Fingerprints as evidence

Definition: Impressions made when fingertips come into contact with surfaces.

Significance: Because each person’s fingerprints are unique, positive identification of unknown persons can be achieved through fingerprint analysis. Law-enforcement agencies often rely on fingerprinting to identify the persons involved in crimes, both victims and suspects.

The patterns of tiny ridges and valleys that appear on the fingertips and toes of human beings are unique to each individual. These fingerprints and toe prints are formed on a fetus around the fourth month of gestation and remain unchanged throughout the individual’s lifetime. They do not change with age—the print patterns of a one-hundred-year-old person are identical to those that person had in the womb and at birth. Even identical twins do not share identical fingerprints and toe prints, although they may possess ones that are notably similar. If fingerprints are temporarily altered through some sort of accidental trauma or through calculated procedures intended to obliterate them, the body repairs them, and within a short time new prints identical to the original prints are formed. It is the uniqueness of such prints that makes them indispensable tools in law-enforcement investigations.

Immediately below the surface of the skin of the fingertips, on which patterns of ridges and valleys appear, are sweat glands from which small concentrations of skin oils are expelled through the pores. This sweat is composed mostly of water, but about 1.5 percent of it contains a combination of salts and various chemicals that the body produces. When a person’s fingertips touch a surface, fingerprints formed by this sweat are left behind. The water in the sweat evaporates quickly, but a slight residue of salts and chemicals remains on whatever is touched. (It should be noted that, in contrast with the fingerprints of adults, those of young children often disappear almost instantly, for reasons that are not fully understood.)

Early Uses of Fingerprints

Fingerprints have been used as means of identification from the earliest recorded times. Egyptologists exploring artifacts dating back four thousand years, to the times of the pharaohs, have found that artisans working in clay often marked their work with their thumbprints before the clay was fired. On some occasions, an artisan would place an entire handprint on the soft surface of a pot about to be fired. Fingerprints and handprints were used as means of identification in China more than two thousand years ago, when a largely illiterate population accepted such marks as substitutes for written signatures on legal documents.

It was recognized in early cultures that no two people bore the same fingerprints, even though the classification of types of fingerprints probably did not occur until the late seventeenth or early eighteenth century, when an English plant morphologist, Nehemiah Grew, recognized the ridge patterns in fingerprints and announced his findings accompanied by remarkably detailed drawings of a variety of such patterns. In 1823, Jan Evangelista Purkynê, a Bohemian physiologist, wrote a lengthy paper in which he described in depth a number of friction ridge patterns found in a variety of fingerprints. A nineteenth century British book illustrator, Thomas Bewick, used finger- or thumbprints in works that he published as reliable means of identification.

William James Herschel, a British colonial working in India in the mid-nineteenth century, made extensive use of fingerprints to identify the people with whom he was working and to prevent them from receiving multiple benefits by simply appearing and claiming them. Once they signed in by providing their fingerprints, it was easily determined who they were and whether they had received what was due them. Henry Faulds, a Scottish physician working in India, became fascinated by fingerprints and corresponded with Charles Darwin, the evolutionist, about their uniqueness, suggesting that they might be used effectively in the investigation of crimes. A similar suggestion was made by Thomas Taylor of the U.S. Department of Agriculture, who published an article in which he contended that fingerprints and palm prints might be useful in identifying criminals.

After English scientist Francis Galton grew interested in the Faulds-Darwin correspondence about fingerprints, Herschel shared with Galton some of his material on fingerprints. In 1892, Galton published Finger Prints, which became a classic in its field. In this book, Galton presented exhaustive evidence that every fingerprint of every individual is unique and asserted that the ridge patterns on a person’s fingers do not change throughout a lifetime.

Edward R. Henry, a British official who served as the inspector general of police for the Indian province of Bengal in the late nineteenth century, read Galton’s book and soon began a correspondence with Galton, who shared with him much of the material he had obtained through his contacts with Herschel, Faulds, and Darwin. Henry devised a system of classifying and analyzing fingerprints that was widely used in colonial India and in Great Britain. Henry’s book Classification and Uses of Finger Prints, published by the Indian government in 1897, became, like Galton’s earlier book, a landmark in its field.

