That’s the takeaway from this well-sourced piece by Radley Balko that is entirely worth your time.

We’ve all seen stories, both in life and in fiction, about how bad guys got caught by fiber analysis, or bite analysis, or ballistics reports that conclusively prove that a given recovered bullet came from a specific gun, or whatnot. Turns out? None of that is backed by science, and when examined in blind tests the so-called pattern-matching experts tend not to do any better than a 50% accuracy rate. The root of the problem is that these “sciences” have typically been invented by law enforcement to achieve conviction, and those same organizations are DEEPLY unwilling to re-examine the accuracy of these kinds of evidence because, for them, convictions are FAR more important than anything so quotidian as justice.

The current exemplar of this attitude is the assistant general counsel of the FBI crime lab, a guy named Jim Agar, who is actively telling analysts

how to circumvent judges’ restrictions on unscientific testimony. He even suggests dialogue for prosecutors and analysts to recite if challenged. Most controversially, Agar advises analysts to tell judges that any effort to restrict their testimony to claims backed by scientific research is tantamount to asking them to commit perjury.

People are in PRISON over this stuff. People have been EXECUTED. It’s only been with the advent of DNA that the unreliability of pattern-matching evidence has really come to light, which is definitely good news — except one remaining are of pattern matching is not nearly as likely to be disproven by DNA: firearm tracing.

Guns work at a remove. You can shoot someone without being near enough to deposit DNA. And in the absence of DNA evidence to disprove an unethical prosecutor’s pattern-matching-backed theory, you might be up shit creek. Especially since, as the Agar memo shows, law enforcement organizations care far, far more about winning cases than they do about accuracy. Even at the FBI.