Image: Harris Hawthorne Wilder and Bert Wentworth. Personal Identification: Methods for the Identification of Individuals, Living Or Dead. Boston: Richard G. Badger, The Gorham Press, 1918, 188. Public domain.
“No Two Finger Prints Are Alike”
Since its invention in the 19th century, modern fingerprint identification has relied upon the assumption that by examining a person’s fingerprints one can identify that individual with certainty and to the exclusion of all others. This assumption has, in turn, rested on another assumption: that no two people have fingerprints that are exactly identical in terms of the form and configuration of their patterning. For example, we find a statement of this idea in the instructional materials of the University of Applied Science, an institution that provided training in fingerprint identification in the United States during the early decades of the 20th century:
There is one point of which we are absolutely convinced, and that is: no two finger prints are alike. We might take hundreds of thousands of prints from as many persons and there would be no two that we could call absolutely alike in every detail. There might be two or even many more of them that would have the same general design, but even then we would find on careful examination that they were vastly different, or so much so that we could easily point out the difference when we compare the two prints [1].
It was on the basis of this notion of the individual uniqueness of fingerprints that police, forensic experts, and prison officials have been so confident that they could identify individuals using this technique. It is important to be clear about what “uniqueness” means in this context, however. When people make the claim that fingerprints are unique to the individual, they do not mean that there are no two people with the same number or configuration of arches, loops, and whorls on their fingers, because, in fact, there are. These pattern-types are illustrated below.
Rather, saying that no two people have identical fingerprints really means that no two people share identical configurations of fingerprint minutiae (referred to as “Galton details” in the accompanying image). A recent treatment of the subject defines minutiae as “[events] along a ridge path, including bifurcations (points at which one friction ridge divides into two friction ridges), dots (isolated friction ridge units that have lengths similar [to] their widths), and ridge endings (the abrupt end of ridges)” [2].
Image: Harris Hawthorne Wilder and Bert Wentworth. Personal Identification: Methods for the Identification of Individuals, Living Or Dead. Boston: Richard G. Badger, The Gorham Press, 1918, 125. Public domain.
Fingerprint minutiae are the minute characteristics of friction ridge skin that make the forensic use of fingerprint identification possible: even two people who have the same number of arches, loops, and whorls on their fingers will have different configurations of minutiae. As illustrated in the image below, fingerprint examiners can compare the types and locations of minutiae appearing in two fingerprints (or a crime scene fingermark and a fingerprint taken from a suspect) when attempting to determine if they came from the same person. Historically, this has been a commonly used approach in the field of latent fingerprint evidence.
Image: Francis Galton. Finger Prints. London: Macmillan and Co., 1892, