Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers


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Science and technology constantly evolve; forensic tests become ever more discriminating, cheaper and easier to use, and more freely available to law enforcement. Courts and legislators must not be complacent if they are to keep pace with scientific innovation. Unfortunately austerity-blighted Britain may be storing up serious trouble for the future. All forms of judicial evidence are inherently fallible: witnesses are sometimes dissembling or forgetful, or sincere and credible yet wrong.

Confessions may be false or made under duress. As reliance on scientific evidence grows so too will the number of miscarriages of justice that stem from forensic science. Scientific evidence is in some areas peculiarly vulnerable to unreliability and misinterpretation.

Challenging the Admission of Forensic Evidence

To begin with, scientific evidence is circumstantial. In other words, it typically leaves considerable scope for interpretation. Scientific samples are prone to degradation and contamination, as demonstrated by recent high-profile cases in the UK and Australia in which contaminated samples falsely incriminated innocents. Presented in court, there is a constant danger that it will be misrepresented or misunderstood by lawyers, judges, or jurors. These difficulties are compounded whenever scientific evidence offered by one legal team is contradicted, or given a different interpretation, by counter-expertise advanced by the opposing side.

There will always be a risk of error, attributable to human fallibility, that must be accepted regardless of our efforts to detect miscarriages of justice. But one aspect of modern forensic science evidence is genuinely novel.

DNA for Defence Lawyers

But the arrival of DNA profiling in the mids has seriously disrupted this way of thinking. They are statements of random match probability RMP , the probability that a person would match the DNA at the crime scene? It was soon realised that, far from a weakness of DNA profiling in contrast to other well-established techniques, in fact DNA profiling represents a truly scientific approach, whereas orthodox forensic practice rested on a fallacy.

It is never possible to conclusively identify a person from sets of matching characteristics, unless the characteristics measured are known to be absolutely unique in the population. This kind of uniqueness probably does not exist in the real world — as graphically demonstrated by recent fingerprint miss-attributions in Scotland and in the US.

Yet the impression is that many forensic scientists have failed to grasp these radical implications, and continue to make claims for their evidence that lack foundation in either logic or science. For an effective prescription for institutional reform there needs to be an intelligent diagnosis of the existing ailments. Simplistic solutions, or those predicated on superficial misunderstandings of criminal procedure, are liable to do more harm than good. Unfortunately, in a moment of penny-pinching madness that future governments may regard with incomprehension, the UK coalition government closed down the world-famous Forensic Science Service , arguing — quite improbably — that the private sector would fill the gap.

The results, William Thompson, an attorney and a criminology professor at the University of California at Irvine, told a KHOU 11 reporter, were terrifying: It appeared that Houston police technicians were routinely misinterpreting even the most basic samples. Carol Batie watched the entire segment, rapt.


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As soon as it ended, she e-mailed KHOU A few days after reporting the crime, the woman spotted Sutton and Adams walking down a street in southwest Houston. She flagged down a passing patrol car and told the officers inside that she had seen her rapists.

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Police detained the boys and brought them to a nearby station for questioning. From the beginning, Sutton and Adams denied any involvement. Adams was 5 foot 11 and pounds. Sutton was three inches taller and 25 pounds heavier, the captain of his high-school football team. The DNA evidence was harder to refute. Having seen enough prime-time TV to believe that a DNA test would vindicate them, Sutton and Adams had agreed, while in custody, to provide the police with blood samples. The blood had been sent to the Houston crime lab, where an analyst named Christy Kim extracted and amplified DNA from the samples until the distinct genetic markers that swim in every human cell were visible, on test strips, as a staggered line of blue dots.

A vaginal swab contained a complex mixture of genetic material from at least three contributors, including the victim herself. In , a jury found Sutton guilty of aggravated kidnapping and sexual assault.

Failures in Forensic Science and DNA Evidence: A View From the Bench

He was sentenced to 25 years in prison. But what could I do? Batie was starting to think her son would never be freed. But the KHOU 11 segment, the first of a multipart investigative series on the Houston crime lab, encouraged her. In the course of their work on the series, Raziq and his team had uncovered a couple of close calls with wrongful conviction—in one case, a man had been falsely accused, on the basis of improperly analyzed DNA evidence, of raping his stepdaughter.

