DNA first became criminal evidence in the mid-1980s. An English murder inquiry in 1986 and 1987 was the first criminal use, and in 1987 a Florida man named Tommie Lee Andrews became the first American convicted with it.1Nature Medicine. Genetic fingerprinting Acceptance wasn’t instant, though, and courts spent years arguing over how labs handled the evidence before it became routine.
The First Case Solved With DNA
Lynda Mann was murdered in Narborough, Leicestershire, in November 1983.2Wikipedia. Colin Pitchfork Dawn Ashworth was murdered in nearby Enderby in July 1986.3Wikipedia. Colin Pitchfork Police suspected that one person had killed both girls. The Enderby case became the first application of DNA profiling in a criminal investigation.4Nature Medicine. Genetic fingerprinting
A local young man had confessed, but his DNA did not match the crime-scene sample, so he was cleared. The first DNA case began with an exoneration, not a conviction.5the Guardian. Killer breakthrough – the day DNA evidence first nailed a murderer
With the confession ruled out, police moved to a population screen. Local men in the surrounding villages were asked to give blood so their DNA could be compared with the crime-scene sample. The screening worked by elimination. Each man whose profile didn’t match was cleared, and the pool narrowed. Testing so many men at once had never been tried, and it raised early questions about consent.
Colin Pitchfork got a coworker to give a sample in his place, and he was caught only after the swap was overheard.6the Guardian. Killer breakthrough – the day DNA evidence first nailed a murderer Pitchfork became the first person convicted of rape and murder using DNA profiling.7Wikipedia. Colin Pitchfork
In 1986 the idea that a body fluid could point to one person was brand new. Blood typing could only narrow suspects to large groups. Newspapers called the new method “genetic fingerprinting” to compare it to the familiar fingerprints people already trusted.
The First US Conviction in 1987
In 1987 Tommie Lee Andrews became the first American convicted in a case using DNA evidence.8PBS. When Did DNA Evidence Become Evidence in Criminal Cases? It took two trials. The first ended without a DNA-based conviction, as jurors struggled with the new science. Weeks later the state tried him for an attack on Nancy Munroe.9WESH 2. WESH 2 Investigates: How Florida prosecutors used forensic science in Tommie Lee Andrews’ case This time the jury understood and accepted the DNA evidence and found him guilty.10WESH 2. WESH 2 Investigates: How Florida prosecutors used forensic science in Tommie Lee Andrews’ case
Explaining DNA to a jury was hard because nobody in the box had heard of DNA matching. It had to be put in everyday terms, including the odds of a random match. The second trial succeeded partly because the explanation was clearer.
A DNA match isn’t a witness statement. Jurors must trust a chain of steps they can’t see, from sample collection to lab comparison to statistical conclusion. In 1987 almost no juror had any background to judge those steps, so the first trial’s difficulty reflected how new the idea was, not a flaw in the evidence. England used DNA to find a suspect; Florida used it to convict a man already charged.
Early Court Battles Over DNA
People v. Castro in 1989 was the most publicized case rejecting DNA evidence.11Case Western Reserve Law Review. DNA Evidence It was also one of the first cases where the defense seriously challenged DNA.12Case Western Reserve Law Review. DNA Evidence The court accepted DNA testing in general and rejected only that lab’s results.13Case Western Reserve Law Review. DNA Evidence Castro showed that flaws could exist in at least one lab’s procedures, even though Castro later pleaded guilty.14Case Western Reserve Law Review. DNA Evidence
That is the common mistake about early rejections: “DNA was thrown out” doesn’t mean DNA doesn’t work. The targets were sloppy handling, not the genetics. A correct test on a mishandled sample can still mislead. Defense lawyers learned to attack collection and lab steps, and that scrutiny is a main reason modern standards exist.
