Fingerprint

This article is about human fingerprints. For other uses, see Fingerprint (disambiguation). "Thumbprint" redirects here. For other uses, see Thumbprint (disambiguation).

A fingerprint is an impression left by the friction ridges of a human finger. Finding partial fingerprints at a crime scene is an important method of forensic science. Moisture and grease on a finger can leave fingerprints on surfaces such as glass or metal. Fingerprints can also be made on purpose by transferring ink or other substances from the ridges onto a smooth surface such as paper. Fingerprint records usually include the pad on the last joint of each finger and thumb, and fingerprint cards also often show parts of the lower joints.

Human fingerprints are detailed, unique, hard to change, and last throughout a person's life. This makes them useful as long-term markers of identity. Police and other authorities may use them to identify people who want to hide who they are, or people who are unable to identify themselves, such as after a natural disaster.

Their use as evidence has been challenged by academics, judges and the media. There are no uniform standards for point-counting methods. Academics have argued that the error rate in matching fingerprints has not been adequately studied, and that fingerprint evidence has no secure statistical foundation. Researchers have also studied whether experts can focus objectively on feature information in fingerprints without being misled by extraneous information, such as context.

Biology

Fingerprints are marks left on surfaces by the friction ridges on a person's finger. Matching two fingerprints is one of the most widely used and most reliable biometric techniques. Fingerprint matching looks only at the obvious features of a fingerprint.

Fingerprints are mostly water, about 95 to 99 percent. They also contain other substances, including amino acids, proteins, glucose, lactase, urea, pyruvate, fatty acids, sterols, and ions such as chloride, sodium, potassium, and iron. Friction ridges are raised parts of the epidermis on fingers, toes, the palm, and the sole of the foot. These ridges form around the 15th week of fetal development and stay until after death, when decomposition begins. They help amplify vibrations when fingertips touch an uneven surface, sending signals to sensory nerves that sense fine textures. The ridges may also help grip rough surfaces and improve contact in wet conditions.

Genetics

Scientists generally agree that the dermatoglyphic patterns on fingertips are passed down from parents. The fingerprints of monozygotic twins are very similar, though not identical. Fingerprints of dizygotic twins are much less alike. Scientists have found significant heritability for 12 dermatoglyphic traits. Current models suggest Mendelian inheritance, with extra effects from either additive or dominant major genes. Genes mostly decide the general type of pattern, while environmental factors cause slight differences in each fingerprint. One study suggested that about 5% of the total variability comes from small environmental effects, although it only measured total ridge count. One model suggests that a buckling instability in the basal cell layer of the fetal epidermis forms the ridges, and blood vessels and nerves may also play a role. Another model suggests that changes in the fluid around each developing finger affect how cells grow. These factors affect each finger differently, so no two fingerprints are identical, though they keep similar patterns.

Studying inheritance is difficult because there is a huge variety of phenotypes, and classifying a pattern is often subjective. Total ridge count has been suggested to follow a polygenic pattern of inheritance, but other research challenges this idea. The arch pattern may be inherited as an autosomal dominant trait, and further research suggests that a major gene or multifactorial inheritance is responsible. The whorl pattern may be controlled by a single gene or a group of linked genes. Specific genes have also been connected to fingerprints. Ridge counts showed a link to chromosome 5q14.1 for the ring, index, and middle fingers. In mice, variants of the gene EVI1, also called MECOM, were linked to dermatoglyphic patterns. Genome-wide association studies found single nucleotide polymorphisms in the gene ADAMTS9-AS2 that appear to affect the whorl pattern. This gene encodes antisense RNA, which may block ADAMTS9. In February 2023, a study identified WNT, BMP, and EDAR as signaling pathways that help form the main ridges on fingerprints. WNT and BMP have an opposite relationship, which was explained by a Turing reaction-diffusion system.

Classification systems

"Dactylographer" redirects here. For the racehorse, see Dactylographer (horse).

Before computers, fingerprints were stored in large paper filing systems called fingerprint repositories. A classification system sorts fingerprints by their features, so a fingerprint can be compared with a smaller group in a database instead of with every print. Early systems grouped fingers by their general ridge patterns. The Roscher System was developed in Germany and used in Germany and Japan. The Vucetich System was developed in Argentina and used across South America. The Henry Classification System was developed in India and used in most English-speaking countries.

