Forensic science

• 1. Introduction

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  Forensic science has been changing the outcome of trials for centuries, but it took a Scottish surgeon working in a Tokyo hospital in 1880 to plant a seed that would eventually transform criminal justice worldwide. Henry Faulds published his first paper on fingerprints in the journal Nature, proposing a method to record them with printing ink. He offered the concept to the Metropolitan Police in London, but they dismissed it outright. What follows is the story of how a sprawling set of disciplines, from toxicology to DNA analysis, slowly converged into the field we call forensic science. How did ancient Chinese officials and 19th-century French police officers build a system that today can exonerate the innocent and identify killers from a single drop of blood? And what happens when that system gets it wrong?

• 2. Ancient Roots

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  In 1248, a Chinese director of justice named Song Ci wrote a book that would echo through the centuries. Xi Yuan Lu, translated as Washing Away of Wrongs, laid out methods for determining cause of death at a time when criminal trials relied almost entirely on forced confessions and witness testimony. Song Ci described how to distinguish drowning from strangulation, how to use sunlight and vinegar under a red-oil umbrella to reveal hidden injuries on corpses, and how to calculate time of death by accounting for weather and insect activity.

  One case in the book illustrates how far ahead of his time Song Ci was. A murder victim had been killed with a sickle. An investigator gathered all suspects and had each one bring his sickle to a single location. The investigator had already tested various blades on an animal carcass to confirm the weapon type. Flies, attracted by traces of blood, gathered on a single sickle. Its owner confessed.

  Across the ancient world, other methods were developing that anticipated modern lie detection. In ancient India, suspects were made to fill their mouths with dried rice and spit it out. In ancient China, accused persons had rice powder placed in their mouths. In middle-eastern cultures, the accused briefly licked hot metal rods. The reasoning was that fear and guilt reduce saliva production, leaving a dry mouth. A person whose tongue was severely burned, or whose mouth retained abundant rice, was considered likely guilty.

• 3. Science Enters the Courtroom

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  In Lancaster in 1784, a man named John Toms stood trial for murdering Edward Culshaw with a pistol. When investigators examined Culshaw's head wound, they found a pistol wad: a piece of crushed paper used to secure powder and shot in a muzzle-loading firearm. It matched perfectly with a torn newspaper found in Toms's pocket. The physical evidence convicted him.

  In Warwick in 1816, a farm laborer was tried for drowning a maidservant in a shallow pool. Police found footprints in the damp earth near the water, along with an impression from corduroy cloth bearing a sewn patch, and scattered grains of wheat and chaff. When investigators examined the breeches of a nearby farm laborer who had been threshing wheat, the cloth matched the impression exactly.

  These English cases belonged to a broader shift taking place across Europe during the Enlightenment. The use of torture to force confessions was being curtailed. Belief in the occult ceased to carry weight in courts. In 1773, Swedish chemist Carl Wilhelm Scheele devised a method for detecting arsenious oxide in corpses. German chemist Valentin Ross expanded on this work in 1806, learning to detect arsenic in the walls of a victim's stomach. The scientific study of poisons, what we now call forensic toxicology, was taking shape.

  James Marsh brought that science into a celebrated courtroom drama in 1832. He was called by the prosecution in a murder trial to testify about arsenic poisoning. The defendant, John Bodle, was accused of lacing his grandfather's coffee with arsenic. Marsh confirmed the presence of arsenic using the established test, but the sample deteriorated before the jury could evaluate it. Bodle was acquitted. Unsatisfied with that outcome, Marsh developed a far more sensitive method: combining a suspected arsenic sample with sulfuric acid and arsenic-free zinc to produce arsine gas, which, when ignited and passed over a cold surface, left a distinctive silvery-black metallic deposit. The test, which Marsh first described in The Edinburgh Philosophical Journal in 1836, could detect as little as one-fiftieth of a milligram of arsenic.

• 4. Fingerprints and Identification

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  Sir William Herschel began using thumbprints on documents as a security measure in 1858 while working for the Indian Civil Service. His aim was to prevent the rampant repudiation of signatures. By 1877, at Hooghly near Kolkata, he had extended the practice to contracts, deeds, and the fingerprints of government pensioners, using them to prevent relatives from fraudulently collecting a pensioner's payments after death.

  Henry Faulds had dismissed fingerprints at the Metropolitan Police, so he wrote to Charles Darwin with a description of his method. Darwin, too old and ill to pursue it, passed the information to his cousin Francis Galton, who spent ten years studying the subject. Galton published a detailed statistical model of fingerprint analysis in his book Finger Prints and calculated that the chance of two different individuals sharing identical fingerprints was about 1 in 64 billion.

  Argentine chief police officer Juan Vucetich acted on Galton's research. In 1892, after studying Galton's pattern types, Vucetich established what is considered the world's first fingerprint bureau. That same year, a woman named Francisca Rojas of Necochea was found injured at home; her two sons had been killed. She accused a neighbor, who withstood brutal interrogation without confessing. Vucetich's colleague Inspector Alvarez found a bloody thumb mark on a door at the scene. When compared to Rojas's own prints, it matched her right thumb exactly. She confessed.

  In Calcutta in 1897, a Fingerprint Bureau was formally established after the Council of the Governor General approved its use for classifying criminal records. Two Indian fingerprint experts working there, Azizul Haque and Hem Chandra Bose, developed a classification system that was eventually named after their supervisor, Sir Edward Richard Henry. The Henry Classification System was adopted in England and Wales when the first United Kingdom Fingerprint Bureau opened at Scotland Yard in 1901.

