DNA evidence is incredibly helpful. Both to persuade a jury to convict and, occasionally, to exonerate.
Usually, DNA evidence is just a start. Especially in "cold hit" cases. You identify a suspect, either independently or through a DNA match, and then you build up a case from there. Witnesses. Some physical evidence. Maybe a confession. The combination of all of these results in a conviction.
I haven't seen a case in which basically the only evidence against the defendant was the result of a "cold hit" off the DNA.
A taxicab driver, Jose Martinez, gets killed by a shot to the back of his head. No one can identify the person who shot him, or who got in the cab. No fingerprints. No gun. Nothing. The only evidence that exists is some blood in the back seat of the taxicab. Blood that presumably came from the person who shot Mr. Martinez, since the taxicab likely crashed shortly after the driver was shot, resulting in the back seat passenger (likely) being thrown against the front seats and bleeding.
Detectives enter the DNA from the blood into a database. No hits. But a year later, a cold hit comes up. Xiong. Who lives within a mile or so of the crime scene. So they interrogate him. Nothing. No evidence. Nothing from any search. No confession. No independent evidence at all. Just the DNA evidence and a bump on Xiong's head. Maybe from being hit in an alley, maybe from a long-ago accident in a taxicab, maybe from something else. Essentially, it's all about the DNA. Xiong's DNA matches the blood in the cab, and the probability that it would match any particular random person is low.
Proof of guilt beyond a reasonable doubt?
The jury decides it is, and convicts Xiong of first-degree murder, and he's sentenced to life without the possibility of parole. The Court of Appeal affirms. Holding that the DNA evidence alone, even from a cold hit, is sufficient to establish guilt beyond a reasonable doubt.
That's even true despite the fact that the prosecution never introduced evidence about the probability of a random match between the DNA in the taxicab and the DNA in a random Asian. There's a very low probability of a match between the suspect DNA in the taxicab and that of a random Caucasian, African-American, or Hispanic. There are readily available databases of Asian DNA -- the ethnic group to which the defendant belongs (Hmong) -- but the expert here didn't use any of them. That's despite the fact that, presumably, if there's a DNA match between the blood and Xiong, there's likely a higher probability of a similar match to an Asian (with similar DNA) than to someone in a different ethnic group.
No matter. The DNA evidence alone is good enough. Cold hits are themselves sufficient to establish guilt.
My money's on Xiong being guilty. But isn't it a scary prospect if he isn't?
It's a cold hit. If your (or my) DNA matches the blood -- and everyone admits it's possible, just not very likely -- then we go to prison too. Until we die. There may be absolutely no evidence against us other than the random hit. But that's enough. We're convicted entirely on the basis of our DNA.
And, at some point, a random hit happens that identifies someone innocent. It's a mathematical (as well as human) certainty. They go to prison forever as well.
As I said, I've never seen a case that's based entirely on a cold hit. I have now. And it frightens me. And would frighten me even more if I thought that there's someone out there who may well have 15 loci in their DNA that are the same as mine.
Which there may well be.
For all of us.