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 a mark left by the friction ridges on a human finger. Finding partial fingerprints at a crime scene can help solve crimes. This is an important part of forensic science. Oil and moisture on a finger can leave marks on things like glass or metal. People can also make fingerprints on purpose by pressing ink or other substances from their fingers onto smooth paper. Fingerprint records usually include the pad on the last joint of each finger and thumb. Fingerprint cards often show parts of the lower joints too.

Fingerprints are detailed and unique. They are hard to change and stay the same for a person's whole life. This makes them useful for identifying someone over many years. Police and other officials may use them to identify people who are hiding who they are, or people who cannot say who they are, such as after a natural disaster.

Some people have questioned whether fingerprints should be used as evidence. There are no single standard methods for comparing them. Some scientists believe the error rate in matching fingerprints has not been studied well enough. They also feel that fingerprint evidence does not have a strong statistical basis. Researchers have also studied whether experts can look at the details of a fingerprint fairly, without being influenced by other information, such as the situation around it.

Biology

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

Fingerprints are mostly water, about 95 to 99 percent. They also contain other substances, such as amino acids, proteins, glucose, lactase, urea, pyruvate, fatty acids, sterols, and minerals like chloride, sodium, potassium, and iron. The ridges are raised parts of the epidermis on fingers, toes, the palm, and the sole of the foot. They start forming around the 15th week before birth and stay until after a person dies. The ridges help make vibrations stronger when fingertips touch a bumpy surface. This sends signals to sensory nerves, which help us feel fine textures. The ridges may also help us grip rough surfaces and hold on better when things are wet.

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 strong 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 small differences in each fingerprint. One study suggested that only a small part of the variation comes from the environment, 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 exactly the same, though they keep similar patterns.

Studying inheritance is hard because there are many different phenotypes, and deciding what type of pattern a fingerprint has can depend on opinion. Total ridge count has been suggested to follow a polygenic pattern of inheritance, but other research questions 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. Some 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, changes in 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 makes antisense RNA, which may block ADAMTS9. In February 2023, a study identified WNT, BMP, and EDAR as signals that help form the main ridges on fingerprints. WNT and BMP work in opposite ways, which was explained by a Turing reaction-diffusion system.

Classification systems

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

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

The Henry system sorts fingerprints into three basic patterns: loops, whorls, and arches. About 60–65 percent of fingerprints are loops, 30–35 percent are whorls, and about 5 percent are arches. Loops are named for the direction 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 based on whether each finger has a whorl. This puts each set of ten fingerprints into one of 1024 groups.

Fingerprint identification

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

In 2024, research using deep learning neural networks found that fingerprints from different fingers of the same person could be matched to that person with 99.99% confidence. This was different from what many people had assumed. Planned prints are usually made with black printer's ink rolled onto white card, or recorded on glass using live scan. A "latent print" is one left by chance on an object or a wall. Latent prints cannot be seen easily and are often incomplete. They may need powder, chemicals, or special lights to be seen. Sometimes an ordinary bright flashlight is enough. Many things change how a print looks, such as the skin, the pressure, and the surface it touches. Because every situation is different, fingerprint examiners need a lot of 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 left on a surface is called a latent fingerprint. Moisture and grease on fingers leave latent prints on things like glass. These prints are hard to see, so they may need special treatment. Methods include dusting with powder, 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 a brush are used, and then clear tape lifts the print off. Partial prints that are easy to see are called patent prints. 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. There are two main kinds: solid-state and optical. A finger bends and stretches when it touches a surface, so each scan can look a little different. To solve this, touchless 3D fingerprint scanners were developed as of 2010. After death, fingerprints may still be taken from a body during an autopsy. Dried skin can be softened with water, or baby powder can be brushed on to make the ridges show.

Police have used fingerprints 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. Even identical twins, who share DNA, do not have the same fingerprints. Hidden prints are hard to see, so a "developer," such as a powder or chemical, is often needed to make them visible. At crime scenes, simple powders or chemicals may be used in situ. Labs can use more complex methods.

