by Roger Koppl
Most people think DNA evidence is bulletproof. Molecular biologist and forensic scientist Dan Krane has very nice presentation on some of the problems that can arise in this area. When the biological sample is mixed, degraded, or small, the evidence can be ambiguous, which allows subjective judgment to enter. Recent events in Baltimore point to another problem that has nothing to do with such ambiguity.
On September 27th, the Baltimore Sun reported, “In at least nine homicide, sex assault and burglary cases, Baltimore police detectives instructed crime lab technicians not to follow up on convicted criminals’ DNA found on evidence at crime scenes because they determined it was not relevant to their investigations.” The police have a plausible explanation. “Police spokesman Sterling Clifford played down the significance of the discovery, saying detectives routinely make judgment calls on evidence.” Moreover, says Clifford, “Very often those crime scenes are enormous, sometimes covering entire city blocks.”
The spokesman sounds plausible. The trouble is that the system gives discretion to one group of actors, the police, who may not always view evidence in the right light, as another story in the Sun illustrates. A woman is killed and her ex, Randy Golden, is convicted for the crime. Golden “was convicted on the account of a young, drug-addicted prostitute who testified that she witnessed the murder.” Now, “a city crime lab technician had clipped fingernails from [the victim’s] body, checked them for DNA and found a match to another criminal.” This DNA evidence was not pursued by the police, however, who (apparently) felt sure they had their man in Golden. Nor was it made known to the prosecutor or Golden’s defense attorney. Guess whose DNA was found? Yep, a friend of the eye witness. It kinda looks like the witness blamed the wrong guy on purpose to help her friend. Notice that the legal process of “discovery” cannot reveal information unknown to the prosecution.
The problem here should be familiar to economists, namely, monopoly. In most jurisdictions in the US we have put monopoly forensics under the control of the police. Economists appreciate the lesson that monopoly is not generally a good idea.
I think it is important to recognize poor police decisions about DNA do not necessarily reflect corruption, ill will, or indifference to the truth. It is the right and proper job of an investigating officer to formulate a theory of the case. Once such a theory has been formed, the universal human disposition to confirmation bias reduces the chance that disconfirming evidence will be given adequate weight. The issue is not getting the right people in the job, but giving them the right incentives, which is another lesson economists can appreciate.