He says the problem isn't limited to Texas. He says the newest, best analysis method — called "probabilistic genotyping" — takes time to roll out, and that's put labs in a quandary.

"There's this interim time that cases are coming up and the analyst has to do something with it, and they know by definition that there is a better approach," Inman says.

Meanwhile, the justice system's hunger for DNA evidence just keeps growing. There are now police departments that have made swabbing for DNA part of their routine.

"We collect DNA evidence daily," says Jim Ferraris, deputy chief in Salem, Ore. His department has taken advantage of quicker testing provided by the state lab, and he says every officer in town is now trained to collect DNA. They even swab stolen cars and burgled homes.

"The doorjamb area, the point of entry — we'd swab that area," says Ferraris. "Let's say the dresser got rifled, we'd look on the handles. DNA has been folded into the fabric of what we do every day here."

All that swabbing has paid off. They've found DNA links between crimes, and they've found suspects. Ferraris believes the DNA-swabbing has led to a decrease in property crimes.

At the same time, these "touch samples" can be very challenging for the labs. When you take a sample from a doorjamb, the sample may include DNA from several people, in roughly equal proportions, all mixed together.

Faced with ambiguous samples like that, results can vary. A lab using one method may find a match, while another lab, using a more conservative analysis, may judge the same sample to be inconclusive.

In the world of scientific research, this would be seen as normal; scientific doubt is considered part of the process. But NYU law professor Erin Murphy says in the world of courts and lawyers, those doubts aren't always understood.

"We have this tendency in criminal justice to look for solutions to the most vexing problem, which is, 'How do we find dangerous people and stop them?' And when we look for those solutions, we like for them to be perfect," Murphy says.

I feel like we're clinging to a life raft of DNA, and saying, 'Well this one will save us!'

Erin Murphy, NYU law professor

Murphy has written a new book about this called Inside the Cell: the Dark Side of Forensic DNA. She says people should understand that DNA analysis can involve "subjectivity," and she worries about the tendency of juries to look past doubt.

"[Juries] are even willing to go a step further and say, 'We'll convict on the basis of DNA, even in the face of evidence — non-DNA evidence — that this isn't the perpetrator," Murphy says.

She has a pet theory about this. She says the public has seen other kinds of evidence discredited over the years, such as bite mark analysis and hair analysis. Even eyewitness testimony is now seen as unreliable. So we look to DNA.

"I feel like we're clinging to a life raft of DNA and saying, 'Well this one will save us!' " Murphy says. "But I think that both wrongly inflates DNA's capabilities, and it also is just overlooking the part of the story of these old techniques, where they were used in our system for decades without challenge."

If we now push this technology too hard — say, by prosecuting someone on nothing more than a genetic match — then DNA may also be headed for the kind of reassessment that has battered the reputation of older types of physical evidence.

Copyright NPR 2021.