Forensic laboratories

There are hundreds of ways to find fingerprints, but many are only used for study. About 20 are truly useful and are used in advanced fingerprint laboratories around the world. Some, such as ninhydrin, diazafluorenone and vacuum metal deposition, are very sensitive and are used in real cases. Some chemicals react only with certain substances. For example, ninhydrin and diazafluorenone react with amino acids. Others, such as ethyl cyanoacrylate, work in different ways. Vacuum metal deposition uses gold and zinc. It is not specific, but it can detect layers of fat as thin as one molecule.

Simpler methods, such as using fine powders, stick to the oily marks that fingers leave behind. Fresh fingerprints are mostly water, but this water can evaporate quite quickly and may be almost gone after 24 hours. Many fluorescence techniques have been developed, mainly to make chemically developed fingerprints easier to see. Infrared lasers can even show fingerprints in 3D without using chemicals. A complete manual of these methods was last published in 2013 by the UK Home Office Scientific Development Branch. It is widely used around the world. A technique proposed in 2007 aims to identify a person's ethnicity, sex, and eating habits.

Limitations and implications in a forensic context

The surface a fingerprint is left on matters a lot. On hard, smooth surfaces, the marks sit on top and can be smudged by something touching them. On porous surfaces, such as paper, the marks soak into the material. In both cases, the print may be useless to experts or may be destroyed.

Experts need a clear print to see the ridge patterns and their features. How well they can study a print depends on how clear it is.

In court, many people have argued that fingerprint identification should be seen as an opinion rather than a fact. Many have also said that it is accepted in court today mainly because the rules for accepting evidence were fairly loose when fingerprints were first used in the legal system. Only a limited number of studies have been done to test the science behind the process.

Crime scene investigations

A new method called the scanning Kelvin probe (SKP) can record fingerprints without touching them. It also does not need developers. This means a fingerprint can be recorded while the item stays in good condition, so it could later be tested for DNA.

In 2010, a prototype that could be used in crime investigations was being developed at Swansea University. The research drew a lot of interest from the British Home Office and many police forces in the UK and other countries. People hope that the tool could one day be made in large enough numbers for investigators around the world to use.

Detection of drug use

Fingerprints can hold tiny traces of chemicals from the body. These include metabolites, which are the substances the body makes when it breaks other substances down. Scientists can find these traces and use them in forensic science. For example, the fingerprints of tobacco smokers contain traces of cotinine, a nicotine metabolite, and also traces of nicotine itself. Results like these need careful reading, because simply touching a tobacco product can leave the same traces.

Scientists can make a smoker's fingerprint glow. They first treat it with tiny gold particles called nanoparticles that have antibodies for cotinine attached. Then they add a glowing dye that is also attached to cotinine antibodies. A non-smoker's fingerprint stays dark. As of 2010, scientists were testing the same method to find coffee drinkers, cannabis smokers, and users of various other drugs.

Police force databases

Many police agencies in the United States use a method called Wavelet Scalar Quantization (WSQ). It is a way to store fingerprint pictures so they take up less space. The pictures are saved at 500 pixels per inch (ppi). WSQ was developed by the FBI, the Los Alamos National Lab, and the National Institute of Standards and Technology (NIST). For very detailed fingerprints recorded at 1000 ppi spatial resolution, police, including the FBI, use JPEG 2000 instead.

Validity

Fingerprints found at a crime scene or on evidence can be used in forensic science to find out who touched a surface. This might be a suspect, a victim, or someone else. In the late 1800s, police agencies began using fingerprints to identify people. This was more reliable than body measurements, especially for people with a criminal record who sometimes used a false name. For about the past 100 years, governments around the world have used fingerprints to identify criminals.

