Why do we need a statute of limitations when we have DNA?

Sometimes I wonder whether the advent of DNA testing has been such a great thing for criminal defendants. Sure, it's been great for all those exonerated defendants. And in newer cases, it has undoubtedly prevented many suspects from being wrongly charged. But those cases in which the presence of DNA clearly points to a person's guilt (or innocence where there isn't a match) are only a percentage of the cases involving DNA.

Prosecutors always bemoan the "CSI effect", whining that juries expect them to provide physical evidence and won't convict without it. But I think there's another side to the CSI effect, which is that juries accept physical evidence, especially DNA, as absolute proof of guilt regardless of the circumstances. The presence of DNA often isn't very probative of guilt. We leave our DNA all over the place. And DNA comparison isn't quite the exact science CSI makes it out to be. But you wouldn't know any of that from the way we bow down to DNA as the gold standard of proof. If we have your DNA, we have you.

(See the Amanda Knox trial in Italy: her DNA is on a knife we think was used in the crime, so she must be guilty. But, the knife was in a kitchen that she had cooked in, so isn't there a perfectly innocent explanation for the presence of her DNA on that knife?)

Now, prosecutors want to be able to charge a DNA profile, even when they have no idea who the profile belongs to. The idea is that with some kinds of crimes, we know that we want to charge whoever left that DNA behind at the crime scene, but we don't know that individual's name. So, we charge "John Doe" and attach the DNA information. Otherwise, if the DNA profile can't be charged, the statute of limitations will expire and the perpetrator will never be able to be prosecuted. (The statute of limitations is satisfied as long as the charge is filed before the time expires.) But this way, we can file the charges in time and then pursue the trial if we ever locate the guy.

I know Kansas has tried this in the past, but the charging documents were not sufficient, though our Supreme Court left open the possibility that a DNA profile could be charged if it were done better. According to this NY Times article, several states explicitly allow these types of John Doe charges. Kansas now has a different way to extend the statute of limitations in cases where a DNA match is made much later. We have a statute that authorizes charging an individual with certain sex offenses within a year of matching the DNA profile to a specific suspect.

Last week, we learned that a man had finally been charged in a 1990 attack on a young girl in Texas. Miraculously, the girl had survived, so the top charge is attempted capital murder. While the rest of the internet rejoiced that the man who left that poor girl for dead had been caught, my lawyer brain kept wondering about the statute of limitations. (Well, that and the presumption of innocence, but reminding people that he is still entitled to that presumption usually proves futile.) Of course this was a case where the DNA left behind on the girl's body was finally matched to a name. I have yet to find any explanation of what allows for such an untimely prosecution in the Texas case.

I know it feels really good to find the guy who hurt that Texas girl or the women mentioned in the New York Times article. But I'm still troubled by these very late prosecutions. We have statutes of limitations for a reason. It's really hard to defend yourself against a charge when everything happened 20 years ago. Good luck trying to remember where you were or who you were with on that Tuesday. Good luck trying to find witnesses who might actually remember something that doesn't jibe with the state's theory of your guilt. What difference could that make, though, when there's DNA, right? You can't possibly have been somewhere else or done anything else that day because we have your DNA.

But DNA evidence, like any other evidence, is still only as good as the people testifying about it: the people who collected it, transported it, stored it, cut it, and tested it. And there still is a human element to declaring a DNA match. If we're trying people 20 years later, we've most likely lost any meaningful opportunity to probe the methodology used by those people who handled the evidence because memories fade, computer systems crash and scramble data, paper files get lost or damaged. If we try people so long after the crime, it becomes very difficult to uncover any problems with the evidence or the methodology used and we're left, then, with the result being almost uncontestable. We say we have your DNA and you have no good way to show that we are wrong, so you must be guilty. Ultimately, I think that's a dangerous attitude to take about DNA evidence, especially given the National Academy of Science's recent findings about the state of forensic crime labs.

I'm all for prosecuting rapists and other violent criminals, but I'm not sold on the idea that we should make statute of limitations exceptions for DNA evidence. Statutes of limitations exist for a reason: because evidence goes bad. DNA evidence really isn't all that different. It isn't infallible. Like any other kind of evidence, the witnesses presenting it need to be questioned and cross-examined and put to the test. And like with any other kind of evidence, that's a lot harder to do 20 years after the fact.