The Evidence That Lied: Forensic Myths and Wrongful Convictions

In twenty-four percent of all wrongful convictions in the United States, misleading forensic evidence was a factor. Last week, you heard the stories of people who lost years to mistaken identity, and today we examine one of the most powerful engines of that injustice: the evidence that was presented as science. For decades, in courtrooms across the country, experts took the stand. They presented evidence that seemed irrefutable. Microscopic hair comparisons that allegedly linked a suspect to a crime scene. Analyses of bite marks on a victim’s skin, declared to be a unique match, just like a fingerprint.

To a jury, this testimony sounded definitive. It was presented as fact, as the cold, hard truth. And it sent hundreds of people to prison. Here is what those juries were never told. The scientific foundations for many of these techniques were weak, or in some cases, non-existent. There were no validated error rates for bite-mark comparison. There was no way to say for sure that a single hair found at a scene belonged to one person and one person ONLY. The testimony often exceeded the limits of what the science could actually support. It gave the appearance of certainty. And in a courtroom, that appearance is a powerful thing.

The incentive, for prosecutors, was a conviction. For the so-called experts, it was a career. The one who lost was the person sitting at the defense table. The unravelling began quietly, in labs, with the advent of post-conviction DNA testing. This was a science with verifiable standards and vanishingly small error rates. And it began to tell a different story. In case after case, DNA proved that the person convicted by bite marks or hair analysis could not have committed the crime. Keith Harward, convicted in 1982 on bite-mark evidence, was exonerated in 2016. Ray Krone, convicted in 1992, was freed in 2002.

Carlton Lewis was exonerated in 2023, after thirty-one years in prison. The truth was always there, waiting in the biological evidence. It just took decades for the science to be able to read it. The problem was never just one bad expert or one flawed test. It was systemic. In Canada, the Goudge Inquiry examined the work of a single forensic pathologist, Doctor Charles Smith, whose errors led to multiple wrongful convictions. The inquiry produced one hundred and sixty-nine recommendations for reform. In the United States, there is no equivalent federal body. Texas has a commission to review unreliable techniques.

But for the most part, oversight is a patchwork. The American Board of Forensic Odontology finally declared a moratorium on using bite-mark evidence in 2016. That was years after DNA had already proven it was unreliable. So what does this all add up to? It adds up to the 635 cases where the National Institute of Justice found that forensic science errors contributed to a wrongful conviction. It adds up to the fact that in more than half of all the cases overturned by the Innocence Project, faulty science was a key ingredient. Each of these cases should have been treated as what one expert, Doctor John Morgan, calls a "sentinel event." A warning light.

A signal that the system itself has a deficiency that needs to be fixed. Because the cost of getting it wrong is measured in human lives. The goal was never just to prove one person's innocence. It was to ensure the same mistakes don't happen again. That work continues, one case, one hearing, one piece of evidence at a time.