Some academics, judges, and members of the media have asked whether fingerprint evidence is valid. In the United States, fingerprint experts have not agreed on one set of rules for deciding that a fingerprint belongs to a certain person. Some countries require experts to find a set number of matching points before they accept a match. England requires 16 points, and France requires 12. Critics say that counting points may only look at where certain features appear. Some academics also say that the error rate in matching fingerprints has not been studied well. They think fingerprint evidence may not have a strong statistical basis. Researchers have also looked at whether experts can focus only on the fingerprint itself, without being affected by other information about the case. In theory, fingerprints can also be faked and placed at a crime scene.

Professional certification

Fingerprints helped start the first group for people who study fingerprints, the International Association for Identification (IAI), in 1915. In 1977, the IAI created the first certification program for these experts. It was called the Certified Latent Print Examiner program. People who met strict rules received certificates. A certificate could be taken away if a person's work was not good enough. Other forensic fields later started their own certification programs.

History

People have found fingerprints on ancient clay tablets, seals, and pottery, and on the walls of Egyptian tombs. In ancient China, officials used fingerprints to check government documents. Around 200 BCE, people in Babylon used fingerprints to sign written contracts. Today, fingerprints can be taken from 3D scans of cuneiform tablets using the GigaMesh Software Framework. Law officials under Hammurabi took the fingerprints of people who had been arrested. During China's Qin dynasty, officials collected hand and foot prints as evidence. The Iranian doctor Rashid-al-Din Hamadani wrote in his Jami al-Tawarikh that "no two individuals have fingers exactly alike." Historians still debate whether ancient peoples knew that fingerprints could identify a single person. From the mid-1600s, Marcello Malpighi of the University of Bologna described the ridges, spirals, and loops in fingerprints. In 1788, Johann Christoph Andreas Mayer became the first European to conclude that each person's fingerprints are unique. Jan Evangelista Purkyně identified nine fingerprint patterns in 1823, and Georg von Meissner studied friction ridges in 1853.

In 1858, Sir William James Herschel began using fingerprints in Hooghly, near Kolkata in India. He used them on contracts and deeds to prevent people from denying their signatures, which is called repudiation. In 1880, Henry Faulds published his first paper on using fingerprints to identify people. Later, he suggested the idea to the Metropolitan Police in London, but they did not accept it at that time. Before the early 1890s, police forces in the United States and on the European continent could not reliably identify criminals, so they could not easily keep track of their criminal record. In 1892, Francis Galton published a detailed statistical model for identifying people by fingerprints. That same year, Juan Vucetich, an Argentine chief police officer, created the first method for recording people's fingerprints in files. That year, fingerprints were also used to solve a murder case, the first known case of its kind. Later, in 1897, a Kolkata bureau used fingerprint evidence. There, Azizul Haque and Hem Chandra Bose helped develop a classification system named after their supervisor, Sir Edward Richard Henry. In 1901, Scotland Yard began taking fingerprints from people after a method using iodine fumes was developed. In 1902, the Scheffer case became the first case in which a person was identified, arrested, and found guilty of a murder using fingerprint evidence. Alphonse Bertillon helped with this case. Since the early 1900s, fingerprint identification for law enforcement has been considered essential in the United States. It has also helped identify people after natural disasters and anthropogenic hazards, when body identification is needed. The FBI manages a database called the Integrated Automated Fingerprint Identification System (IAFIS). OBIM and the DHS also keep large fingerprint records. In 1910, Edmond Locard set up the first forensic lab in France. Some criminals wear gloves to avoid leaving fingerprints. However, gloves can leave their own unique prints, which can be matched to glove prints found at other crime scenes. In many jurisdictions, wearing gloves while committing a crime can itself be punished 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 many UK school children were being fingerprinted by their schools. Often, their parents did not know about it or agree to it. By 2007, many schools were using these systems. Under the UK Data Protection Act, schools do not have to ask parents for permission. Parents who do not want their children fingerprinted can only complain about each school. In 2010, the European Commission said it had "significant concerns" about this practice. It may break the European Union data protection rules.

