Friday, August 2, 2013

New analyses undermine perception of DNA infallibility

Rags-against-riches case highlights technology's pitfalls

If you trust in the technology of DNA matching, it was an impeccable case:

Lukis Anderson's DNA matched that found on the fingernails of a San Francisco Bay Area millionaire killed in a home-invasion robbery. Based on the match to Anderson’s sample in the DNA database, the homeless man was arrested on a potential capital murder charge and spent five months in jail.

Fortunately for him, Anderson had an airtight alibi: He was lying in a hospital bed miles away, drunk to the point of unconsciousness. He also had no known connection to Raveesh "Ravi" Kumra, a cell phone entrepreneur and former winery owner who was killed during a home-invasion robbery near San Jose, California.

Although Anderson's attorneys initially thought there might have been a mix-up at the crime lab – the most common cause of erroneous DNA matches – an investigation ruled out improprieties. This despite the fact that, in an ironic twist, the technician who handled the DNA evidence in the case was previously implicated in a crime lab scandal in nearby San Francisco.

Prosecutors think they have solved the mystery: The paramedics who responded to Kumra’s home were the same two who had brought Anderson to the hospital via ambulance about two hours before the home-invasion attack on Kumra began. They likely inadvertently transferred Anderson’s DNA to Kumra via their equipment or clothing.

The local prosecutor called the case unique. But this is far from the first time that cross-contamination has led to a wrongful DNA match.

One of the strangest, most infamous and most embarrassing cases was the "Phantom of Heilbronn." A mystery woman was linked to six murders and dozens of other crimes across Germany and Austria through DNA found on everything from a heroin syringe to a cookie to a stolen car. Desperate police turned to profilers, a monetary reward and even fortune-tellers and psychics to no avail. Finally, after 15 years, the case was cracked: Evidence collection kits had accidentally been contaminated by a worker at a cotton swab factory. Forensic swabs are sterilized, but sterilization does not kill DNA.

In Australia, meanwhile, a 20-year-old man named Farah Jama was convicted and spent time in prison for a rape that likely never even took place. The same forensic officer had collected his DNA in an unrelated matter a day before collecting DNA from a woman who was found unconscious at a Melbourne nightclub. The woman had no recall of events and never claimed she was assaulted; nonetheless, Jama -- who didn’t know the woman and denied ever setting foot inside the nightclub -- spent 15 months in prison before his conviction was overturned.

Potential contamination of DNA evidence also factored into the reversal of Amanda Knox’s conviction in the odd Italian case that received international scrutiny. (Stay tuned on that convoluted case, by the way; the acquittal has now been overturned and a retrial in abstentia is scheduled to begin next month.)

The fact that an airtight alibi did not prevent Alexander from languishing in jail for five months, with a potential death sentence hanging over his head, highlights the problem of blind faith in the reliability of DNA evidence. As Osagie K. Obasogie, a law professor at Hastings School of Law in San Francisco and a senior fellow at the Center for Genetics and Society, argues in a compelling New York Times op-ed:

[T]he certainty with which prosecutors charged Mr. Anderson with murder highlights the very real injustices that can occur when we place too much faith in DNA forensic technologies. In the end, Mr. Anderson was lucky. His alibi was rock solid; prosecutors were forced to concede that there must have been some other explanation. It’s hard to believe that, out of the growing number of convictions based largely or exclusively on DNA evidence, there haven’t been any similar mistakes.
Chance matches more common than thought

But there may be bigger and more ominous problem than the rare transfer errors. The claim that random DNA matches are just about impossible, promoted by crime shows like CSI and powerfully influential in court, turns out to be flat-out wrong. As DNA databases become more and more massive, so too do the odds of chance hits.

An audit of Arizona’s 65,000-profile DNA database turned up almost 150 matching pairs, collected from different people. The California case of John Puckett is frequently cited as an example of misleading over-claiming about the reliability of DNA matches. Puckett is serving life due to a cold hit in a 1972 killing. Jurors heard testimony that there was only a one-in-a-million chance of a coincidental match. But, as Obasogie points out, that figure is misleading, according to an analysis by the National Research Council:
It reflects the chance of a coincidental match in relation to the size of the general population (assuming that the suspect is the only one examined and is not related to the real culprit). Instead of the general population, we should be looking at only the number of profiles in the DNA database. Taking the size of the database into account in Mr. Puckett’s case (and, again, assuming the real culprit’s profile is not in the database) would have led to a dramatic change in the estimate, to one in three.

This overdue recognition of the fallibility of DNA technology is causing some to call for greater oversight and to rethink the idea of allowing convictions based solely upon cold hits from DNA evidence.

Obasogie's final warning is profound:
For far too long, we have allowed the myth of DNA infallibility to chip away at our skepticism of government’s prosecutorial power, undoubtedly leading to untold injustices. In the Anderson case, thankfully, prosecutors acknowledged the obvious: their suspect could not have been in two places at once. But he was dangerously close to being on his way to death row because of that speck of DNA. That one piece of evidence -- obtained from a technology with known limitations, and susceptible to human error and prosecutorial misuse -- might mistakenly lead to execution at the hands of the state should send chills down every one of our spines. The next Lukis Anderson could be you. Better hope your alibi is as well documented as his.

Related blog post: DNA science on trial (April 17, 2009)

Blogger note: As always, it was great meeting blog subscribers during my seminar and training tour at Bond University in Queensland and the American Psychological Association convention in Honolulu. Thanks to all of you who attended and participated. The trainings were great fun; now it's back to the old grindstone as I head home and get back to work.

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