Fingerprints and other friction ridges of the skin have a persistent structure that often leaves characteristic evidence at crime scenes. Latent print examiners compare this evidence with inked copies of friction ridge skin from a known suspect to determine whether these two patterns originate from the same source. This examination process uses computer databases for initial screening, but all evidence presented in court is based on human comparisons. Experts must establish their credentials in order to testify, and recent vision science work has suggested that experts possess visual mechanisms that novices do not. However, these experts have also shown evidence of biases, and critics have begun to question the evidentiary value of fingerprints.
Sources of Fingerprint Evidence
Impressions left by volar skin, or the ridged skin of the palmar surfaces of hands and fingers and plantar surfaces of feet and toes, play a major role in forensic science. Commonly known as fingerprints, palm prints, or footprints, recordings of this skin are often used as a form of physical evidence to link a person to a particular item or location, such as a crime scene.
Perspiration, oil, blood, or other substances are often present on the skin and are deposited on surfaces such as plastic, wood, metal, or glass during touches, which might leave a recording of details of the ridge, crease, scar, and imperfection patterns from the skin. The evidence is recovered using a variety of development techniques to make the latent image, or undeveloped print, visible. These include powders, cyanoacrylate glue, chemicals, or stains that adhere to or react with the residues of the print. A variety of different lights and filters can also be used to visualize a latent print.
The basic challenge of fingerprint (more formally known as friction ridge) evidence derives from the fact that any latent print recovered from a crime scene will vary in appearance from every other latent print and from every standard print. A standard print can be obtained using a variety of techniques, from black-inked prints on a white card to electronic imaging of the volar surfaces. The intent of the standard recording is to obtain a clear set of prints from a known-source individual for comparison with the unknown-source latent prints. In some cases, these variations in appearances are trivial, and the match between an unknown latent and known inked print appears obvious. However, latent or standard prints with low quality or quantity of details are a challenge to examine, and thus the field of forensic latent print examinations uses established procedures, practices, guidelines, and methodology to support the examination of latent prints.
Friction ridge skin develops its structure in utero by means of biological, chemical, and physical processes of pattern formation known as reaction-diffusion. This process ensures that ridges form in a roughly parallel configuration and tend to orient orthogonally to lines of stress that occur during the fetal development of structures known as volar pads in fingers, palms, soles, and toes. The resulting ridges form patterns of loops, whorls, and arches in the pads of the developing finger tip (distal phalanges). Additional structure is provided by the development of ridges that form bifurcations and ridge endings, or minutiae. These minutiae are often coded when fingerprints are entered into computer databases such as the Integrated Automated Fingerprint Identification System, maintained by the U.S. Federal Bureau of Investigation.
The interactions of chemistry, physics, and biology in pattern formation in nature support the belief within science that no two natural patterns will ever be exactly alike. All the internal and external developmental noise, interactions, and timings that occur will cause the anomalies that become a part of the configurations within natural pattern structures. All natural patterns in volar skin will be unique. This includes the ridges, furrows, creases, and pores and their anomalies and textures that make up the skin. Scarring will provide new unique features to the skin. The homeostatic regeneration of skin maintains the form and function of the features of the volar surface in persistent configural and sequential arrangements.
Basis of Testimony
The uniqueness and persistence of friction ridge skin is the rule of support for the proposition that an individual can be determined as having touched a particular surface. With this rule or law, the next step is to examine the latent and standard prints and determine whether the latent print was made by the person who made the standard print. The two prints are comparatively measured with each other. The first-level detail of general direction of ridge flow is examined, followed by examination of the second-level detail of lengths of individual ridge paths with their endings and bifurcations and, if needed, the examination of the third-level detail of edges, textures, and pore positions of the ridges. The details are examined to determine whether they correspond in sequences, shapes, and configurations in both prints. The examination results either in a determination that the person made the latent print (individualization) or that the person did not make the latent print (exclusion), or no determination is made whether the person made the latent print (inconclusive). This individualization or exclusion determination has, in principle, a philosophical problem: Comparisons between the latent print and all prints in the world are impractical. However, in practice this has been overcome with a high degree of certainty (although there are criticisms of this conclusion, which we will discuss in a later section). The expert makes the determination that there is definite agreement between the configural and sequential arrangement of details in the two images, indicating that they were made by the same unique and persistent source. The individualization decision basically comes down to the expert judgment that the recovered latent print is so similar to the inked print that it could only have come from the same person. Stated in a different way, the claim is made that there is no more similar print from any other source among all the prints in the world, which is of course impractical to test. In practice, the individualization often comes down to the expert rendering the opinion that the degree of match between the latent and the inked print is typical of known training, competency, and proficiency individualizations and casework peer-reviewed individualizations and is closer than any close correspondence from another source that the expert has ever seen or expects to see. Because this judgment is based on prior experience, presenting a conclusion in court depends on the expert establishing his or her credentials, which has become a major portion of latent print testimony and is discussed next.
The latent print expert examiner should have some basic knowledge before conducting case work, rendering conclusions, and testifying in court. This includes understanding the source of images, volar skin, and its unique and persistent features. The examiner also must understand the basics of fetal development, homeostasis, growth, aging, wound healing, scarring and imperfections of the volar skin, and the uniqueness of pattern formations in nature. Moreover, the examiner must understand distortions of the skin and variations in appearances of latent or standard prints or images. Latent and standard print development, capture, and imaging techniques must be understood to understand the variations in appearances. The examination method within the latent print community of analysis, comparison, and evaluation (ACE), possibly followed by verification (V), is the method used in conjunction with the sufficiency and judgment threshold of quality and quantity (QQ) of details in the images. Furthermore, the examiner must understand examination method and sufficiency and judgment thresholds. In addition, the examiner must understand the history of latent print examinations and latent print communities, the role of a community within science, and the role of the expert within a scientific community. Finally, the latent print examiner must be trained to be competent and demonstrate accuracy and proficiency within the community.
