Skip to main content

DNA Evidence


Among the many new tools that science has provided for the analysis of forensic evidence is the powerful and controversial analysis of deoxyribonucleic acid, or DNA, the material that makes up the genetic code of most organisms. DNA analysis, also called DNA typing or DNA profiling, examines DNA found in physical evidence such as blood, hair, and semen, and determines whether it can be matched to DNA taken from specific individuals. DNA analysis has become a common form of evidence in criminal trials. It is also used in civil litigation, particularly in cases involving the determination of paternity or identity.

History and Process of DNA Analysis

DNA, sometimes called the building block or genetic blueprint of life, was first described by the scientists Francis H. C. Crick and James D. Watson in 1953. Crick and Watson identified the double-helix structure of DNA, which resembles a twisted ladder, and established the role of DNA as the material that makes up the genetic code of living organisms. The pattern of the compounds that constitute the DNA of an individual life-form determines the development of that life-form. DNA is the same in every cell throughout an individual's body, whether it is a skin cell, sperm cell, or blood cell. With the exception of identical twins, no two individuals have the same DNA blueprint.

DNA analysis was first proposed in 1985 by the English scientist Alec J. Jeffreys. By the late 1980s, it was being performed by law enforcement agencies, including the federal bureau of investigation (FBI), and by commercial laboratories. It consists of comparing selected segments of DNA molecules from different individuals. Because a DNA molecule is made up of billions of segments, only a small proportion of an individual's entire genetic code is analyzed.

In DNA analysis for a criminal investigation, using highly sophisticated scientific equipment, first a DNA molecule from the suspect is disassembled, and selected segments are isolated and measured. Then the suspect's DNA profile is compared with one derived from a sample of physical evidence to see whether the two match. If a conclusive nonmatch occurs, the suspect may be eliminated from consideration. If a match occurs, a statistical analysis is performed to determine the probability that the sample of physical evidence came from another person with the same DNA profile as the suspect's. Juries use this statistical result in determining whether a suspect is guilty or innocent.

Although DNA analysis is sometimes called DNA fingerprinting, this term is a misnomer. Because the entire DNA structure of billions of compounds cannot be evaluated in the same way that an entire fingerprint can, a "match" resulting from DNA typing represents only a statistical likelihood. Thus, the results of DNA typing are not considered absolute proof of identity. A DNA nonmatch is considered conclusive, however, because any variation in DNA structure means that the DNA samples have been drawn from different sources.

An example from the early 1990s illustrates the way in which DNA evidence is used in the criminal justice system. After a Vermont woman was kidnapped and raped in a semi-trailer truck, police identified Randolph Jakobetz, a truck driver, as a suspect in the crime. Officers searched the trailer that Jakobetz had hauled on the night of the crime and found hairs matching those of the victim. After arresting Jakobetz, law enforcement officials sent a sample of his blood to the FBI laboratory in Washington, D.C., for DNA analysis and for comparison with DNA taken from semen found in the victim shortly after the crime.

At Jakobetz's trial, an FBI expert testified that the blood and semen samples were a "match," concluding that there was one chance in 300 million that the semen samples could have come from someone other than Jakobetz. Based on this and other strong evidence, Jakobetz was convicted and sentenced to almost 30 years in prison.

Jakobetz appealed the decision, claiming that DNA profiling was unreliable and that it should not be admitted as evidence. In the first major federal decision on DNA profiling, the U.S. Court of Appeals for the Second Circuit upheld the lower court's decision to admit the DNA evidence (United States v. Jakobetz, 955 F.2d 786 [2d Cir. 1992]). The U.S. Supreme Court later declined to hear an appeal.

The Jakobetz case illustrates the way in which the probabilities generated by DNA analysis can be used as devastating evidence against a criminal suspect. Juries have tended to view the statistical results of this analysis as highly incriminating, which has caused many defense attorneys to challenge the validity of the results, and many prosecuting attorneys to defend them. At the same time, defense lawyers have used DNA analysis as evidence to reverse the convictions of their clients.

Legal History of DNA Evidence

In general, state and federal courts have increasingly accepted DNA evidence as admissible. The first state appellate court decision to uphold the admission of DNA evidence was in 1988 (Andrews v. Florida, 533 So. 2d 841 [Fla. App.]), and the first major federal court decision to uphold its admission occurred in Jakobetz. By the mid-1990s, most states' courts admitted DNA test results into 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.

further readings

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.

No court has rejected DNA evidence on the grounds that the underlying scientific theory is invalid. However, some courts have excluded it from evidence because of problems with the possible contamination of samples, questions surrounding the significance of its statistical probabilities, and laboratory errors. Several states have passed laws that recognize DNA evidence as admissible in criminal cases, and others have enacted laws that specifically admit DNA evidence to help resolve civil paternity cases.

The admissibility of novel scientific evidence such as DNA profiling is governed by two different judicial tests or standards: the Frye, or general acceptance, standard, and the Daubert, or relevancy-reliability, standard. The Frye test, which comes from the 1923 case Frye v. United States 293 F. 1013 (D.C. Cir.), holds that the admissibility of evidence gathered by a specific technique (such as DNA analysis) is determined by whether that technique has been "sufficiently established to have gained general acceptance in the particular field in which it belongs." In Frye, the Court of Appeals for the District of Columbia Circuit ruled that a lie-detector test using a blood-pressure reading was not admissible as evidence. By the 1970s, 45 states had adopted this common-law standard for the admission of novel scientific evidence.

