Who needs that new-fangled DNA anyway?

December 16, 2008

by Roger Koppl

It seems some police and prosecutors do not view DNA evidence as friendly to their cause.  I’ve come across two cases in which the police won’t use available DNA evidence.  Earlier I blogged about Baltimore, where the police were ignoring DNA evidence.  Now the Chicago Tribune reports, that in Lake County, Illinois, “authorities have sometimes pressed for convictions even when the DNA doesn’t match a suspect.”  The story recounts several episodes, including two in which the child victims “were found with semen in their bodies, and in both cases DNA tests revealed genetic profiles that do not match the defendants’ DNA.”  In one of those cases the authorities claimed the semen could have been picked up when the child played in the woods at spot known for romantic encounters.

In the Lake County and Baltimore case we have another illustration of the technical obsolescence of forensic fraud, I’ve discussed in an earlier blog. Susan Feigenbaum and David Levy (1996. “The Technical Obsolescence of Scientific Fraud,” Rationality and Society, 8: 261-276) point out that you can induce bias by running several tests, each unbiased when considered individually, and selectively reporting only the results tending to favor your preferred conclusion.   I’m beginning to wonder if some police and prosecutors are applying a similar logic by refusing to consider potentially exculpatory DNA evidence.  

I’ve been pushing for an end to the epistemic monopoly of crime labs.  In these cases, however, the monopoly power exists upstream in the police and prosecution.  

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