Deoxyribonucleic acid (DNA) is genetic material mainly stored in the cellular nuclei of organisms. DNA strands contain identifying characteristics unique to each individual, enabling an application known as genetic fingerprinting, DNA fingerprinting, or DNA profiling. This technique allows law enforcement personnel to use DNA evidence recovered from crime scenes to make positive identifications of criminal suspects.
DNA profiling belongs to a branch of law enforcement known as forensic science, which applies scientific knowledge, principles, analysis, and practices to criminal investigations and legal cases. Genetic fingerprinting investigations use DNA evidence recovered from crime scenes, which can be collected from bodily fluids, skin cells, hair, and other forms of trace biological evidence. The identifying characteristics of the genetic material can then be compared against information in a DNA database, which contains the genetic profiles of specific individuals, most of whom have already had contact with law enforcement or the justice system. Alternately, investigators can obtain DNA samples from suspects, with or without the suspect's knowledge and willful cooperation, then compare those samples against samples recovered from a crime scene to determine whether they have a match. For instance, they might collect trace hair, skin, or saliva samples from an individual suspected of a crime and use them to try to place the suspect at a crime scene.
Genetic fingerprinting is commonly used in investigations involving homicide, sexual assault, and other major crimes involving close physical contact between a perpetrator and a victim. It can also be used to place a suspect at a crime scene. DNA evidence can also be used to identify unnamed crime victims, establish positive physical links between a victim and a suspect, determine paternity of a child, or exonerate wrongfully convicted individuals.
History and Early Uses of DNA Technology by Law Enforcement
Biologists, physicists, and other scientists first began to study DNA in detail in the twentieth century. Rosalind Franklin (1920–1958), Maurice Wilkins (1916–2004), James Watson (1928–) and Francis Crick (1916–2004) made significant breakthroughs during the early 1950s. Watson and Crick developed a model for using DNA to identify the specific genetic code of organisms, winning a Nobel Prize in 1962 for their work. Concurrently, forensic science was becoming an increasingly prominent feature of law enforcement and criminal investigations, with French criminologist Edmond Locard (1877–1966) founding the world's first modern crime lab. Locard also articulated a famous theory known as the Locard exchange principle, which posits that criminals almost always unwittingly leave evidence of their presence at a crime scene or take evidence of their presence at the scene with them when they leave. These developments helped shape DNA technology's emergence as a major asset to criminal investigators.
However, it was not until 1987 that genetic fingerprinting was first used to secure the conviction of a criminal suspect. That year, British police enlisted the assistance of DNA expert Alec Jeffreys (1950–) during an investigation in the sexual assault and murder of two Leicestershire girls. Jeffreys helped design a public campaign to collect DNA samples from volunteers, which were compared against DNA evidence linked to the unidentified suspect in the case. Eventually, a man came forward and revealed that a friend had paid him a large sum of money to submit a DNA sample in his place. The person who paid for the falsified sample was later identified as Colin Pitchfork (1960–). After investigators obtained a genuine sample of Pitchfork's DNA, he was conclusively determined to be the killer and subsequently confessed to both murders.
Tommie Lee Andrews (1964–) was the first American to be convicted of a crime using DNA evidence. In 1987 prosecutors used genetic fingerprinting to definitively identify Andrews as a serial sexual assault suspect wanted for dozens of crimes. Within a few years of Andrews's arrest, both state and federal law enforcement agencies were building DNA databases. In 1989 Virginia became the first state to compel individuals convicted of felonies to submit DNA samples for inclusion in the state's database. Today, DNA databases exist in all fifty US states alongside nationwide databases operated by the Federal Bureau of Investigation (FBI). All fifty states also have laws authorizing the mandatory collection of DNA samples from individuals convicted of certain violent felonies.
Expanding the Use of DNA Technology
Criminal investigators found many applications for DNA technology after its entry into the law enforcement mainstream. Two New York City public defenders launched the Innocence Project at Yeshiva University's Benjamin N. Cardozo School of Law in 1992. The project was built on a novel principle: if DNA evidence could be used to convict criminals, it could also be used to prove the innocence of wrongfully convicted individuals. By the end of the year, the Innocence Project had exonerated ten people serving prison terms for crimes they did not commit. It continues to operate today and has succeeded in reversing the wrongful convictions of 367 people as of September 2019.