The U.S. Supreme Court overruled use of the Frye test in federal courts in its 1993 decision Daubert v. Merrell Dow, 509 U.S. 579, 113 S. Ct. 2786, 125 L. Ed. 2d 469. In Daubert, the Court held that the federal rules of evidence, enacted in 1975, govern the admission of novel scientific evidence in federal courts. It found that Frye provides too stringent a test and that it is incompatible with the federal rules, which allow the admission of all evidence that has "any tendency to make the existence of any fact that is of consequence to the determination of the action more probable or less probable than it would be without the evidence" (Fed. R. Evid. 401). The Court found that judges have a responsibility to "ensure that any and all scientific testimony or evidence admitted is not only relevant, but reliable."

In general, courts that have used the Daubert standard have been more likely to admit DNA evidence, although many jurisdictions that have relied on Frye have permitted it as well. Nearly all cases in which DNA evidence has been ruled inadmissible have been in jurisdictions that have used Frye.

States are free to adopt their own standards for the admission of evidence, and have increasingly adopted the Daubert standard. By 1995, the number of states using the Frye standard had dropped to 23, while 21 had adopted the Daubert standard.

Current Issues Surrounding Use of DNA Evidence

A report issued by the justice department in 2002 indicated that two-thirds of chief prosecutors in the United States rely on DNA testing during investigations and trials. The use of DNA evidence has exonerated at least ten individuals who were wrongly convicted of murder and faced the death penalty, while the sentences of more than 100 others convicted of lesser crimes were overturned based upon DNA evidence. The FBI maintains a database that may be used to compare DNA samples from unsolved state and federal crimes. Since its inception in 1992, the FBI's database has made more than 5,000 matches, thus allowing law enforcement officials to solve crimes that might not have been solved without the use of DNA.

The FBI crime laboratory dominated research in forensic sciences for much of the 1980s and 1990s. However, allegations surfaced in 1995 that suggested scientists at the crime lab had tainted evidence related to the 1993 bombing of the World Trade Center in New York City. A former chemist in the lab, Frederic White-hurst, testified before the House Committee on the Judiciary that the FBI had knowingly drafted misleading scientific reports and pressured FBI scientists to commit perjury by backing up the false reports. These allegations injured the FBI's reputation and led to speculation in the late 1990s that prosecutors could not rely on the FBI's analysis of DNA evidence.

Even as the FBI rebuilt its reputation, other questions surrounding the use of DNA evidence have arisen since the late 1990s. In 1999, the department of justice issued a report stating that evidence from at least 180,000 unsolved rape cases had not been submitted for testing. A 2002 report by USA Today suggested that several thousand pieces of evidence from rape and homicide cases had not been submitted for DNA testing, so they do not appear in the FBI's database. In 2000, Congress allocated $125 million to support the national DNA database system, including $45 million designated to allow states to test evidence from unsolved crimes. However, several states claim that their law enforcement officials are so swamped with current cases that they cannot test older, unsolved cases. Moreover, a small number of states—primarily New York, Florida, Virginia, and Illinois—have aggressively developed their own DNA databases and have contributed heavily to the FBI's system. These states accounted for more than half of the FBI's DNA matches between 1992 and 2002.

Use of DNA evidence to overturn criminal convictions remains a common topic of discussion among legal and criminal justice experts, as well as the popular media. One of the most closely followed cases involved the convictions of five young men for the rape of a jogger in Central Park in New York City in 1989. The five men in the case, dubbed the "Central Park Jogger Case," served sentences ranging from seven to eleven years for the incident. However, another man, Matias Reyes, who was convicted for murder in 1989, confessed to the rape. Testing confirmed that the semen found in the victim and on the victim's sock matched Reyes's DNA.

Upon receiving the new evidence, the New York County district attorney's office asked the New York State Supreme Court to overturn the convictions of the five men. Several groups, including women's rights groups, cited this case as an example of why law enforcement should be more proactive in pursuing unsolved rape cases through the use of DNA testing.

further readings

Bennett, Margann. 1995. "Admissibility Issues of Forensic DNA Evidence." University of Kansas Law Review 44 (November).

"Confronting the New Challenges of Scientific Evidence: DNA Evidence and the Criminal Defense." 1995. Harvard Law Review 108 (May).

Federal Bureau of Investigation. 1994. Handbook of Forensic Science. Washington, D.C.: U.S. Government Printing Office.

National Research Council. 1992. DNA Technology in Forensic Science. Washington, D.C.: National Academy Press.

Wright, Eric E. 1995. "DNA Evidence: Where We've Been, Where We Are, and Where We Are Going." Maine Bar Journal 10 (July).


Forensic Science.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"DNA Evidence." West's Encyclopedia of American Law. . 21 Sep. 2017 <>.

"DNA Evidence." West's Encyclopedia of American Law. . (September 21, 2017).

"DNA Evidence." West's Encyclopedia of American Law. . Retrieved September 21, 2017 from

Learn more about citation styles

Citation styles gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, cannot guarantee each citation it generates. Therefore, it’s best to use citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.