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DNA Evidence: How Infallible is it?

A recent article in the Washington Post shed some light on a new technique used by law enforcement officers to take advantage of the tremendous power of DNA analysis technology to catch criminals on the loose. Facing the challenge of trying to identify criminals without a viable DNA sample, officers are using samples from family members of suspected criminals in order to compare these family DNA samples to known samples of criminals.

Specifically, many states are beginning to use DNA taken from crime scenes to compare DNA not only of previously convicted criminals in their databases, but also of the family members of suspects. Though in some instances, privacy laws would prevent an analyst from revealing a potential suspect to investigators, some experts say that this new broader application of DNA sample matching could raise the number of suspects identified through DNA by as much as 40%.

Many are decrying the widespread practice of finding suspects through family members’ DNA samples, claiming that officers are turning family members into unknown and unwitting informants, and it is an example of evidence misuse (taking evidence for one stated purpose and using it for another). It could also work to further stigmatize innocent individuals who are related to criminals, many of whom may be working to shed the burden of this association.

But this discussion is operating on the assumption that DNA evidence is infallible, or at least overwhelmingly persuasive. In fact, biometric information such as fingerprints or DNA are considered circumstantial evidence, according to the rules of evidence in federal criminal law as well as most state laws.

Why is this the case? Well, primarily, it’s because these identifiers that are commonly believed to be nearly flawless in determining ID, are in fact imprecise, especially fingerprints. A judge in Baltimore declared that fingerprint evidence presented in a case did not meet the standards of evidence to be admitted.

But CSI fans will be quick to point out that DNA evidence is a different story, that DNA in fact far exceeds fingerprints in terms of identification accuracy. Firstly, the certainty associated with DNA evidence is often misunderstood. DNA can definitively eliminate a potential suspect, but can only deal in probabilities when it comes to positively matching a suspect. A DNA analyst will determine the likelihood of a DNA match in terms of how many persons have that sequence in their genes; when the probability exceeds the population of the earth (1 in approximately 6.6 billion), then a DNA match is considered exact. But analysts can’t always determine with that level of accuracy.

Secondly, there are many other factors to consider that are often obscured by the unshakeable faith that the public has come to place in the accuracy of DNA testing. Take the treatment of crime scenes, for example. Law enforcement officers and forensic scientists are careful with crime scenes, but they’re just error-prone humans. Evidence left at the scene of a crime is in fact circumstantial evidence, and a positive or negative match of DNA is only as accurate in determining guilt as the rest of the case that law enforcement builds.

One unfortunate incident that demonstrates the fallacy of putting too much faith in DNA evidence is the case of Mark Minick. The 26-year-old was charged in October 2007 with the rape of a teenage girl seven years earlier while she was spending time at a hospital where Minick worked as porter.

Minick had been serving a year-long prison sentence for robbery when he was taken in and questioned in regard to the rape. Detectives who interrogated him claimed they had irrefutable DNA evidence that tied him to the scene of the crime in the form of a hair they found in the girl’s room. (Of course, as a hospital porter, it would have been easy for Minick to unwittingly leave a hair in that room or any room during the course of his job.)

What police didn’t tell Minick was that the girl had described her attacker as “black, large and tall.” Minick is short (5’6″ according to news reports), thin and white. Focusing so closely on matching the DNA in the case, these law officers had gone so far as to use their circumstantial evidence to trump the description provided by the only eyewitness to the crime. While the scientific case had been certain, it hadn’t been used properly with traditional policing methods to form a case.

Perhaps law officers themselves aren’t immune to the CSI effect.

DNA evidence is indeed the most precise method we have in identifying crime scene samples and matching them to criminal records, but whether or not it’s an effective criminal detection and conviction tool depends largely on how it’s used. New techniques like using the DNA of family members may help identify or eliminate potential suspects effectively, but with that powerful tool comes additional responsibility for investigators to use that evidence properly.

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