The Henry system sorts fingerprints into three basic patterns: loops, whorls, and arches. These make up about 60–65, 30–35, and 5 percent of all fingerprints. Loops are named for the side toward which their tail points. Ulnar loops point toward the pinky side, near the ulna, the lower arm bone. Radial loops point toward the thumb side, near the radius. The system also uses a "primary classification number," a fraction made from whether each finger has a whorl. This puts each set of ten fingerprints into one of 1024 groups.

Fingerprint identification

Fingerprint identification, also called dactyloscopy, ridgeology, or hand print identification, compares two sets of friction ridge skin marks (minutiae) from human fingers, toes, palms, or soles. It checks whether they could have come from the same person. Because the ridges form in a random way, no two prints are ever exactly alike in every detail, even when two are taken one after the other from the same hand. An expert, or an expert computer system working under threshold scoring rules, decides whether two prints are likely to have come from the same finger, palm, toe, or sole.

In 2024, research using deep learning neural networks found, contrary to "prevailing assumptions," that fingerprints from different fingers of the same person could be matched to that person with 99.99% confidence. Intentional prints are usually made with black printer's ink rolled onto white card, or recorded digitally on glass using live scan. A "latent print" is one left by chance on an object or a wall. Latent prints are invisible to the eye and often incomplete, so they may need powder, chemicals, or special lights to be seen. Sometimes an ordinary bright flashlight is enough. Many factors change how a print looks, including the skin, the pressure, and the surface it touches. Because every situation is unique, fingerprint examiners need extensive training. The scientific study of fingerprints is called dermatoglyphics or dactylography.

Fingerprinting techniques

Exemplar prints, also called known prints, are fingerprints taken on purpose from a person. This may happen when someone joins a system or is arrested for a suspected crime. After an arrest, one print is usually taken from each finger, rolled from one edge of the nail to the other. Flat prints of the four fingers of each hand and of each thumb are also taken. These prints can be collected with live scan or with ink on paper cards.

In forensic science, a partial fingerprint lifted from a surface is called a latent fingerprint. Moisture and grease on fingers leave latent prints on surfaces such as glass. Because these prints are not clearly visible, they may need chemical treatment, such as powder dusting, spraying ninhydrin, iodine fuming, or soaking in silver nitrate. The method depends on the surface. On nonporous surfaces like glass, metal, or plastic, a fine powder and brush are used, and then clear tape lifts the print off. Partial prints that are easy to see are called patent prints, and chocolate, toner, paint, or ink on fingers can leave them. Prints found in soft materials like soap, cement, or plaster are called plastic prints.

Capture and detection

Getting a clear fingerprint image is the most important step in a computer system that checks fingerprints. Fingerprint readers measure the difference between ridges and valleys, and they come in two main families: solid-state and optical. Because the skin bends and stretches when a finger touches a surface, each scan can look a little different. To solve this, non-contact or touchless 3D fingerprint scanners were developed as of 2010. After death, skin dries and cools, so fingerprints may still be taken from a body during an autopsy. Dried skin can be boiled to rehydrate it, or baby powder can be brushed on to make the ridges show.

Fingerprints have been used by police since the late nineteenth century to identify suspected criminals and crime victims. In the 1930s, investigators in the United States first found hidden fingerprints on fabrics, especially inside gloves that criminals had thrown away. Each person's ridge patterns are unique and do not change over time, and even identical twins, who share DNA, do not have the same fingerprints. Because these hidden prints are hard to see, a "developer," such as a powder or chemical, is often needed to make them visible. Crime scenes may use simple powders or chemicals applied in situ, while more complex lab methods were reporting that 50% or more of recovered prints had been identified as of 2010.

Forensic laboratories

Although there are hundreds of reported ways to find fingerprints, many are only of academic interest. Only around 20 are really effective and used in the more advanced fingerprint laboratories around the world. Some, such as ninhydrin, diazafluorenone and vacuum metal deposition, are very sensitive and used in real cases. Some chemicals react only with certain substances, such as ninhydrin and diazafluorenone reacting with amino acids. Others, such as ethyl cyanoacrylate, work in different ways. Vacuum metal deposition, which uses gold and zinc, is not specific, but it can detect fat layers as thin as one molecule.

Simpler methods, such as applying fine powders, stick to the oily marks that fingers leave behind. Water makes up over 90% of a fresh fingerprint's weight, but it can evaporate quite quickly and may be mostly gone after 24 hours. Many fluorescence techniques have been introduced, mainly to make chemically developed fingerprints easier to see. Infrared lasers can even show fingerprints in 3D without chemicals. A comprehensive manual of these methods was last published by the UK Home Office Scientific Development Branch in 2013 and is widely used around the world. A technique proposed in 2007 aims to identify a person's ethnicity, sex, and dietary patterns.

