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DNA Evidence

Dna Evidence: Boon Or Boondoggle For Criminal Justice?

Since its first use in the late 1980s, DNA evidence has been a subject of controversy in the U.S. criminal justice system. Although courts have increasingly allowed DNA analysis to be admitted as evidence, doubts about the propriety of such evidence remain. In general, the debate over DNA evidence pits those, such as prosecutors and law enforcement officials, who are eager to use it as a tool to fight crime, against those, particularly defense attorneys, who claim that it is unreliable and will lead to the wrongful conviction of innocent people.

Law enforcement officials and prosecuting attorneys are quick to identify the benefits of DNA evidence for the criminal justice system. DNA evidence, they argue, is even more useful than fingerprinting, with several advantages over that more traditional tool of investigation. DNA evidence is more readily available in criminal investigations than are legible fingerprints because body fluids and hair are more likely to be left at the scene of a crime. DNA evidence is also "robust"; that is, it does not decay or disappear over time. The DNA in a piece of physical evidence such as a hair may be examined years after a crime.

Law enforcement officials have confidence in the reliability of DNA analysis performed by commercial and government forensic laboratories. They maintain that innocent people have no need to worry about the use of DNA evidence in the legal system. In fact, they argue, DNA evidence will help to ensure that innocent suspects are not convicted because the DNA of such suspects will not match that taken from crime-related samples.

Proponents of DNA evidence fear that successful courtroom attacks on its reliability will erode public confidence in its use, giving the state less power in bringing criminals to justice. But most remain confident that it will be a permanent part of criminal investigation. According to Eric E. Wright, an assistant attorney general for Maine,"[T]he history of forensic DNA evidence consistently and ever increasingly demonstrates its reliability. It has been subjected to savage scrutiny unlike any FORENSIC SCIENCE before, and it has survived. Soon the only wonder about DNA evidence will be: What was all the fuss about?"

Defense attorneys and others who are skeptical about DNA evidence strongly disagree with many of these claims. While generally accepting the scientific theory behind DNA evidence, including its ability to exculpate the innocent suspect, they assert that it is not nearly as reliable in practice as its proponents claim. They argue that DNA evidence may be unreliable for any number of reasons, including contamination owing to improper police procedures and faulty laboratory work that may produce incorrect results.

Barry C. Scheck is a leading critic of DNA evidence. A professor at the Benjamin N. Cardozo School of Law, a defense attorney in several notable cases involving DNA evidence, and an expert for the defense in the celebrated 1995 murder trial of O. J. SIMPSON, Scheck has led the movement for increased scrutiny of DNA evidence. Conceding that "there is no scientific dispute about the validity of the general principles underlying DNA evidence," he nevertheless argued that serious problems with DNA evidence remained. He found particular fault in the work of forensic laboratories and pointed to research that showed that as many as one to four percent of the DNA matches produced by laboratories were in error. Laboratories denied such claims.

Scheck also criticized the procedures used by laboratories to estimate the likelihood of a DNA match. Because juries consider the probabilities generated by the labs—figures such as one in 300 million or one in 5 million—when assessing the validity of DNA results, it is important to ensure that they are accurate.

DNA critics assert that statistical estimates of a match may be skewed by incorrect assumptions about the genetic variation across a population. In some population subgroups, they claim, individuals may be so genetically similar that a DNA match is more likely to occur when comparing samples drawn from within that subgroup. Examples of such subgroups are geographically isolated populations or tightly knit immigrant or religious communities. Other problems may occur in cases where suspects are closely related to one another. Critics call for more research on population substructures and DNA similarities within them, in order to get a better understanding of statistical properties.

In response to these arguments, proponents of DNA analysis maintain that the importance of frequency calculations has been overrated. They claim that such calculations are, if anything, conservative. Furthermore, they argue that a match itself is more important than a frequency calculation and that questions of how to calculate frequency should not mean that DNA evidence is inaccurate.

DNA critics call for a number of other procedures to make DNA testing more accurate. They advocate sample splitting, a procedure by which samples of physical evidence are sent to two forensic laboratories in order to better guard against mistaken matches. They also ask that all DNA laboratories be required to undergo proficiency testing through blind trials. Such trials would have laboratories analyze DNA samples without knowing whether the analysis was being done for an actual investigation or for evaluation purposes only. Blind trials would yield error rates for each laboratory that could be given to a jury to help it weigh the significance of DNA evidence. Blind trials would also provide incentives for laboratories to lower their error rates.

Criminal defense lawyers have also called for state-funded access to the services of experts who can evaluate the handling and analysis of DNA evidence. These "counter experts" would give the defense a chance to scrutinize DNA evidence more closely. Defense attorneys also assert the need for access to laboratory records and physical samples for retesting. Providing this access would require the state to preserve samples.

Prosecutors and attorneys have continued to identify new uses for DNA in law enforcement and in the legal system. In July 2001, a Milwaukee, Wisconsin, appeals court judge upheld the validity of a criminal warrant for the arrest of "John Doe 12," issued for a 1994 rape case just days before the STATUTE OF LIMITATIONS was to expire. What made the warrant noteworthy was that the suspect was identified only by his DNA profile. This was the first known case in which prosecutors sought arrest warrants based solely on a DNA description. When a DNA evaluation matched the DNA of "John Doe 12" with Bobby Richard Dabney Jr., the state replaced "John Doe" with Dabney's name. Dabney's attorney sought to dismiss the claim because Dabney was not named in the original complaint until after a six-year statute of limitations had expired. A Milwaukee County Circuit Court Judge denied the motion to dismiss the case.

In September 2001, the Wisconsin state legislature effected new changes to the statute of limitations. This legislation expressly addresses DNA evidence and extends the time limits for such cases. The amendments permit prosecution any time within 12 months of the time a DNA match results in a probable identification of a person.

In another legal "first," attorneys for plaintiff Nanette Sexton Bailey of West Palm Beach, Florida, used DNA evidence found on bed sheets to allege ADULTERY on the part of her husband in a pending DIVORCE matter. Five years into their marriage, the couple mutually agreed to amend their prenuptial agreement to include a "bad boy clause," guaranteeing Sexton $20,000 per month for her husband's infidelity. When she found a nightgown and stained bed sheets in their home, she wrapped them in a plastic bag. When the sheets and nightgown were examined by a Denver laboratory, it confirmed that the DNA on the items belonged to another woman. Although the husband eventually challenged the "bad boy" clause, the judge ruled that the DNA evidence was admissible as evidence of the adultery.

Science may eventually solve many of the problems regarding DNA evidence. In the meantime, debate over its use has already led to changes that will allow courts and juries to better assess the guilt or innocence of criminal suspects.


Committee on DNA Forensic Science. 1996. The Evaluation of Forensic DNA Evidence. Washington, D.C.: National Academy Press.

Federal Judicial Center. 2000. Reference Manual on Scientific Evidence. New York: Lexis Publishing.

National Institute for Justice. 2001. Understanding DNA Evidence: A Guide for Victim Service Providers. Washington, D.C.: National Institute for Justice.


Forensic Science.

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