In 2007, the UK government thought about fingerprinting all children aged 11 to 15. The prints would be added to a government database as part of a new passport and ID card plan. 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. He said it helped schools keep track of 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 supported by 85 Members of Parliament asked for a full public discussion. After a Conservative and Liberal Democratic coalition government was formed in May 2010, the UK ID card scheme was cancelled. Security experts also had serious worries. The Ottawa Police in Canada told parents who were worried about their children being kidnapped to fingerprint them.

Absence or mutilation of fingerprints

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

Some medicines and conditions can also affect fingerprints. The anti-cancer medicine capecitabine may cause fingerprints to be lost. Swelling of the fingers, such as from bee stings, can sometimes make fingerprints disappear for a short time. They come back when the swelling goes down. As people get older, their skin becomes less stretchy. This means many senior citizens have fingerprints that are hard to capture. Fingerprints can also be erased permanently. Criminals could possibly use this to avoid being convicted. Methods include plastic surgery. John Dillinger tried to erase his fingerprints with acid. However, 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 made for law enforcement, and later they became more common in everyday life. Making these systems reliable was hard, so scientists studied them a lot in pattern recognition and image processing. A fingerprint is unique because of its overall pattern and small breaks in the ridges called minutiae. In the 2000s, minutiae were seen as the most reliable part of a fingerprint, so they became the most common basis for automatic checking. 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 think 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. 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, and ultrasound scanners use sound waves. Thermal scanners sense temperature differences between the ridges and valleys.

Since 2000, electronic fingerprint readers have been added as consumer electronics security tools. Fingerprint scanners could be used for login authentication and for identifying computer users. However, some simpler sensors could be fooled by fake fingerprints made from gels. In 2006, fingerprint sensors became popular in the laptop market. 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. In April 2014, Samsung released the Galaxy S5, which had a sensor on its home button. In September 2015, Apple's iPhone 6S had 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. In 2018, Synaptics showed sensors built into the whole touchscreen of the Vivo X21 UD smartphone.

Algorithms

Matching algorithms compare a saved fingerprint template with a new fingerprint. This helps check who someone is for authentication. The algorithms can compare the original images, or they can compare certain features. Before matching, the image is cleaned up by removing extra noise, so the fingerprint is easier to see.

Pattern-based algorithms compare the basic fingerprint shapes, called arch, whorl, and loop. The two images must line up in the same direction. To do this, the algorithm finds a central point and centers the image on it. The template stores the type, size, and direction of these patterns. The new fingerprint is then 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. One study found that even with a very strong microscope, it can be hard to tell a koala's fingerprints apart from a human's.

In fiction

Mark Twain's memoir Life on the Mississippi (1883) is mostly about the time the author spent on the river. It also tells about parts of his later life. It includes tall tales and stories that were said to have been told to him. One story describes a murder solved by a thumbprint.

Twain's novel Pudd'nhead Wilson, published in 1893, has a courtroom story that depends on fingerprints. Fingerprints appear often in crime stories too.

Sir Arthur Conan Doyle wrote "The Norwood Builder" (1903). It is a short story about his famous detective, Sherlock Holmes, and it takes place in 1894. In the story, a bloody fingerprint helps Holmes find the real criminal and save his client.

The British writer R. Austin Freeman wrote his first Thorndyke novel, The Red Thumb-Mark, in 1907. In it, a bloody fingerprint is left on a piece of paper next to a package of diamonds in a safe. After the diamonds are stolen, Dr. Thorndyke investigates and defends the accused man, whose fingerprint matches the one on the paper.

In the television show Bonanza (1959–1973), the character Hop Sing uses what he knows about fingerprints to free Little Joe from a murder charge. In the 1997 movie Men in Black, Agent J has to remove his fingerprints by pressing his hands on a metal ball. The agency sees this as necessary to hide the identities of its agents. In the 2009 science fiction movie Cold Souls, a mule who smuggles souls wears fake fingerprints made of latex so she can get past airport security. She can change her identity by changing her wig and her latex fingerprints.