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Just how reliable is DNA evidence in violent crime prosecutions?

As we discussed on this blog at the time, in June the U.S. Supreme Court ruled there is no constitutional bar to a law enforcement agency setting up a colossal DNA database and requiring all those arrested for “serious crimes” to submit DNA samples for inclusion. Law enforcement can then use the database to compare the arrestees’ DNA to any DNA evidence collected in unsolved violent crimes and sex offenses.

According to a recent New York Times op-ed by a law professor at the University of Californiaa Hastings, the main problem with this is the “myth of DNA infallibility” that far too many Americans buy into. Even when DNA evidence stands directly contradicts other reliable evidence, she says, jurors tend to believe that DNA tests are foolproof. Prosecutors who unknowingly or intentionally exploit that false belief could be responsible for wrongful convictions and miscarriages of justice.

While DNalysis is a valuable scientific tool both for law enforcement and for those seeking to exonerate the falsely accused, much more oversight is needed to prevent its misuse. The best way for prosecutors to ensure that DNA evidence is reliable, the professor points out, is to use it only to corroborate other evidence.

The main reasons DNA evidence might lead to a false conviction are crime scene contamination and transference, laboratory errors, and misleading testimony about its significance.

For an example of contamination, the professor cites a case in which German and Austrian police spent 15 years searching for what they thought was a female serial killer after trace amounts of the same woman’s DNA were found at six different murder scenes in the two countries. In 2009 they found that woman -- who was a factory worker at the company that produced cotton swabs for the police.

More troubling is testimony by police officers or lab techs that exaggerate the dependability of DNA tests. For example, witnesses often testify about the statistical likelihood of someone else having DNA which matches the crime scene evidence as closely as the defendant’s does. In one case, the witness said there was only a 1 in 1.1 million.

Even an FBI expert committee concedes that statistic was misleading because it purported to compare the defendant’s DNA to that of the general public. In fact, the defendant’s DNA was only compared it only to that of other suspects in the DNA database. An independent analysis found the chance was 1 in 3.

If our country will increasingly be relying on DNA evidence to convict people of serious and violent crimes, we owe it to ourselves to learn more about the reliability of that evidence. Prosecutors should never take a citizen’s liberty by exploiting myths.

Source: The New York Times, "High-Tech, High-Risk Forensics," Osagie K. Obasogie, July 24, 2013

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