In recent decades, DNA evidence has become something of a pop-culture phenomenon. It can be found in popular magazines, in children's books, and on television. All too often, DNA evidence is shown as irrefutable fact, a scientific fingerprint that allows for a black-and-white determination of guilt or innocence. Unfortunately, such depictions are inaccurate and dangerous.
On October 12, 2017, the state of Texas executed Robert Lynn Pruett. Warden James Jones carried out the death sentence that Pruett received more than fifteen years earlier for the 1999 murder of prison guard Daniel Nagle. (1) At first glance, the case against Pruett seemed strong. Pruett was serving a ninety-nine-year sentence for a 1995 murder, and Pruett and Nagle often clashed. In fact, one of Nagle's last acts as an officer was to reprimand Pruett. During the trial, one of Pruett's fellow prisoners testified that he saw Pruett kill Nagle. Especially important to the case was the DNA evidence: a lab analyst testified that he found that Pruett's DNA was on the murder weapon. Based on this evidence, the jury convicted Pruett after a weeklong trial and sentenced him to die. (2)
On appeal, however, the evidence used at trial to connect Pruett to the crime was much less convincing. There was an eyewitness, but he was a notorious jailhouse informant and received a reduced sentence in exchange for his testimony. Similarly, while officers found a murder weapon, it was never established that Pruett possessed it. The entire case boiled down to DNA, and even that was inconclusive. During Pruett's appeal, a second lab analyst examined the DNA on the weapon and determined that it could have come from thirty percent of the employees at the jail, and thirty-one percent of the population of the United States. (3) It was unconvincing evidence of guilt. Nonetheless, the scant DNA evidence, along with the inmate testimony, was enough for Pruett to be executed. (4)
Unfortunately, Pruett's case is not unusual, as faulty DNA collection or analysis persists across the entire criminal justice system. Indeed, this paper could have begun with the stories of any number of individuals exactly like Pruett, revealing an ugly truth: while the science behind DNA testing is strong, the gathering and application of DNA evidence is systematically flawed. These weaknesses lead to improper prosecution, provide an opportunity for evidence tampering and, as in Pruett's case, can result in misidentification and wrongful execution. Despite the serious consequences of misapplied DNA evidence, the justice system has not fully examined these issues. This paper attempts to address wrongful convictions by calling for a temporary moratorium on the presentation of DNA evidence at trial, implemented until federal standards are put in place to regulate the presentation of DNA evidence, and until robust studies indicate the prevalence of DNA transfer. (5) This paper will demonstrate that DNA analysis schemes are dangerously flawed and that they allow for various outcomes, ranging from understandable human error to the exploitation of an already unstable system. (6) It will then argue that the current reliance on DNA analysis differs from what its legislative creators intended, and will propose solutions to current problems in DNA testing procedures, including amending jury instructions, minimum prosecution guidelines, and handling standards. A moratorium is the best way to achieve such a holistic review.
