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June 04, 2013

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Dan Markel

Michelle, you've given lots of thought to this issue--more than I have. Do we know how many Type 1 errors have been fixed by these DNA databases so far?
Is there a substantial risk that in many of the situations, prosecutors will argue that the presence of someone else's DNA only shows that the crime may have been committed by more than one person, and thus, we might not see as many type 1 error fixes as we think/hope?

Michelle Meyer

Hi Dan,

Of course hundreds of Type I errors have been fixed by DNA, but these usually occur through a convict managing to secure the right to use newly available technology to test the crime scene DNA in his case against his own DNA. I take it, in your first question, that you're asking instead about the power of offender databases (as opposed to crime scene and convict DNA) to correct Type I errors -- that is, to exonerate. I don't have numbers, but when I was researching these cases ten years ago, at least, the fact pattern usually worked something like this: After the convict is permitted to have himself tested against the crime scene DNA and no match results, then law enforcement will usually test the newly processed crime scene DNA against the offender database, and often they will get a hit for someone else already in the system. In some cases, the no-match for the convict is enough by itself to overturn the conviction or convince prosecutors to drop the charges, and the hit in the offender database is gravy. Here is such a case: http://arstechnica.com/civis/viewtopic.php?f=23&t=628338

But in other cases, as you suggest in your second question, prosecutors (or judges) aren't convinced, for a variety of reasons, that a no-match alone exonerates the convict. Whether they are convinced or not depends on both the details of the case (e.g., how likely it is that there were multiple assailants; how likely it is that crime scene DNA could have been innocently left) and also on the individual prosecutor's discretion and willingness to admit s/he was (probably) wrong.

For instance, in 2001, it was revealed that sperm left on the body of the Boston Strangler's final victim did not match that of Albert DeSalvo, who had been suspected of being the strangler (and was killed in jail while serving time for an unrelated offense). Prosecutors responded by saying that this didn't posthumously exonerate DeSalvo, who could have killed the final victim without raping her, and that it only suggested that at least one other person was present when she was killed. Compare that case with that of Jeffrey Grant, incarcerated awaiting trial in 1999 for being the Long Beach rapist. The same DNA was left at two rape scenes, and prosecutors were shocked when Grant's DNA didn't match. Still, they dropped all charges, saying, "You can speculate that whoever did these crimes might have had an accomplice, but it is only speculation, and you can't prosecute someone based on speculation."

In these circumstances, the convict's case for postconviction relief usually becomes stronger if his no-match is coupled with a hit in the database to someone else -- preferably, someone with an MO that fits the instant crime and with no ties to the convict. That's more probative than a no-match alone, and so this is where I see offender databases, as currently constructed, sometimes correcting (as opposed to preventing) Type I errors.

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