  French police officer Alphonse Bertillon had already created a parallel approach to identification. Dissatisfied with the ad hoc methods for identifying captured criminals in France in the 1870s, Bertillon built a system based on measuring 20 parts of the human body. In 1884, over 240 repeat offenders were caught using the Bertillon system. Though fingerprinting soon supplanted his central methods, Bertillon's other contributions, including the mug shot and the systematic photography of crime scenes, remain standard practice.

• 5. DNA Changes Everything

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  Sir Alec Jeffreys developed forensic DNA analysis in 1984, recognizing that variation in the genetic sequence could identify individuals and distinguish one person from another. The first real test of that discovery came in a double murder in the small English town of Narborough, Leicestershire. In 1985, a 15-year-old schoolgirl named Lynda Mann was raped and murdered at Carlton Hayes psychiatric hospital. Police recovered a semen sample but found no suspect.

  In 1986, Dawn Ashworth, also 15 years old, was raped and strangled in the nearby village of Enderby. Forensic evidence showed both crimes shared the same blood type. A man named Richard Buckland became a suspect: he worked at Carlton Hayes, had been seen near the second murder scene, and knew details about the body that had not been released. He confessed to Dawn Ashworth's murder but denied killing Lynda Mann. Jeffreys was brought in to analyze the semen samples from both crimes. He found no match between the samples and Buckland. Buckland became the first person in history to be exonerated through DNA evidence. The semen samples from both murders matched each other, confirming a single killer.

  To find the perpetrator, investigators collected DNA samples from the entire male population of the town: more than 4,000 men aged 17 to 34. A breakthrough came when a friend of a man named Colin Pitchfork was overheard saying that Pitchfork had persuaded him to provide a sample under Pitchfork's name. Pitchfork was arrested in 1987. His DNA profile matched both semen samples. The case led directly to the creation of searchable DNA databases, including the national FBI database and the European Network of Forensic Science Institutes.

• 6. Institution and Controversy

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  Hans Gross, an Austrian criminal jurist, wrote a handbook for coroners, police officials, and military policemen in 1893 that is generally credited as the founding document of criminalistics as an organized field. His work combined psychology, physical science, and crime scene photography into a single system. He went on to found the Institute of Criminalistics in 1912 as part of the University of Graz Law School. Similar institutes spread across the world.

  In 1909, Archibald Reiss founded the Institut de police scientifique at the University of Lausanne, the first school of forensic science in the world. A year later, Dr. Edmond Locard persuaded the Police Department of Lyon to give him two attic rooms and two assistants, and he founded what may have been the first criminal laboratory in the world. Locard formulated the guiding principle of the field: "Every contact leaves a trace."

  Yet the 20th century also revealed how badly forensic science could fail. Comparative bullet-lead analysis was used by the FBI for over four decades, beginning with the assassination of John F. Kennedy in 1963. The FBI's own internal studies, along with an outside review by the National Academy of Sciences, eventually determined the technique was unreliable due to improper interpretation. The FBI abandoned it in 2005.

  Forensic dentistry faced similar scrutiny. A 1999 study by a member of the American Board of Forensic Odontology found a 63 percent rate of false identifications when examining bite marks. In at least three cases, bite-mark evidence was used to convict people of murder who were later freed by DNA evidence. The Innocence Project's database of DNA exonerations found that forensic science errors contributed to roughly 39 to 46 percent of wrongful convictions, according to figures cited alongside the US Department of Justice.

  In Houston in 2002, an audit of the police department's forensic lab revealed fabricated evidence that had led to the wrongful conviction of a man named George Rodriguez for rape. When the lab's former director was asked about the scope of the problem, he estimated that between 5,000 and 10,000 cases could have been affected by improper work.

• 7. Beyond the Crime Scene

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  Forensic science has moved well outside the courtroom. The International Committee of the Red Cross uses forensic methods to clarify the fate of people missing after armed conflict, disasters, and migration, as part of its work on Restoring Family Links. Identifying missing persons can make it possible for families to grieve and move forward.

  The Argentine Forensic Anthropology Team works to clarify the fate of people who disappeared during Argentina's military dictatorship between 1976 and 1983. The International Commission on Missing Persons applied forensic science to find those who went missing during the conflicts in the Balkans. Recognizing all of this, a group of experts in the late 1980s produced a UN manual on investigating extrajudicial executions, known as the Minnesota Protocol, which was revised and republished by the Office of the High Commissioner for Human Rights in 2016.

  In the United States as of 2025, there are an estimated 18,500 forensic science technicians. The National Institute of Standards and Technology oversees several programs in the field, including CSAFE, a center of excellence in forensic science, and the Organization of Scientific Area Committees for Forensic Science. Privacy debates have opened up around genetic genealogy databases, which law enforcement has used to identify suspects by comparing crime-scene DNA with the profiles of their relatives. A study by researchers including Christi Guerrini found that most survey respondents supported police searches of such databases when the crimes involved violence or offenses against children, though the same research noted that genetic testing customers skew wealthy and white, while those identified as offenders through such databases skew low-income and Black. The Supreme Court's 2009 ruling in Melendez-Diaz v. Massachusetts, decided by a 5-to-4 margin, added another layer of accountability: crime laboratory reports may not be used against defendants at trial unless the analysts who produced them testify and submit to cross-examination.