Defrosting Cold Cases

The Washington Monthly has an excellent article by Michael Bobelian about issues related to DNA that are unknown to many and kept under wraps by few. However, the few do it very well.

This article is about the statistical chance that a DNA match is found when running a sample in a cold case through DNA databases. The media and the numerous TV shows make you believe that the chance of hitting a match is rare hence when you find a match…bingo! You have the suspect who left the physical evidence. Case closed, right? Wrong!

This scenario is only correct when police use fresh DNA material to link a crime directly to a suspect identified through eyewitness accounts or other evidence. Then the chances of accidentally hitting on an innocent person are extraordinarily slim. But, when suspects are found by combing through large databases, the odds are exponentially higher.

“Over the past quarter century, DNA evidence has transformed criminal justice, freeing hundreds of innocent people and helping unravel countless crimes that might otherwise have gone unsolved. It has also captivated the public imagination: the plots of popular TV crime shows often hinge on the power of DNA to crack impossible cases, which has helped to give this forensic tool an air of infallibility—a phenomenon known in criminal justice circles as “the CSI effect.” This fail-safe image is not entirely unfounded, especially when it comes to traditional applications of DNA evidence. But increasingly DNA is being used for a new purpose: to target the culprits in cold cases, where other investigative options have been exhausted. All told, U.S. law enforcement agencies have conducted more than 100,000 so-called cold-hit investigations using the federal DNA database and its state-level counterparts, which hold upward of 7.6 million offender profiles. In these instances, where the DNA is often incomplete or degraded and there are few other clues to go on, the reliability of DNA evidence plummets—a fact that jurors weighing such cases are almost never told. As a result, DNA, a tool renowned for exonerating the innocent, may actually be putting a growing number of them behind bars.

When analyzing DNA, scientists ideally focus on thirteen markers, known as loci. The odds of finding two people who share all thirteen is roughly on par with those of being hit by an asteroid—about one in a quadrillion in many cases. But the fewer the markers, the higher the probability that more than one person will match the same profile, since relatives often share a number of markers and even perfect strangers usually share two or three. Such coincidental matches are more than a theoretical possibility, as the Chicago police can attest. In 2004, detectives investigating a string of robberies on the city’s North Side found some skin cells that the culprit had left behind at one crime scene, which contained six DNA markers. When they ran this profile against Illinois’s offender database, they found it matched a woman named Diane Myers. There was just one problem: when the burglaries in question were committed, Myers was already in jail, serving time on drug charges.

Searches of databases elsewhere have revealed similarly unsettling numbers. In 2006, for instance, a Chicago judge ordered a search of the Illinois database, which contained 233,000 profiles. It turned up 903 pairs with nine or more matching DNA markers. Among geneticists and statisticians, these findings have eroded faith in the FBI’s DNA rarity statistics, which were based on data from just 200 or 300 people and are used by crime labs across the country.

Juries in cold-hit cases are rarely, if ever, presented with evidence on the high probability of coincidental DNA matches. This is partly because most defense attorneys don’t understand the underlying statistical problems. When they see DNA evidence, they assume the case against their client is airtight and start praying for a plea bargain. In the rare instances where defense teams challenge the government figures, judges tend to reject their arguments. Few lawyers are savvy enough about genetics and statistics to make persuasive cases, and even those who are have trouble getting judges to comprehend the complex underlying concepts. And because courts are bound by precedent, each time a judge decides to bar information about the shortcomings of DNA evidence, he or she makes it more difficult for defense teams in other cases to get this evidence before juries.

Outside scientific circles, this perilous distortion of DNA evidence has gotten little attention. This is partly because the underlying mathematics can be difficult to grasp for those with no training in science or statistics. But there may also be another factor at play: so far, those who have been swept up in cold-hit investigations have mostly been convicted felons and sex offenders, because theirs were the only profiles in the databases, and the possibility that people who have committed vicious crimes might be getting shabby treatment from the courts is far less likely to stir public outrage. But the list of groups cropping up in these databases is expanding rapidly.

Last year, California and at least fourteen other states started cataloging DNA, not just from convicted felons, but from anyone arrested for a felony. At the same time, the FBI began collecting DNA from detained immigrants and anyone arrested for a federal offense, including those charged with petty misdemeanors, such as loitering on federal property.

As a result, more than a million new profiles are being added to our nation’s offender databases each year, and as DNA testing becomes more routine, it is likely that these systems will grow to include an even wider cross-section of the public. Of course, as the number of profiles in the databases swell, so do the odds of accidentally fingering innocent people.”

Please read the entire article here.

Categories: Forensics, Miscarriages of Justice, News: Cold Cases, Unidentified, Unsolved

Tags: Actual Innocence, Autopsy, Capital Punishment, Crime Labs, Crime Scene, DNA, DNA Database, Evidence, Expert Testimony, FBI, Forensics, Miscarriage of Justice, Police, Prisons, Rape kits, Unsolved Homicide, Victim, Witnesses, Wrongful Convictions