Sunday Long Read: The Science Of Error

The science of DNA testing has been proven time and time again.  The problem is human error: lab technicians who make mistakes, overloaded crime labs with no funding, backlogs of evidence and police who mishandle all kinds of samples, by accident or even on purpose. The thing is in crime lab after crime lab in America, all of that has now become commonplace.

DNA analysis has risen above all other forensic techniques for good reason: “No [other] forensic method has been rigorously shown able to consistently, and with a high degree of certainty, demonstrate a connection between evidence and a specific individual or source,” the National Research Council wrote in an influential 2009 report calling out inadequate methods and stating the need for stricter standards throughout the forensic sciences.

The problem, as a growing number of academics see it, is that science is only as reliable as the manner in which we use it—and in the case of DNA, the manner in which we use it is evolving rapidly. Consider the following hypothetical scenario: Detectives find a pool of blood on the floor of an apartment where a man has just been murdered. A technician, following proper anticontamination protocol, takes the blood to the local crime lab for processing. Blood-typing shows that the sample did not come from the victim; most likely, it belongs to the perpetrator. A day later, the detectives arrest a suspect. The suspect agrees to provide blood for testing. A pair of well-trained crime-lab analysts, double-checking each other’s work, establish a match between the two samples. The detectives can now place the suspect at the scene of the crime.

When Alec Jeffreys devised his DNA-typing technique, in the mid-1980s, this was as far as the science extended: side-by-side comparison tests. Sizable sample against sizable sample. The state of technology at the time mandated it—you couldn’t test the DNA unless you had plenty of biological material (blood, semen, mucus) to work with.

But today, most large labs have access to cutting-edge extraction kits capable of obtaining usable DNA from the smallest of samples, like so-called touch DNA (a smeared thumbprint on a window or a speck of spit invisible to the eye), and of identifying individual DNA profiles in complex mixtures, which include genetic material from multiple contributors, as was the case with the vaginal swab in the Sutton case.

These advances have greatly expanded the universe of forensic evidence. But they’ve also made the forensic analyst’s job more difficult. To understand how complex mixtures are analyzed—and how easily those analyses can go wrong—it may be helpful to recall a little bit of high-school biology: We share 99.9 percent of our genes with every other human on the planet. However, in specific locations along each strand of our DNA, the genetic code repeats itself in ways that vary from one individual to the next. Each of those variations, or alleles, is shared with a relatively small portion of the global population. The best way to determine whether a drop of blood belongs to a serial killer or to the president of the United States is to compare alleles at as many locations as possible.

Think of it this way: There are many thousands of paintings with blue backgrounds, but fewer with blue backgrounds and yellow flowers, and fewer still with blue backgrounds, yellow flowers, and a mounted knight in the foreground. When a forensic analyst compares alleles at 13 locations—the standard for most labs—the odds of two unrelated people matching at all of them are less than one in 1 billion.

With mixtures, the math gets a lot more complicated: The number of alleles in a sample doubles in the case of two contributors, and triples in the case of three. Now, rather than a painting, the DNA profile is like a stack of transparency films. The analyst must determine how many contributors are involved, and which alleles belong to whom. If the sample is very small or degraded—the two often go hand in hand—alleles might drop out in some locations, or appear to exist where they do not. Suddenly, we are dealing not so much with an objective science as an interpretive art.

And suddenly, a tool that is a lock for a conviction is increasingly something that can be wrongly determined or even falsified into a wrongful conviction.  DNA analysis is a tool and a powerful one, but any tool can be misused, and that's where we need far more oversight.