In 1993 the US Supreme Court in Daubert replaced the old “general acceptance” test for scientific evidence in federal courts.15Journal of Forensic Sciences. The Case Against <i>Daubert:</i> The New Scientific Evidence “Standard” and the Standards of the Several States Under the old test, a judge could mostly ask whether the relevant scientific community accepted a method. That could shut out new but sound techniques or let in widely used but weak ones. Now judges looked at whether a method was reliable, not just widely used. As a gatekeeper, a judge asks whether a method can be tested, has been reviewed by other scientists, and has a known error rate. For DNA, labs had to be ready to show their procedures and how often they could go wrong, not simply point to the technique’s popularity.
The 1995 Simpson trial was the first major trial where science was the focus of proceedings.16NIST. How the O.J. Simpson Trial Inspired My Career in Forensic Science It questioned many lab and police practices of the time, especially how evidence was collected.17NIST. How the O.J. Simpson Trial Inspired My Career in Forensic Science Labs and police tightened handling procedures afterward.
How DNA Testing Changed Since the 1980s
Early DNA testing cut DNA into pieces whose lengths vary between people because of repeated sections.18National Institute of Justice. Crime Scene and DNA Basics for Forensic Analysts | DNA Typing by RFLP Analysis It was slow, needed much larger samples, and could take up to a month.19Thermo Fisher Scientific. RFLP Analysis It could still work on bone exposed to burial or water for months.20PubMed. Restriction fragment length polymorphism (RFLP) analysis on DNA from human compact bone
Newer testing copies tiny amounts of DNA and reads short repeated sections. Copying works by making many duplicates of chosen stretches of DNA, so a few cells can yield enough material to read. That is why a touched object or a small stain can now be tested, where the first method needed a visible stain of blood or semen.
Only with this approach did DNA become sensitive and reliable enough for efficient databases.21Handbook of Analytical Separations. Forensic DNA typing technologies: a review In 1995 the UK set up the first national DNA database of people and crime scenes.22PubMed. A brief history of the formation of DNA databases in forensic science within Europe The US equivalent is CODIS, which lets US labs compare profiles nationwide.
A “match” means labs compare a set number of spots on the DNA, not the whole genome. US databases use 20 core spots. Matching all of them in a full, clean profile makes a coincidence extremely unlikely, except with identical twins; partial or mixed profiles carry weaker odds. A match shows whose DNA it is, not how or when it got there.
This is also why old cold cases are now solvable. Tiny or old samples that were once useless can be tested, and evidence kept in storage since the 1980s can be rerun. Databases let a decades-old sample match a recent arrestee. Rapid DNA machines can profile a cheek swab automatically in roughly two hours, which brings testing to the booking desk.
How DNA Has Freed the Wrongly Convicted
By July 2016, 342 people had been exonerated through DNA analysis.23NIJ Journal. Wrongful Convictions and DNA Exonerations: Understanding the Role of Forensic Science By 2020 the count of US DNA exonerations reached 375.24Innocence Project. DNA Exonerations in the United States (1989 – 2020)25PubMed. The genetics of innocence: analysis of 194 U.S. DNA exonerations
A typical exoneration starts when stored evidence from an old trial is retested with modern methods. The DNA fails to match the convicted person, and sometimes it matches someone else in the database. Courts then vacate the conviction, often after years of appeals.
Retesting usually begins when a convicted person or a legal group asks for it, and that request can be refused or delayed. Testing depends on evidence having been kept, and in some cases it was lost or destroyed, which can end a claim before it starts. Laws about access to post-conviction testing differ from state to state, so the route to testing depends partly on where the conviction took place.
These cases also revealed problems with other kinds of evidence. Many involved mistaken eyewitness identification. Some involved false confessions, as in the very first DNA case. They pushed reforms in lineups and interrogations.
Can DNA Evidence Mislead Today
With sensitive testing, who the DNA belongs to is hardly ever seriously questioned in court.26PubMed. Secondary DNA transfer by working gloves How the DNA reached the crime scene is questioned more often.27PubMed. Secondary DNA transfer by working gloves A person’s DNA can be found on an object they never handled.28PubMed. Secondary DNA transfer by working gloves Some people also shed far more DNA than others, which affects how much transfers.29PubMed. Secondary DNA transfer by working gloves
One route is DNA carried by a go-between. You shake hands with someone who then touches an object, and your DNA can ride along on their skin or gloves.