In 1994 Congress passed the DNA Identification Act. The legislation established national DNA identification databases containing the genetic information of individuals convicted of certain serious crimes, as well as DNA samples recovered by investigators at crime scenes and DNA samples from unidentified deceased individuals. These databases continue to operate as the National DNA Index System (NDIS) and the Combined DNA Index System (CODIS). Law enforcement officials also began to issue what are informally known as John Doe warrants, which are arrest warrants for unnamed individuals identified only by their DNA profiles. They are usually used in cases where investigators have the DNA profile of a suspect but have not yet matched it to an individual and must issue a warrant before the applicable statute of limitations expires and makes it legally impossible to prosecute the criminal should he or she be identified at a later date.
In 2004 federal lawmakers had passed the Justice for All Act, which included specific measures to improve the accuracy of trials involving defendants facing the death penalty. Title IV of the Justice for All Act is known as the Innocence Protection Act, and its Subtitle A mandates the use of DNA testing to provide death row inmates with an exoneration opportunity.
During the 2010s, law enforcement officials began to make increasing use of data from websites that map genealogy to search for DNA links to criminal suspects. The unregulated practice involves comparing DNA samples from crime scenes against voluntarily submitted genetic information on these sites, which can create pools of potential matches if a database contains genetic samples from an as-yet-unidentified suspect's close or distant relatives. This technique remains ethically controversial, however, with opponents claiming that it violates privacy. However, the high-profile 2018 arrest of Joseph James DeAngelo (1945–) stands as an example of the technique's potential. DeAngelo is alleged to be the infamous Golden State Killer, also known as Mr. Cruel, the East Area Rapist, and the Original Night Stalker. DeAngelo's arrest appeared to end a forty-year search for this notorious serial rapist and killer after genetic evidence submitted by his relatives was drawn from a privately operated genealogy website's database and used to link him to multiple crimes, including at least twelve murders.
Challenging DNA Evidence
While DNA evidence often appears irrefutable, there are many examples of criminal cases in which it has been successfully challenged. A 1989 court decision, believed to be the first of its kind in the United States, saw a judge rule a critical DNA test inadmissible in a murder case. The judge made his ruling on the grounds that testing procedures used to apparently show the suspect's wristwatch contained traces of the victim's blood were flawed and could not be submitted as testimony. In New York v. Castro, the New York Supreme Court set an important legal precedent that helped launch the so-called DNA Wars, in which complex court proceedings aimed to establish the validity and limitations of genetic evidence in criminal cases.
In the mid-1990s, O. J. Simpson (1947–) was found not guilty of two homicides in one of the highest-profile criminal trials in US history. One key aspect of the defense arguments revolved around allegations that the Los Angeles police had mishandled DNA evidence used in the trial. Barry Scheck and Peter Neufeld, two members of Simpson's defense team, developed an early familiarity with DNA evidence when they founded the Innocence Project in the years immediately preceding Simpson's trial. Scheck and Neufeld were among the first lawyers to identify best practices for handling forensic material and presenting such evidence in court.
Beyond the technicalities of DNA testing and allegations of mishandling of genetic evidence, legal challenges often center on claims of police or laboratory incompetence, improper training, cross-contamination, or law enforcement bias. Despite its reputation for near infallibility, DNA evidence can also be misrepresented, misinterpreted, or ambiguous. DNA samples decay over time, and older samples may be able to produce only partial genetic profiles that are vulnerable to dispute.
Another shortcoming of genetic fingerprinting used in law enforcement and criminal prosecution stems from what is colloquially known as the CSI effect. Taking its name from the popular television franchise CSI: Crime Scene Investigation, the CSI effect refers to how courts and jury members often display near-unshakable confidence in DNA evidence. The phenomenon illustrates links between representations of genetic fingerprinting science in the popular media, where DNA evidence is all but unquestionable, and the way genetic evidence is treated in courtrooms, even in cases where there are valid reasons to scrutinize, doubt, or dismiss it.