But in those instances, attorneys had managed to demonstrate the problems before their clients were sent to prison. Thompson had been studying forensic science for decades. His wife took the transcripts, and Thompson took the DNA tests. Almost immediately, he found an obvious error: In creating a DNA profile for the victim, Kim had typed three separate samples, two from blood and another from saliva. The resulting DNA profiles, which should have been identical, varied substantially. As Thompson had predicted, the results confirmed that Sutton was not a match. In the spring of , more than four years after his arrest, Sutton was released from prison.

His mother was waiting for him at the gates, her eyes bright with tears. Young confessed that in , he and an accomplice had raped a Houston woman in her Ford Expedition. In January , Young pleaded guilty to the crime. Christy Kim was fired from the Houston crime lab, but reinstated after her lawyer argued that her errors—which ranged from how she had separated out the complex mixture to how she had reported the odds of a random match—were a product of systemic failures that included inadequate supervision.

Kim could not be reached for comment. M odern forensic science is in the midst of a great reckoning. Since a series of high-profile legal challenges in the s increased scrutiny of forensic evidence, a range of long-standing crime-lab methods have been deflated or outright debunked. Last year, the bureau admitted that it had reviewed testimony by its microscopic-hair-comparison analysts and found errors in at least 90 percent of the cases.

A thorough investigation is now under way. DNA typing has long been held up as the exception to the rule—an infallible technique rooted in unassailable science. Unlike most other forensic techniques, developed or commissioned by police departments, this one arose from an academic discipline, and has been studied and validated by researchers around the world. The method was pioneered by a British geneticist named Alec Jeffreys, who stumbled onto it in the autumn of , in the course of his research on genetic sequencing, and soon put it to use in the field, helping police crack a pair of previously unsolved murders in the British Midlands.

A new era of forensics was being ushered in, one based not on intrinsically imperfect intuition or inherently subjective techniques that seemed like science, but on human genetics. Several private companies in the U. Defense attorneys protested that DNA typing did not pass the Frye Test, a legal standard that requires scientific evidence to have earned widespread acceptance in its field; many prominent academics complained that testing firms were not being adequately transparent about their techniques.


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And in , during the murder trial of O. Simpson, members of his so-called Dream Team famously used the specter of DNA-sample contamination—at the point of collection, and in the crime lab—to invalidate evidence linking Simpson to the crimes. But gradually, testing standards improved. Crime labs pledged a new degree of thoroughness and discipline, with added training for their employees.

Misleading DNA Evidence 1E

Analysts got better at guarding against contamination. Extraction techniques were refined. The FBI created its codis database for storing DNA profiles of convicted criminals and arrestees, along with an accreditation process for contributing laboratories, in an attempt to standardize how samples were collected and stored. And a lot of people were ready to move on. Now convinced that DNA analysis, provided the evidence was collected cleanly, could expose the racism and prejudice endemic to the criminal-justice system, the two attorneys set about applying it to dozens of questionable felony convictions.

They have since won exonerations using DNA testing; in the majority of the cases, the wrongfully convicted were black. Prosecutors lie. While helping to overturn wrongful convictions, DNA was also becoming more integral to establishing guilt. The number of state and local crime labs started to multiply, as did the number of cases involving DNA evidence. The database has since grown to include more than 15 million profiles, which contributed to tens of thousands of investigations last year alone.

In , Donald E.

Public Defenders

Shelton, a felony trial judge in Michigan, published a study in which 1, randomly summoned jurors in the city of Ann Arbor were polled on what they expected prosecutors to present during a criminal trial. Three-quarters of the jurors said they expected DNA evidence in rape cases, and nearly half said they expected it in murder or attempted-murder cases; 22 percent said they expected DNA evidence in every criminal case. A researcher in Australia recently found that sexual-assault cases involving DNA evidence there were twice as likely to reach trial and 33 times as likely to result in a guilty verdict; homicide cases were 14 times as likely to reach trial and 23 times as likely to end in a guilty verdict.