Limitations and implications in a forensic context

The surface a fingerprint is left on matters a great deal. On hard, non-porous surfaces, the marks are not soaked into the material and can be smudged by another surface. On porous surfaces, the marks soak into the material. In both cases, the print can end up being of no use to examiners or be destroyed. Experts also need a clear print to correctly identify ridge patterns and their features, so how well they can analyze a print depends on its clarity. In a court setting, many have argued that fingerprint identification should be treated as opinion rather than fact. Many have said it is only accepted in court today because the standards for admitting evidence were quite low when it was added to the legal system. Only a limited number of studies have been done to confirm the science behind the process.

Crime scene investigations

A new method called the scanning Kelvin probe (SKP) can record fingerprints without touching them and without using developers. This means fingerprints could be recorded while the material stays intact, so it could later be tested for DNA.

During 2010, a prototype that could be used in forensic work was under development at Swansea University. The research drew strong interest from the British Home Office and many police forces across the UK and internationally. The hope is that the instrument could eventually be made in large enough numbers for forensic teams worldwide to use.

Detection of drug use

Fingerprints can hold tiny traces of chemicals from the body, including metabolites, which are the substances the body makes when it breaks other substances down. These traces can be detected and used in forensic science. For example, the fingerprints of tobacco smokers contain traces of cotinine, a nicotine metabolite, and also traces of nicotine itself. Care is needed when reading these results, because simply touching a tobacco product can leave these traces too.

Scientists can make a smoker's fingerprint glow by treating it with gold nanoparticles that have cotinine antibodies attached, and then with a glowing dye attached to cotinine antibodies. A non-smoker's fingerprint stays dark. As of 2010, the same method is being tested for identifying heavy coffee drinkers, cannabis smokers, and users of various other drugs.

Police force databases

Most law enforcement agencies in the United States use Wavelet Scalar Quantization (WSQ), a wavelet-based way to store fingerprint images at 500 pixels per inch (ppi) so they take up less space. WSQ was developed by the FBI, the Los Alamos National Lab, and the National Institute of Standards and Technology (NIST). For fingerprints recorded at 1000 ppi spatial resolution, law enforcement, including the FBI, uses JPEG 2000 instead of WSQ.

Validity

Fingerprints collected at a crime scene, or on items of evidence, have been used in forensic science to identify suspects, victims, and other people who touched a surface. In the late 1800s, fingerprint identification became an important system in police agencies because it was more reliable than body measurements for identifying people with a criminal record, sometimes listed under a false name. For about the past 100 years, governments around the world have used fingerprinting to identify criminals.

Some academics, judges, and media have questioned whether fingerprint evidence is valid. In the United States, fingerprint examiners have not agreed on uniform standards for matching a fingerprint to one person. In some countries, examiners must match a set number of identification points before a match is accepted. England requires 16 points and France requires 12. Critics say that counting points can focus only on where certain features appear. Some academics also say that the error rate in matching fingerprints has not been studied well, and that fingerprint evidence may not have a secure statistical basis. Researchers have also studied whether experts can focus only on the fingerprint features without being influenced by other information, such as the situation around the case. Fingerprints can, in theory, be faked and placed at crime scenes.

Professional certification

Fingerprinting helped start the first organization for forensic professionals, the International Association for Identification (IAI), in 1915. In 1977, the IAI created the first professional certification program for forensic scientists, called the Certified Latent Print Examiner program. People who met strict rules received certificates, and those certificates could be taken away if a person's work was not good enough. Other forensic fields later started their own certification programs.

History

Fingerprints have been found on ancient clay tablets, seals, and pottery, and on the walls of Egyptian tombs. In ancient China, officials used fingerprints to check government documents, and in about 200 BCE they were used to sign written contracts in Babylon. Fingerprints from 3D-scans of cuneiform tablets are now extracted using the GigaMesh Software Framework. Law officials under Hammurabi took the fingerprints of people who had been arrested, and during China's Qin dynasty, officials collected hand and foot prints as evidence. The Iranian physician Rashid-al-Din Hamadani wrote in his Jami al-Tawarikh that "no two individuals have fingers exactly alike." Whether ancient peoples understood that fingerprints could uniquely identify people has been debated. From the mid-1600s, Marcello Malpighi of the University of Bologna identified ridges, spirals and loops in fingerprints. In 1788, Johann Christoph Andreas Mayer became the first European to conclude that fingerprints are unique to each individual. Jan Evangelista Purkyně identified nine fingerprint patterns in 1823, and Georg von Meissner studied friction ridges in 1853.

