By Ryan Singel | The feds will soon be collecting about one million DNA samples a year under a new program that lets federal agents collect cheek swabs from citizens merely arrested for any federal crime or from any non-citizen detained by federal agents — including visitors to the country who have visas.
The intent is build a massive database of DNA samples (.pdf) that police can use to catch rapists and murderers, but even the innocent should fear being in the database, due to the vagaries of how cold case DNA searches can easily pinpoint an innocent person.
Thanks to an amendment in the Violence Against Women Act of 2005 that was sponsored by Sen. Jon Kyl (R-Arizona), the feds now have the authority to immediately take DNA from any arrestee or ‘detained’ non-citizen and immediately upload it to the FBI’s CODIS database. That database is currently fed by federal law enforcement agencies and all 50 states, a few of which collect and upload DNA samples from people arrested, but not convicted of a crime.
DNA profiles are composed of 13 genetic markers that are meant not to reveal genetic makeup or disease. Like fingerprints, DNA are very powerful and scientifically sound evidence, when used to connect a known suspect to evidence found at the scene of the crime. Jurors are easily persuaded to accept the DNA link for someone who had already been suspected of a crime scene when told the odds against a false identification are 1 in millions or billions.
But DNA is far less certain when you compare one sample against all of the profiles in the database typically known as one-to-many. In that case the chances that a match between a DNA sample — especially an incomplete one — and a person in a DNA database could nab an innocent person has different math. Very different math.
So if you have a probability of 1 in 1.1 million chance of people having a certain sequence of DNA markers and you have a database of 550,000 people, you have a 50% chance of making a match. That’s great, if you know that the perpetrator is in that database. But what it also means is that as you start testing DNA profiles against more and more people, the chances that you will match an innocent person to a DNA profile from a crime scene gets higher.
A recent L.A. Times story about a cold case prosecution of a 1972 rape and murder in California, where 30 years later, police matched a DNA sample from the scene to that of a convicted rapist in its 338,000 profile strong DNA database. Given the number of markers that were used there was a one in three chance that some profile in the database would match. In this case, it matched John Puckett, who lived in the same city.
The jury however, wasn’t told about the probability that someone in the database would match against the profile (The L.A. Times story erroneously says that there was a 1/3 chance that someone innocent would be fingered in such a search. If one knew for a fact that every person in the database were innocent, then there was a 1 in 3 chance that an innocent person would get fingered, but in Puckett’s case, one simply knows that there was a one in three chance someone in the California criminal database would be fingered.)
And that’s a problem when the government starts collecting millions of DNA samples, sticking them in a massive database and finding ‘cold hits.’
Imagine the innocent man facing down a jury of his peers, hoping that they understand something about statistics.
The Justice Department is taking comments on the proposed DNA rules until Monday, May 19.