The Science of DNA
Before examining issues related to DNA testing, it is necessary to understand what DNA is and how it is used in forensic investigations. DNA is the famous foundational molecule of genetics, the molecular building block of life. In technical terms, DNA stands for deoxyribonucleic acid, a long string of nucleotides--genes--located within almost every cell of the human body. (7) DNA is a set of instructions for human development, determining stature, hair color, and eye color, among other qualities. (8) Experts use DNA when conducting forensic investigations because it contains segments known as short-tandem repeats (STRs). (9) While every person has STRs, each person's STRs are located in slightly different positions on the DNA strand. (10) An individual's unique arrangement of STRs is known as a genetic profile. Any body fluids or skin cells, for instance, left at a crime scene can be processed to reveal their STR pattern, which can then be compared to an STR sample taken from the suspect. (11) This process is exact because no two people--except identical twins--have exactly the same STR profile. (12)
DNA analysts use several methods to test DNA gathered from a crime scene or from an individual. In the most common approach, known as polymerase chain reaction testing, an analyst uses a machine to replicate and then rinse cells with enzymes to draw out the DNA and separate it into STRs. (13) These STRs are placed on gel plates, which separate the DNA into thirteen bands. (14) A second method, amplified fragment length polymorphism testing, mixes small samples of DNA, called trace DNA, with more powerful enzymes in order to draw out DNA from the smaller number of cells. (15) An analyst then compares the STR bands on the gel plate containing the evidence with an STR plate made from a suspect's DNA. If the genetic material from a crime scene matches a suspect's profile, analysts conclude that the suspect "cannot be excluded," meaning that he or she may potentially be the source of the DNA--a match. (16) If the gel plates are not identical, the suspect "cannot be included," meaning that they were not the source of the sample. (17) If the DNA from the crime scene yields fewer than thirteen STRs, it is termed a "partial profile." (18)
Given the obvious potential for identifying criminals, it was not long after its discovery that DNA testing was applied to forensic science. In the late 1980s, the United States Department of Justice (DOJ) determined DNA testing to be "the most feasible and reliable" method of determining the perpetrator of a crime. (19) Scientists at the DOJ reached this conclusion because DNA testing is "scientifically valid," meaning that if DNA testing is carried out properly, a scientist can make an accurate identification. (20) If an STR gel plate matches another STR plate, according to the DOJ, the cells must come from the same place. (21) Once lab reports are completed and evidence analyzed, data is input into the National DNA Index System, which is a shared database among the federal government, the District of Columbia, U.S. territories, and all fifty states. (22)
Since the introduction of DNA evidence, DNA testing has become so firmly rooted in the popular culture that it regularly appears in television shows and movies. Especially prominent on the long-running television drama Crime Scene Investigation (popularly known as CSI), DNA evidence is often shown as irrefutable science. This depiction has become embedded in the larger public consciousness, a phenomenon known as the "CSI Effect."
Unfortunately, even if DNA testing reveals a match or even a partial match, the suspect may be innocent. However, the erroneous match is not due to a scientific error. While the outcome of DNA testing appears absolute, the gathering and application of DNA evidence are dangerously flawed. DNA testing may have a nonexistent scientific error rate, but it has a high human one. Problems with DNA Forensics
While testing DNA is relatively precise, our understanding of how DNA interacts with a crime scene is anything but. Evidence is typically collected by amassing samples of genetic material, such as blood and semen, and by "swabbing" the crime scene for less visible material. (23) Such practices are flawed as DNA can transfer from person to person. In one study, researchers found that DNA was transferred from one person to another, and then to a third location, in all tested conditions--including through handshakes. (24) This transfer means that although cells may originate on an individual, that individual may never have been in the location in which his cells are found. (25) The scientists also found that large samples of DNA could remain at a third location for more than two weeks. (26) Trace DNA can linger for years. (27) These DNA remnants allow for a random third party's DNA to end up as evidence in a crime. The study concludes that DNA transfer is not "well understood," but makes it "impossible and/or impractical" to "identify with certainty the biological source" of any DNA sample. (28) DNA technology is effective at determining the physical source of DNA. (29) DNA transfer, however, makes it difficult to guarantee that the source of the DNA--a suspect--was actually present at the location at all, much less at a specific time.
Furthermore, although the science of DNA testing is objective, the standards for comparing forensic STR gel plates to human profiles are not, leading to a situation where human understanding of DNA results is flawed. Henry C. Lee, a noted professor and forensic science expert, wrote that there are "no generally accepted or national standards for determining what constitutes a match." (30) In short, what looks like a match can depend on who is conducting the analysis. While a scientific result should be empirical, limitations in technology and training often mean that a random analyst determines the result of a DNA test. In fact, two different analysts at two different labs in the same state may draw different conclusions--interpreting the results in two different ways--when analyzing DNA and artifacts while the DNA sample is tested, even if they are using the same technology and have the same education. (31) In one study, seventeen laboratory analysts were shown a sample featuring eleven STR bands and asked to determine if that sample came from the same source as a complete DNA profile. (32) One analyst found that a test profile...