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Junk Science Puts Innocent People in Prison and Keeps Them There

by Matt Clarke

It has been years since the National Academy of Sciences and the President’s Council of Advisors on Science and Technology published studies casting serious doubt on courtroom claims of practitioners of “pattern matching” forensics, a broad field that includes comparative analyses of such items as hairs, fibers, bite marks, shoe sole impressions, “tool marks,” and tire treads. Yet people remain in prison whose convictions were based on those and other largely discredited forensics, such as “shaken baby syndrome.”

The friction between the sciences and courts results from a fundamentally different philosophy. Courts honor the primacy of precedent, i.e., courts view earlier opinions as controlling and strive for finality, so that once appeals are lost, a conviction becomes virtually unassailable. In contrast, science is a learning process in which questioning the past and constantly revising beliefs to comport with new data and understanding is considered a virtue. If a new theory shows past thinking to be in error, it is adopted. On the other hand, the legal system tends to bury its mistakes, prizing the finality of convictions and preserving the public’s faith in the judicial system above the continual search for the truth.

Perhaps the most thoroughly discredited junk forensic technique is bite-mark analysis. Both basic premises of the technique—that human bite marks are unique and that the skin works well to accurately preserve them—have been disproven. The history of bite-mark analysis shows how such junk science can become accepted by the judicial system and remain accepted despite compelling evidence to the contrary.

In 1975, a California court of appeal decided People v. Marx, the first appellate challenge to bite-mark analysis. Although the defense challenged the scientific validity of the method, the court of appeal upheld its use, ruling it was more a matter of common sense than science. Another court of appeal ruling three years later cited Marx and noted approvingly “the superior trustworthiness of the scientific bite mark approach,” despite the fact Marx stated that the forensic technique was not scientific at all. Within a dozen years, courts of appeal in 21 states had accepted bite-mark analysis without dissent. By 2004, it was accepted in 37 jurisdictions.

Unlike bite-mark analysis, DNA testing was developed in the 1990s by scientists and subjected to rigorous validity testing before it was used in the courts. It soon showed that bite-mark analysis was unreliable. Yet every time bite-mark analysis has been challenged, it has failed. In Wisconsin, the controlling case from 1987 upholding the validity of bite-mark analysis is one in which the conviction was overturned by DNA evidence in 2009, after the wrongly convicted defendant spent 23 years behind bars. At least two other states have controlling cases upholding bite-mark analysis in which the defendant was later exonerated.

The Innocence Project estimates that 45 percent of wrongful convictions involve faulty forensics. In 2015, the FBI estimated that its hair-fiber analysts had testified in about 3,000 cases. Northwestern University’s Medill Justice Project estimated that testimony about shaken baby syndrome contributed to about 3,000 convictions. Yet despite the diagnosis of shaken baby syndrome having been discredited, the National Registry of Exonerations list only 16 shaken baby syndrome convictions that have been overturned.

Those defendants have fared much better than the defendants whose convictions were influenced by other types of bogus science and scientists. Many defendants who can disprove the purported science or discredit the scientists used to convict them still cannot get relief due to the judicial system’s adoration of finality and resultant procedural hurdles such as brief time limits for filing for federal post-conviction relief. 


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