Gregory O’Reilly, chief of the forensic science division for the Cook County public defender’s office, said to the Chicago Tribune:

“There’s a terrific power in the phrase ‘DNA match.’ And there’s a great risk that the jury will put great significance on this when it’s not significant at all.

It doesn’t only run the risk of convicting an innocent person or letting bad science into the courtroom, but you’re going to undermine the very power that’s behind DNA in the public consciousness. The meaning of ‘match’ will become so trivialized that you’ll mix powerful science with junk science, powerful evidence with junk evidence. It’s going to lead to a lot of confusion. Because this is DNA.”

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