Why DNA Evidence Isn’t Always as Reliable as It Looks on TV

It’s featured in paternity testing segments on lowbrow daytime TV programs, and is practically a cast member on the various CSI shows. When properly done, DNA testing can greatly assist law enforcement, but if not, it can be unreliable evidence.

It can produce amazing results. Take the recent, highly publicized case of the killing of four people in a mansion in Washington, D.C., not far from the official residence of Vice President Biden. A killer held a family of three and a housekeeper captive for hours, forcing the homeowner to have $40,000 in funds from his business delivered to the home, then killed them and set the home afire.
Earlier that day, a nearby Pizza Hut got a delivery call from the home for several pizzas. Police say they were able to identify the suspect now in custody from DNA he left on partly-eaten slices. Due to his lengthy criminal record, previously collected samples of his DNA were in a national database. You’ve probably heard of other near-miraculous feats of DNA recovery and identification.

Modern DNA testing methodology didn’t arrive until 1985, and it took more than two years after that for the first homicide conviction based largely on DNA evidence. Even as DNA testing began to catch on with law enforcement agencies, some juries remained skeptical. Probably the best-known example came in the 1995 acquittal of O.J. Simpson after he was tried on charges of murdering his estranged wife Nicole and a visitor, Ron Goldman.

The ex-football star’s “Dream Team” defense managed to cast enough doubt on whether Los Angeles police had mishandled DNA evidence, or even planted samples of the victims’ blood in Simpson’s vehicle and on articles of his clothing. Arguably, the Simpson defense team’s most valuable members weren’t Johnnie Cochran and Robert Kardashian, but Barry Scheck and Peter Neufeld, DNA-savvy lawyers who a few years earlier had co-founded the Innocence Project. It analyzes closed criminal cases to see if convictions hold up in light of DNA evidence; the project has to date exonerated over 300 persons, some of whom were awaiting execution.

In the Simpson case, the Los Angeles police department badly mishandled DNA evidence both at the crime scene and in the laboratory. DNA samples can degrade to the point they give little or no reliable evidence if exposed to moisture or higher than room-temperature heat. Yet the DNA samples collected from Simpson and the murder victims were put in plastic bags, more susceptible to moisture than the recommended paper envelopes, then left to sit for many hours in sweltering police vehicles.

At the lab, things got even worse. Technicians with little training and no set protocol for handling samples spilled some of Simpson’s blood in their work area and could not remember if they had worn the same gloves while processing samples from O.J. and the two victims, raising the possibility their careless handling had cross-contaminated the samples.

Though police methods have improved since then, some lab problems are still reported. New equipment can process ever-smaller DNA samples (which makes collection or processing errors more severe), and increasingly can recover “touch” samples from door handles, steering wheels or other locations with which many people may have had contact. “Touch” samples are much less reliable than, say, tissue or bodily fluid samples known to come from a single source.

Another issue with DNA testing is familial testing. To try to crack unsolved cases, police may search DNA databases with paternity-marking Y-chromosome data for the nearest possible matches, and then investigate them — and members of their families.

That happened recently when an Idaho police force, working on a 1996 murder with a DNA sample, perused the Ancestry.com database. It didn’t produce an exact match, and the closest match was too old to be a suspect, but he had a son about the right age, a film maker living in New Orleans. The Idaho cops came to Louisiana, got a search warrant, questioned him and took a DNA sample.

The DNA test cleared him, but familial searches are controversial; at least one state has legislated against them. So, in the 20 years since the Simpson case, DNA testing has made advances and is far better understood by the public; even so, caution, rather than automatic acceptance of results, can still sometimes be merited.

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