Let’s do a thought experiment; you are at home and there is a knock at the door. You open it and there is police detective there. He wants to question you about a murder from thirty-one years ago. You know you never murdered anyone, so you consent. Then the detective tells you that your DNA was matched to a sample from the crime scene, would you be worried? You should be.
"Originally posted at Squarestate.net"
The little scenario is exactly what happened to John Puckett. Michael Bobelian has a story in Washington Monthly with the details. In 1972 a nurse by the name of Diana Sylvester was sexually assaulted and murdered. There was an eyewitness who gave a very general description but the case never really went anywhere and relegated to cold case status.
In 2003 a slide with sperm taken from the victims mouth was unearthed and DNA tested. Thirty one years is a long time for DNA to stay viable and the sample was degraded. It produced only five and half of the markers called loci that allow identification of individuals. To have a 100% identification, you need 13, California requires a minimum of seven,. However this sample was run through their criminal database anyway. It produced a match, Mr. Puckett.
There are a lot of problems with this. First off, DNA is not the slam dunk that TV and exoneration's of some death row inmates make it seem. If you have all 13 of the makers there is almost no chance of a false identification. However as you get less and less markers the chances rise, steeply. From the article:
Barlow’s main point of contention was statistics. Typically, law enforcement and prosecutors rely on FBI estimates for the rarity of a given DNA profile—a figure can be as remote as one in many trillions when investigators have all thirteen markers to work with. In Puckett’s case, where there were only five and a half markers available, the San Francisco crime lab put the figure at one in 1.1 million—still remote enough to erase any reasonable doubt of his guilt. The problem is that, according to most scientists, this statistic is only relevant when DNA material is used to link a crime directly to a suspect identified through eyewitness testimony or other evidence. In cases where a suspect is found by searching through large databases, the chances of accidentally hitting on the wrong person are orders of magnitude higher.
The reasons for this aren’t difficult to grasp: consider what happens when you take a DNA profile that has a rarity of one in a million and run it through a database that contains a million people; chances are you’ll get a coincidental match. Given this fact, the two leading scientific bodies that have studied the issue—the National Research Council and the FBI’s DNA advisory board—have recommended that law enforcement and prosecutors calculate the probability of a coincidental match differently in cold-hit cases. In particular, they recommend multiplying the FBI’s rarity statistic by the number of profiles in the database, to arrive at a figure known as the Database Match Probability. When this formula is applied to Puckett’s case (where a profile with a rarity of one in 1.1 million was run through a database of 338,000 offenders) the chances of a coincidental match climb to one in three.
Think about that for a second. You are a convicted felon of some kind. Your DNA is in the States database. If they run an seven loci sample through the database, you have a one in three chance of matching the profile. This could lead to your arrest or conviction for a crime that you did not commit.
Now, you might be feeling pretty good about our justice system and think that you will be able to introduce the information about coincidental matches and let the jury sort out your innocence. That is probably a mistake too. Mr. Puckett’s attorney, Bicka Barlow, has a masters degree in genetics and molecular biology from Cornell. She is what you would call an expert on this very area of science. For all of that she was unable to get the judge to admit any evidence of coincidental matching.
Part of the reason is that the FBI does not want to allow this type of information to get into courts. They like the aura of infallibility that DNA evidence has.
Rather than try to sort out the disparities between its numbers and database findings, the FBI has fought to keep this information under wraps. After Barlow subpoenaed the Arizona database searches, the agency sent the state’s Department of Public Safety a cease-and-desist letter, warning that its conduct was "under review." Eventually, the Arizona attorney general obtained a court order to block Barlow’s distribution of the findings. In other instances, the FBI has threatened to revoke access to the bureau’s master DNA database if states make the contents of their systems available to defense teams or academics. Agency officials argue they have done so because granting access would violate the privacy of the offenders (although researchers generally request anonymous DNA profiles with no names attached) and tie up the FBI’s computers, impeding investigations. These justifications baffle researchers. In the December 2009 issue of the journal Science, dozens of biologists, geneticists, and forensic experts urged the FBI to change its secretive policy, saying that there was no way that allowing a handful of researchers to run database searches, each of which takes only a few minutes, would hamper investigations. They also dismissed the agency’s privacy concerns, saying, "The government frequently releases sensitive information under controlled conditions to verified researchers." Krane of Wright State University, who was the letter’s lead author, believes the real reason the FBI has blocked access is to avoid revealing the shortcomings in its own system. "Analysis of the offender database is sure to expose the misconceptions and errors in the method the FBI used to arrive at its rarity statistics," he told me.
In the end Mr. Puckett was convicted. There was almost no other evidence, since the case was 31 years old. The jury never heard the 1 in 3 statistic but was told the FBI’s 1 in 1.1 million statistic. Interviews with the jury afterwords showed this piece of information was decisive.
It seems pretty clear that an innocent man was convicted of a crime he did not commit. This is not very likely to be the last time. The FBI is pushing the expansion of DNA databases. They are taking DNA from all immigrants. States like California are now collecting and storing DNA from everyone arrested for a felony, whether they are convicted or not. With the expansion of the databases the chances for false matches on cold cases just goes up.
Unless the Courts start allowing juries to hear about the problems with cold DNA matches there will be more and more people like Mr. Puckett, arrested, tried and convicted for crimes that they did not commit, based on DNA evidence.
The floor is yours