The earliest official use of fingerprints in the United States occurred in 1903, when New York City’s civil service fingerprinted those taking civil service tests to ensure that those appearing to take the tests were the people they claimed to be. At about the same time, various law-enforcement agencies began using fingerprints to identify criminal suspects.

Three Major Types of Fingerprints

Law-enforcement investigators are generally concerned with three types of fingerprints found at crime scenes: latent, visible, and plastic prints. A latent print is one that is hardly visible. It may be incomplete—that is, only part of the finger has left behind an impression that can be lifted for purposes of identification. Although identifications are made most easily when full sets of fingerprints are available, many crime scenes do not provide investigators with that sort of evidence.

Visible prints are those that are easily seen without magnification because they are left on surfaces that are dusty or in such media as blood, damp paint, or grease with which the fingers have come into direct contact. Plastic prints are those left on soft surfaces such as unfired clay, putty, or cookie dough. Like visible prints, plastic prints are easily visible without magnification.

Latent fingerprints are the most difficult for crime scene technicians to collect for analysis. Just a little more than 98 percent of such prints is composed of water; the small amount that is not water, about 1.5 percent, consists of sugars, proteins, and fatty acids, the total mass of which is negligible, making the prints difficult to detect. With advances in forensic science, sophisticated lasers and chemical treatments have been developed to detect such prints, some of which may have undergone considerable degradation shortly after they were deposited on the surfaces where they are found. It is of the utmost importance that such prints be collected as quickly as possible, before they have degraded to the point that their validity is compromised.

In the United States, most law-enforcement crime scene units are now equipped with portable alternate light source (ALS) kits that are used to detect, through fluorescence, latent prints left at crime scenes. Many such prints are now recovered that in the past would have gone unnoticed.

Collecting Fingerprints

Fingerprints are most reliable when they have been gathered soon enough after their discovery that degradation has not advanced significantly. When latent prints are barely detectable on an object, such as a gun, the prints can sometimes be made much more visible in the crime lab through exposure to the vapors of chemical compounds generally called cyanoacrylates (the main ingredients in superglue). The object is placed in a sealed metal box and subjected to the fumes. This method is also used on objects that have no visible prints but may possibly have been touched by someone at the crime scene.

Crime scene technicians also collect latent prints by brushing surfaces with powders containing chemicals that stick to the surface and present sharp contrasts between the ridges and valleys of the prints. Such prints are then lifted with a specially designed tape and placed on cards for transportation to the lab for analysis.

In cases where latent prints are wet, they are often treated with a small particle reagent (SPR) that clings to the lipid residues of salts and chemicals found in fingerprints. Iodine, slightly heated, produces vapors that can make latent prints visible, but this technique is not used very widely because the resulting prints disappear quickly. If they are not photographed at once, they become unusable.

Prints visible to the naked eye are generally photographed and then entered into computerized databases, where they are compared with existing prints that are on file and are then stored for future reference. Plastic prints, which are similarly easy to see without magnification or special chemical treatment, are also photographed for analysis.

Major Classifications and Distribution of Fingerprints

Most of the fingerprints on record fall into the three different categories that Henry identified toward the end of the nineteenth century: the arch, the loop, and the whorl. To these three general classifications, which Henry divided into subgroups such as the plain or tented arch, the plain loop, the central pocket loop, and the double loop, Henry added a fourth classification, the composite, which has the combined features of all of the other types. The most common of the patterns is the loop. The prints of between 60 and 65 percent of people fall into this category, with another 30 to 35 percent having the whorl pattern. The least common pattern, the arch, is found in approximately 5 percent of the world’s population.