The goal of an examination is to judge whether developed unknown latent prints and known standard prints are sufficient for examination purposes and whether the considered source of the latent print can be determined or excluded. As noted earlier, three conclusions of the comparative examination can be reached: (1) the unknown print was determined to have been made by a specific source or person (individualization); (2) the unknown print was determined not to have been made by a specific source or person (exclusion); (3) no determination was made whether a specific source or person made the unknown print (inconclusive).
When testifying in court, the examiner must be able to present the reasons that qualify him or her to testify as an expert for rendering opinions of judgments of examinations. This qualification requires the judge’s determination of sufficiency of expertise based on the training and experience of the witness. The witness must be prepared to answer questions on qualifications and on anything to do with the science and method of latent print examination.
Criticism of Fingerprint Evidence
Recently, fingerprint evidence has come under intense criticism, and below we discuss the different forms of attack on latent print evidence. Since the United States v. Byron Mitchell case of 2003, in which defense attorneys began challenging the admissibility of forensic latent print examinations, fingerprint evidence has come under attack as an admissible science in the courts. A major issue surrounding fingerprint evidence is in the information content that can be extracted and identified in a latent print. While rolled inked prints taken under controlled conditions are usually very clear and rich in detail and information, latent prints are often inherently less clear, are distorted, and contain considerably fewer details due to the commonly partial nature of the print itself. It is up to the examiner to use his or her expertise to determine whether the latent print contains sufficient information to determine usability. Then, the examiner determines whether the details in the inked print and the latent print agree and have a common source. An individualization is made when the examiner claims that the two prints contain a high enough level of similarity that surpasses the similarity between any two prints from different individuals. However, determining the level of similarity between two prints is left to the examiner to establish on the basis of his or her training, skill, and experience within the forensic comparative science community. This makes fingerprint evidence somewhat different from DNA analysis, which codes a limited range of chemical sequences to establish an identification. Unlike DNA analysis, which has a specific set of known features, fingerprints can be matched on the basis of many different types of features, including minutiae, ridge flow, and even shapes of pores. Because the useful features are more difficult to quantify, it is more difficult to establish a specific statistical model that would provide the probability of an erroneous identification. Thus, the procedures include a subjective element, albeit one that can be verified by third parties. The techniques of comparison and evaluation represent an objective application of documented procedures.
Despite this lack of statistical models, some examiners have made claims as to the “infallibility” of fingerprints, that identifications are “100% positive,” and that the error rate of forensic fingerprint identification is zero. In fact, to date there have been approximately 20 known cases of misidentifications recorded, with some involving qualified examiners. Many of these misidentifications have been so widely publicized that the claim of “zero error rate” and infallibility has come under serious scrutiny. In addition, a series of tests conducted from 1995 to 2001 by a private independent testing service recorded misidentification rates ranging from 3% to 22%. As a result, authors have pushed for blind proficiency testing to reduce the amount of erroneous identifications. Many of these misidentifications are corrected with the use of additional fingerprint evidence, leading to the proposal that the criticism should fall on individual examiners rather than the science of latent print examinations as a whole.
Psychological Research Using Latent Print Examiners
An issue raised recently by Itiel Dror is the possibility that external sources of information about a case can affect the decision made by examiners. Known as confirmation bias or contextual biases, these sources of biases originate from knowledge such as whether other examiners called a particular individualization or whether other sources of evidence link the suspect to the crime. Once this information becomes known, it can be very difficult for an examiner to ignore this evidence. One fortunate aspect of fingerprints is that, unlike eyewitness testimony, they represent a form of physical evidence, and if confirmation is required or any bias is suspected, testimony from a new examiner can be sought.
Despite the attacks on fingerprint examiners and the push for fingerprint evidence to be omitted from the courts as scientific testimony, several authors have argued for demonstrable differences between expert examiners and novices. Recent behavioral and electro-physiological (electroencephalogram, EEG) research by Tom Busey and colleagues has shown that experts appear to perceive fingerprints using the configural process or a holistic process, in which the observer appreciates not only the presence of individual features but the spatial relations between them as well. This process is known to occur when humans process visual information for faces, which produces a characteristic pattern in the EEG trace. Fingerprint examiners demonstrate similar brain-wave activity regarding fingerprints as the general population shows with respect to faces, and this fact suggests that experts recruit similar brain processes to support expertise in the fingerprint domain. This suggests that trained latent print examiners have perceptual abilities not shared by the rest of the population.
- Ashbaugh, D. R. (1999). Quantitative-qualitative friction ridge analysis: An introduction to basic and advanced ridgeology. Boca Raton, FL: CRC Press.
- Busey, T. A., & Vanderkolk, J. R. (2005). Behavioral and electrophysiological evidence for configural processing in fingerprint experts. Vision Research, 45, 431—148.
- Cole, S. A. (2005). More than zero: Accounting for error in latent fingerprint identification. Journal of Criminal Law and Criminology, 95(3), 985-1078.
- Cowger, J. F. (1983). Friction ridge skin, comparison and identification of fingerprints. New York: Elsevier Science.
- Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579, 113 S. Ct. 2786 (1993).
- Dror, I. E., Peron, A., Hind, S., & Charlton, D. (2005). When emotions get the better of us: The effect of contextual top-down processing on matching fingerprints. Applied Cognitive Psychology, 19, 799-809.
- United States v. Byron Mitchell, 02-2859 (3d Cir. September 9, 2003).