As the Nuffield Council on Bioethics, in the United Kingdom, pointed out in a major study on forensic evidence, even the knowledge that the prosecution intends to introduce a DNA match could be enough to get a defendant to capitulate. And in a lot of situations, for a lot of lawyers, it was now too costly and time-intensive to fight. 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. The detectives can now place the suspect at the scene of the crime.

When Alec Jeffreys devised his DNA-typing technique, in the mids, this was as far as the science extended: side-by-side comparison tests. Sizable sample against sizable sample. 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.

The Evaluation of Forensic DNA Evidence.

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 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. A groundbreaking study by Itiel Dror, a cognitive neuroscientist at University College London, and Greg Hampikian, a biology and criminal-justice professor at Boise State University, illustrates exactly how subjective the reading of complex mixtures can be.

In , Dror and Hampikian obtained paperwork from a Georgia rape trial that hinged on DNA typing: The main evidence implicating the defendant was the accusation of a co-defendant who was testifying in exchange for a reduced sentence. Two forensic scientists had concluded that the defendant could not be excluded as a contributor to the mixture of sperm from inside the victim, meaning his DNA was a possible match; the defendant was found guilty. Dror and Hampikian gave the DNA evidence to 17 lab technicians for examination, withholding context about the case to ensure unbiased results.


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All of the techs were experienced, with an average of nine years in the field. Dror and Hampikian asked them to determine whether the mixture included DNA from the defendant. In , the results of the experiment were made public: Only one of the 17 lab technicians concurred that the defendant could not be excluded as a contributor.

Twelve told Dror and Hampikian that the DNA was exclusionary, and four said that it was inconclusive. In other words, had any one of those 16 scientists been responsible for the original DNA analysis, the rape trial could have played out in a radically different way. Some veer close to farce, such as the year hunt for the Phantom of Heilbronn, whose DNA had been found at more than 40 crime scenes in Europe in the s and early s.

The DNA in question turned out to belong not to a serial killer, but to an Austrian factory worker who made testing swabs used by police throughout the region. Most troubling, Murphy details how quickly even a trace of DNA can now become the foundation of a case. In , police in California arrested Lukis Anderson, a homeless man with a rap sheet of nonviolent crimes, on charges of murdering the millionaire Raveesh Kumra at his mansion in the foothills outside San Jose.

Contamination is an obvious hazard when it comes to DNA analysis. But at least contamination can be prevented with care and proper technique. DNA transfer—the migration of cells from person to person, and between people and objects—is inevitable when we touch, speak, do the laundry.

A study showed that sperm cells from a single stain on one item of clothing made their way onto every other item of clothing in the washer. I could pick up a knife at 10 in the morning, but an analyst testing the handle that day might find a stronger and more complete DNA profile from my wife, who was using it four nights earlier. Or the analyst might find a profile of someone who never touched the knife at all.

One recent study asked participants to shake hands with a partner for two minutes and then hold a knife; when the DNA on the knives was analyzed, the partner was identified as a contributor in 85 percent of cases, and in 20 percent as the main or sole contributor. Context is needed. What worries experts like Murphy is that advancements in DNA testing are enabling ever more emphasis on ever less substantial evidence.

A new technique known as low-copy-number analysis can derive a full DNA profile from as little as 10 trillionths of a gram of genetic material, by copying DNA fragments into a sample large enough for testing. The technique not only carries a higher risk of sample contamination and allele dropout, but could also implicate someone who never came close to the crime scene.

Given the growing reliance on the codis database—which allows police to use DNA samples to search for possible suspects, rather than just to verify the involvement of existing suspects—the need to consider exculpatory evidence is greater than ever. But Bicka Barlow, the San Francisco attorney, argues that the justice system now allows little room for caution.

Techs at many state-funded crime labs have cops and prosecutors breathing down their necks for results—cops and prosecutors who may work in the same building. The threat of bias is everywhere.

Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers
Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers
Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers
Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers
Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers
Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers
Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers
Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers
Misleading DNA Evidence. A Guide for Scientists, Judges, and Lawyers

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