For purposes of classification, Henry devised a system that is still in general use by law-enforcement agencies. In a set of ten prints from one person, each print is designated as being from the right or left hand, using the letters R and L, and is further designated using the letter p to identify the small or pinky finger (Rp or Lp), r to identify the ring finger, m to identify the middle finger, i to identify the index finger, and t to identify the thumb.

Examining Minutiae

The uniqueness of fingerprints is attributable to varying ridge characteristics. Fingerprint analysts note the number and types of these characteristics as well as their positions relative to other ridge characteristics. Through careful study of these characteristics, referred to as minutiae, analysts determine what makes individual fingerprints unique and differentiate among prints that closely resemble one another.

Particular types of minutiae that analysts observe include bifurcations, or ridges that divide at some point; delta or triangulated ridges, which resemble the Greek letter for which they are named; lakes, or groups of ridges that divide and then rejoin to form what resemble lakes in their centers; spurs, or ridges that jut out from the main ridge line; and crossovers, or ridges that connect to other ridge lines.

Fingerprint Identification

Law-enforcement investigators often identify individuals based on archived sets of fingerprints that have been obtained previously by law-enforcement or other authorities. In making such a set of fingerprints—ten prints, five from each hand—ink is placed on the person’s fingers and, with a rolling motion, an impression of each finger is made on a fingerprint card. Various law-enforcement agencies and other organizations maintain massive databases of such archived fingerprints for purposes of identification. In criminal cases, fingerprints from crime scenes, suspects, and victims are compared against the prints in these databases. Anyone who is arrested in the United States is fingerprinted, and sets of all arrestees’ prints are added to the Integrated Automated Fingerprint Identification System (IAFIS), the national fingerprint database maintained by the Federal Bureau of Investigation (FBI). Sometimes just part of a single print is available, but even such a minimal impression can be compared with the millions of archived sets in the FBI database. The FBI has estimated that approximately three thousand fugitives are identified each month through IAFIS.

In addition to the routine use of fingerprint identification by law-enforcement agencies in criminal cases, fingerprint comparisons are increasingly being employed to screen persons seeking entry to areas where security is essential. Fingerprints are also used to identify victims of highway and airplane accidents, in which sometimes only fragmentary remains are available for examination.

Training

With advances in technology and increasing emphasis on national security, given the rise in terrorism worldwide, the need for fingerprint analysts in the United States has soared in both the private and public sectors. In addition to the need for trained analysts, police officers in jurisdictions ranging from small towns and villages to huge metropolitan areas need special training in the detection, collection, and preservation of fingerprints.

The FBI is a major provider of training in all areas related to fingerprints. Local police officers who are trained at the FBI Laboratory or similar venues generally become instructors in fingerprinting techniques when they return to their local police departments. They are trained specifically not only in how to preserve crime scenes so that viable fingerprint evidence can be collected but also in how best to obtain fingerprints, particularly the latent prints that are so important in building legal cases against criminal suspects. Police officers must be able to determine the most effective means of gathering fingerprints that will allow prosecutors to introduce the prints as evidence in courts of law.

Bibliography

Baden, Michael, and Marion Roach. Dead Reckoning: The New Science of Catching Killers. New York: Simon & Schuster, 2001.

Bell, Suzanne. Encyclopedia of Forensic Science, rev. ed., Facts On File, 2008.

Cole, Simon A. Suspect Identities:A History of Fingerprinting and Criminal Identification. Cambridge, Mass.: Harvard University Press, 2001.

Conklin, Barbara Gardner, Robert Gardner, and Dennis Shortelle. Encyclopedia of Forensic Science: A Compendium of Detective Fact and Fiction. Westport, Conn.: Oryx Press, 2002.

Lee, Henry C., and R. E. Gaensslen, eds. Advances in Fingerprint Technology. 2d ed. Boca Raton, Fla.: CRC Press, 2001.

Pentland, Peter, and Pennie Stoyles. Forensic Science. Philadelphia: Chelsea House Publishers, 2003.

Saferstein, Richard. Criminalistics: An Introduction to Forensic Science, 12th ed., Pearson Education, 2018.