Even investigators can move DNA around. Disposable gloves worn by investigators can carry DNA from one item to another.30PubMed. Secondary and subsequent DNA transfer during criminal investigation Highly sensitive tests make it likelier to detect DNA left by a go-between.31PubMed. Secondary and subsequent DNA transfer during criminal investigation Juries now weigh how much DNA was found and where on the object, whether the person had innocent contact with the place, and how the evidence was handled after collection.
Mixed samples from several people are now interpreted with statistical software.32PubMed. An assessment of the performance of the probabilistic genotyping software EuroForMix: Trends in likelihood ratios and analysis of Type I & II errors When several people’s DNA is on one item, the profile shows overlapping signals that analysts must separate, and the more people and the less DNA, the harder this becomes. The software weighs how likely the evidence is under different explanations, but it still depends on assumptions the analyst chooses. Different labs or programs can give different strengths of conclusion on the same difficult sample, so such results are worth scrutiny.
Can Police Take Your DNA Without Asking
Courts have consistently found that police may collect DNA a suspect leaves in a public place.33Georgia Law Review. Genetic Privacy & the Fourth Amendment: Unregulated Surreptitious DNA Harvesting They hold there is no reasonable expectation of privacy in saliva left on a coffee cup or cigarette.34Northwestern University Law Review. Reclaiming “Abandoned” DNA: The Fourth Amendment and Genetic Privacy The Supreme Court has not ruled specifically on this kind of collection.35Northwestern University Law Review. Reclaiming “Abandoned” DNA: The Fourth Amendment and Genetic Privacy36Northwestern University Law Review. Reclaiming “Abandoned” DNA: The Fourth Amendment and Genetic Privacy Critics argue a person’s genetic data deserves more protection than trash.
A cheek swab taken directly from a person is different. It is a search that generally requires a warrant or consent, with some exceptions, because it intrudes on the body. A discarded item is treated as property the person gave up, so courts have not applied the same protection.
New Ways DNA Solves Crimes
Crime-scene DNA can be tested for about 850,000 markers like those used by consumer ancestry kits.37PubMed Central. Forensic genealogy, bioethics and the Golden State Killer case Searching public genealogy databases can then find a suspect’s relatives and solve old crimes, as in the Golden State Killer case.38PubMed Central. Forensic genealogy, bioethics and the Golden State Killer case
The search narrows step by step. A partial match points to a distant cousin who uploaded a kit. Investigators build a family tree from public records and narrow to relatives of the right age and location. A genealogy match only shows that two people share some DNA, which can point to a family branch but doesn’t identify a single person. Several people may fit at first, so police collect a candidate’s discarded DNA to confirm. The final step is a direct comparison between the candidate and the crime-scene profile, which is the evidence that can be used in court. Relatives never consented to help police, and rules differ widely between countries.
DNA can also predict eye, hair and skin color with reasonably high accuracy.39PubMed. Forensic DNA Phenotyping: Predicting human appearance from crime scene material for investigative purposes Predictions now include freckles, hair texture, male hair loss, tall stature, ancestry and age.40PubMed. Recent advances in Forensic DNA Phenotyping of appearance, ancestry and age Experts say these predictions need proper rules on use.41PubMed Central. The Use of Forensic DNA Phenotyping in Predicting Appearance and Biogeographic Ancestry They are leads, not proof.
Pet hair, poached animal products and illegal plants can all be DNA-tested.42National Institute of Justice. Principles of Forensic DNA for Officers of the Court Such evidence has been used in cases from murder to dog attacks.43Medicine, Science and the Law. Use of non-human DNA analysis in forensic science: a mini review Across borders, since 2008, EU countries must let each other search their DNA databases.44PubMed Central. Trends in forensic DNA database: transnational exchange of DNA data