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How Infallible Is DNA Evidence?

We believe in DNA. The American public is confident that forensic science can establish an accused criminal’s guilt or innocence, unequivocally through DNA evidence. Some of this confidence derives from popular TV police and courtroom dramas, where a DNA match always proves the case and saves the day. But is DNA really as good as we think it is? Does a DNA match always establish guilt beyond the shadow of a doubt? Recent developments suggest some skepticism may be in order.

Errors in crime lab analysis

DNA evidence is only as good as the procedures used to collect and examine it. This spring, The Washington Post reported that the FBI had notified crime labs around the country that it had discovered errors in data used to calculate the likelihood of a DNA match in criminal cases. The problem concerned the population statistics data used to assess the probability, that DNA retrieved from a crime scene, matches that of a particular individual. Mistakes in the data led to overstating a match’s probability. The FBI attributes the data problems to human error, and technological limitations. Thousands of cases are likely to be involved, but the Post reports that the FBI does not believe that case outcomes are likely to be affected. Crime labs and lawyers, however, are reserving judgment about that.

Mixed DNA concerns

Lawyers are finding that retesting DNA evidence because of the FBI’s error notice is raising other concerns. A Texas Tribune article quotes a Galveston County district lawyer who received cause for alarm along with the results of DNA re-analysis using the FBI’s new population statistics protocols. In addition to the new protocols, the crime lab also used a more conservative analysis in examining what is known as “mixed DNA,” a term referring to a sample that contains DNA from more than one person – for example, from a doorknob or car door. The prosecutor explained that, under the new approach, the likelihood that the defendant in the case was the only source for the relevant DNA, dropped from one in a billion to less than one in one hundred. But not all crime labs in Texas, let alone the rest of the U.S., use that same approach. This means that, where one more conservative lab might find its analysis inconclusive, another might find a definite match. And police are collecting more and more samples of mixed DNA as a matter of course.

FBI hair analysis errors

Yet another Washington Post story reported recently that in an ongoing investigation that began back in 2012, the FBI has admitted that examiners with its own lab overstated forensic DNA hair sample matches in testimony they gave in criminal cases. Their mistakes ultimately favored the prosecution. Thus far, the FBI has reviewed 268 of approximately 2500 cases in which its examiners gave hair analysis evidence. The agency’s own review found that DNA matches were overstated in 95 percent of the 268 cases, including 32 in which defendants received the death penalty.

It is important to note that, in all these cases involving questionable DNA analysis, the presence of DNA may well not be the only evidence supporting a conviction. Certainly, in the Texas case discussed above, the district lawyer claims to have sufficient additional evidence to support his case. But, given the reverence with which DNA is often treated, especially in the popular media with which most jurors are familiar, it is worth noting that DNA analysis may not have entirely earned its reputation as a magic bullet establishing guilt or innocence.

An assertion that DNA evidence is infallible cannot be accepted at face value. If you have been arrested or need help with a criminal case, it is important to have an experienced criminal defense lawyer on your side. Contact the Miami lawyers of The Baez Law Firm today.

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