Faulty or unreliable forensic techniques
There’s nothing more convincing in a criminal trial than an expert witness testifying that he found some kind of trace evidence—a fingerprint, a hair, a shoeprint—at a crime scene that must have been left by the defendant. As compelling as such forensic experts might be, there is a solid consensus in the scientific community that nearly every form of forensic evidence except for DNA is far less reliable than once thought. Unreliable forensic evidence contributed to the wrongful convictions of over 15% of Louisiana and Mississippi exonerees.
In 2009, the National Academy of Sciences (NAS) released a groundbreaking report titled Strengthening Forensic Science in the United States: A Path Forward. The NAS, established in 1863 by Abraham Lincoln, consists of prominent scientists who are commissioned by the U.S. Congress to report to it on scientific matters. For two years, at the request of Congress, the NAS reviewed forensic techniques, and the committee heard testimony from dozens of forensic examiners and federal and state law enforcement agencies, while also performing an exhaustive review of the existing literature on forensic techniques. The NAS’s major finding was that, “[w]ith the exception of nuclear DNA analysis, . . . no forensic method has been rigorously shown to have the capacity to consistently, and with a high degree of certainty, demonstrate a connection between evidence and a specific individual or source.”
Take microscopic hair comparison. We’ve all seen shows like CSI where a person in a white lab coat takes a hair found at the crime scene and places it under a microscope next to the prime suspect’s hair, and voila! they’re identical. But it turns out that, as the scientists from the NAS found, anyone of the same race and with the same hair color has microscopically similar hair, and it’s impossible for an examiner to say conclusively that two hairs “match.” Moreover, there are no studies of how common different hair types are, so an examiner can’t even say what the probability is that a particular hair might have come from a specific person.
That didn’t stop a hair examiner at Anthony Johnson’s trial from testifying that he found a microscopic hair “match.” The examiner said that he compared a hair found at the crime scene with Mr. Johnson’s hair, and he told the jury that he was “about 90%” sure that the hair at the crime scene came from Mr. Johnson. Because there are no frequency statistics for hair characteristics, that number—90%—was a complete fiction. But a person labeled as an expert said in a criminal trial, and Mr. Johnson was convicted and sentenced to life without parole before being exonerated by DNA testing 26 years later.
The following Louisiana and Mississippi exonerees were impacted by faulty or unreliable forensic techniques:
(Video credit to The Innocence Project)
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