Is forensic evidence trustworthy?

Science in fiction affects our ability to understand science in real life. For instance, you might already be familiar with the idea that detective shows on TV, particularly forensics shows like CSI, might be influencing what juries expect to see in a courtroom.

This is called the "CSI effect" and it's hotly debated. Some prosecutors think it has a real impact on jury decisions—if they don't get the fancy, scientific evidence they've been conditioned to expect then they won't convict. Meanwhile, though, empirical evidence seems to show a more complicated pattern. Surveys of more than 2000 Michigan jurors found that, while people were heavily expecting to see some high-tech forensic evidence during trials, that expectation probably had more to do with the general proliferation of technology throughout society. More interestingly, that broad expectation didn't seem to definitively influence how jurors voted during a specific trial. In other words: The jury is still out. (*Puts on sunglasses*)

A FRONTLINE documentary that airs tomorrow centers around an interesting corollary on this issue: Whether or not shows like CSI influence juries to expect more technology, they do present a wildly inaccurate portrait of how accurate that technology is. The reality is, many of the tools and techniques used in detective work have never been scientifically verified. We don't know that they actually tell us what they purport to tell us. Other forensic technologies do work, but only if you use them right—and there's no across-the-board standard guaranteeing that happens.

Even ideas you think you can trust implicitly—like fingerprint evidence—turn out to have serious flaws that are seriously under-appreciated by cops, lawyers, judges, and juries.

Brandon Mayfield, an Oregon lawyer, was at the center of international controversy in 2004 after the FBI and an independent analyst incorrectly matched his prints to a partial print found on a bag of detonators from the Madrid terrorist bombings.

Dror asked five fingerprint experts to examine what they were told were the erroneously matched prints of Mayfield. In fact, they were re-examining prints from their own past cases. Only one of the experts stuck by their previous judgments. Three reversed their previous decisions and one deemed them “inconclusive.”

Dror’s argument is that these competent and well-meaning experts were swayed by “cognitive bias”: what they knew (or thought they knew) about the case in front of them swayed their analysis. The Mayfield case and studies like Dror’s have changed how fingerprints are used in the criminal justice system. The FBI no longer testifies that fingerprints are 100 percent infallible.

Watch a short video that explains more about the flaws in fingerprint analysis.

The Real CSI episode of FRONTLINE airs tomorrow, April 17th. Check out the FRONTLINE website for more information.

Image: Fingerprint developed with black magnetic powder on a cool mint Listerine oral care strip, a Creative Commons Attribution (2.0) image from jackofspades's photostream.