In 1858, Sir William James Herschel started using fingerprints in Hooghly near Kolkata, India, including on contracts and deeds to prevent the repudiation of signatures. In 1880, Henry Faulds published his first paper on using fingerprints to identify people. Faulds later offered the idea to the Metropolitan Police in London, but it was dismissed at that time. Before the early 1890s, police forces in the United States and on the European continent could not reliably identify criminals to track their criminal record. In 1892, Francis Galton published a detailed statistical model of fingerprint identification, and that same year Juan Vucetich, an Argentine chief police officer, created the first method of recording the fingerprints of individuals on file. That year, fingerprints were also used to solve a murder case, which was the first known case of its kind. Fingerprint evidence was later used in a Kolkata bureau set up in 1897, where Azizul Haque and Hem Chandra Bose helped develop a classification system named after their supervisor, Sir Edward Richard Henry. In 1901, Scotland Yard began fingerprinting individuals after a method using iodine fuming was developed. In 1902, the Scheffer case became the first case in which a murderer was identified, arrested, and convicted based on fingerprint evidence, with help from Alphonse Bertillon. Fingerprint identification for law enforcement has been considered essential in the United States since the early 1900s, and it has also helped with body identification after natural disasters and anthropogenic hazards. The FBI manages a database called the Integrated Automated Fingerprint Identification System (IAFIS), and OBIM and the DHS also keep large fingerprint records. In 1910, Edmond Locard established the first forensic lab in France. Criminals may wear gloves to avoid leaving fingerprints, but gloves can leave unique prints too, which can be matched to glove prints at other crime scenes. In many jurisdictions, wearing gloves while committing a crime can itself be prosecuted as an inchoate offense.

Use of fingerprints in schools

Further information: Biometrics in schools

In 2002, the non-governmental organization (NGO) Privacy International warned that tens of thousands of UK school children were being fingerprinted by schools, often without their parents' knowledge or consent. By 2007, it was estimated that 3,500 schools were using such systems. Under the UK Data Protection Act, schools do not have to ask parents for consent. Parents who oppose fingerprinting can only make complaints about individual schools. In 2010, the European Commission expressed "significant concerns" about the practice, which may break the European Union data protection rules.

In 2007, the UK government considered fingerprinting all children aged 11 to 15 and adding the prints to a government database as part of a new passport and ID card scheme. Shadow Home secretary David Davis called the plan "sinister", and Nick Clegg criticised it. The UK's junior education minister, Lord Adonis, defended school fingerprinting for tracking attendance and access to school meals and libraries. He told the House of Lords that the prints would be destroyed once children left the school. An Early Day Motion backed by 85 Members of Parliament called for a full public consultation. After a Conservative and Liberal Democratic coalition government was formed in May 2010, the UK ID card scheme was scrapped. Security experts also raised serious concerns, and the Ottawa Police in Canada advised parents who fear their children may be kidnapped to fingerprint them.

Absence or mutilation of fingerprints

A very rare condition, adermatoglyphia, causes people to be born with no fingerprints. They have smooth fingertips, palms, toes, and soles, but no other medical signs or symptoms. A 2011 study suggested it is caused by the improper expression of a protein called SMARCAD1. Researchers called it immigration delay disease because people without fingerprints can face delays when proving their identity while traveling. Only five families with this condition had been described as of 2011. People with Naegeli–Franceschetti–Jadassohn syndrome and dermatopathia pigmentosa reticularis, which are forms of ectodermal dysplasia, also have no fingerprints, along with other signs such as thin, brittle hair.

Some medicines and conditions can also affect fingerprints. The anti-cancer medication capecitabine may cause fingerprints to be lost. Swelling of the fingers, such as from bee stings, can sometimes cause fingerprints to disappear for a short time until the swelling goes down. As people age, their skin becomes less elastic, so many senior citizens have fingerprints that are hard to capture. Fingerprints can also be erased permanently, which criminals could possibly use to avoid being convicted. Methods include plastic surgery. John Dillinger tried to erase his fingerprints with acid, but prints taken during an earlier arrest and at his death still matched each other almost completely.

