Should we trust DNA?


SFGate | The role of DNA testing in the justice system has seemed unassailable – who can argue with the odds of two people sharing the same genetic markers being, in some cases, as low as 1 in 113 billion? So DNA testing has been used to convict defendants in cases that are otherwise scant of evidence, and it’s been used to spring prisoners who rotted in jail for decades for crimes that they said they didn’t commit.

Now, it turns out that DNA might not be so reliable after all.

Seven years ago, a state crime lab analyst was running tests on Arizona’s DNA database when she found two felons – one white, one African-American – who had nine out of 13 matching loci. According to the FBI, the odds of two unrelated people having a similar match was 1 in 113 billion. The analyst found dozens of similar matches in the years to come, and eventually news of her discovery spread to defense lawyers and the courts, despite aggressive attempts by the FBI to block similar searches. Considering how much we’ve come to rely on DNA tests in the judicial system, the FBI’s resistance to allowing other expert scientists test the accuracy of their official statistics borders on criminal.

It’s also completely illogical. The DNA system isn’t under attack here – it’s the numbers, and the way they’ve been used, that are.

“It isn’t that the DNA technology is problematic,” said David Faigman, the John F. Degardi Distinguished Professor of Law at UC Hastings School of Law. “It’s that the explanation for what the DNA is doing that can be problematic. It tells you something, but perhaps it doesn’t tell you everything that you want to know.”

Statistics of “one in a billion” can be misleading if you are searching through data banks with tens, hundreds of thousands, or even millions of people, Faigman explained. Those kinds of data bank searches are usually used in cold cases – no suspect, no motive, and no witnesses.

“It’s an issue for the legal system,” Faigman said. “Do you allow DNA hits in cold cases absent any other evidence? I would hope that there would be other evidence that’s solid if you’re going to get a conviction.”

Unfortunately, the FBI seems to think that the only way is its way. The bureau has sought to discredit the analyst’s findings and urged state authorities to resist requests for similar searches. FBI officials have even threatened to expel offending states from the national DNA database.

Their response is ridiculous and reprehensible. People’s lives are at stake. They must open their databases to outside scientists, and they must do it now.