Fingerprint verification

Fingerprints can be captured as patterns of ridges and valleys. Because they are unique and permanent, fingerprints became the most widely used biometric identifier in the 2000s. Automated fingerprint verification systems were developed for law enforcement, and they later became more common in everyday uses. Making these systems reliable was a challenge, so it was studied a great deal in pattern recognition and image processing. A fingerprint's uniqueness comes from its overall pattern or from small breaks in the ridges called minutiae. In the 2000s, minutiae were considered the most reliable part of a fingerprint, so they became the most common basis for automatic verification. The most used minutiae were the ridge ending and the ridge bifurcation.

There are three basic fingerprint patterns: the arch, the loop, and the whorl. Family members often share the same general patterns, which led scientists to believe these patterns are inherited. Other features of ridges include ridge endings, bifurcations (where one ridge splits in two), short ridges, islands or dots, lakes, spurs, bridges, deltas, and cores.

A fingerprint sensor is an electronic device that takes a digital image of a fingerprint pattern. This image, called a live scan, is digitally processed into a biometric template (a collection of extracted features) that is stored and used for matching. Many technologies have been used, including optical, capacitive, RF, thermal, piezoresistive, ultrasonic, piezoelectric, and MEMS methods. Optical scanners take a picture with a digital camera, capacitive scanners use electric current to form an image, ultrasound scanners use sound waves, and thermal scanners sense temperature differences between the ridges and valleys of a fingerprint.

Since 2000, electronic fingerprint readers have been introduced as consumer electronics security tools. Fingerprint scanners could be used for login authentication and identifying computer users, but some less sophisticated sensors could be fooled by simple tricks, such as fake fingerprints cast in gels. In 2006, fingerprint sensors became popular in the laptop market, and built-in sensors in laptops such as ThinkPad, VAIO, HP Pavilion, EliteBook, and others also worked as motion detectors for scrolling documents, like the scroll wheel. Motorola with the Atrix 4G in 2011 and Apple with the iPhone 5S in September 2013 were among the first smartphone makers to add fingerprint recognition. HTC then launched the One Max, and in April 2014 Samsung released the Galaxy S5, which had a sensor on its home button. In September 2015, Apple's iPhone 6S included a new Touch ID scanner that could unlock the screen or pay for mobile apps purchases. By 2017, Hewlett Packard, Asus, Huawei, Lenovo, and Apple were using fingerprint readers in their laptops, and in 2018 Synaptics revealed sensors built into the whole touchscreen of the Vivo X21 UD smartphone.

Algorithms

Matching algorithms compare stored fingerprint templates with new fingerprints to check identity for authentication. They can compare the original images directly, or they can compare certain features. Before matching, the image is cleaned up by filtering out extra noise so the fingerprint is clearer.

Pattern-based algorithms compare the basic fingerprint shapes, called arch, whorl, and loop, between a stored template and a new fingerprint. The images must line up in the same direction, so the algorithm finds a central point and centers the image on it. The template stores the type, size, and direction of these patterns, and the new fingerprint is compared with it to see how well they match.

In other species

Some other animals have their own unique prints. These are often animals that climb or grip wet things, such as many primates, including gorillas and chimpanzees, Australian koalas, and water-living mammals like the North American fisher. According to one study, even with an electron microscope, it can be quite difficult to tell a koala's fingerprints apart from a human's.

In fiction

Mark Twain's memoir Life on the Mississippi (1883) is mainly about the author's time on the river. It also recounts parts of his later life and includes tall tales and stories allegedly told to him, including a dramatic account of a murder in which the killer is identified by a thumbprint. Twain's novel Pudd'nhead Wilson, published in 1893, includes a courtroom drama that turns on fingerprint identification. Fingerprints also appear often in crime fiction. Sir Arthur Conan Doyle wrote "The Norwood Builder" (1903), a short story about his famous detective Sherlock Holmes, set in 1894. In it, a bloody fingerprint helps Holmes expose the real criminal and free his client. The British writer R. Austin Freeman's first Thorndyke novel, The Red Thumb-Mark, was published in 1907. It features a bloody fingerprint left on a paper beside a parcel of diamonds in a safe-box. Dr. Thorndyke leads the investigation and defends the accused, whose fingerprint matches the one on the paper, after the diamonds are stolen.

In the television series Bonanza (1959–1973), the Chinese character Hop Sing uses his knowledge of fingerprints to free Little Joe from a murder charge. The 1997 movie Men in Black requires Agent J to remove his ten fingerprints by putting his hands on a metal ball, an action the MIB agency sees as necessary to remove the identity of its agents. In the 2009 science fiction movie Cold Souls, a mule who smuggles souls wears latex fingerprints to fool airport security terminals. She can change her identity by changing her wig and latex fingerprints.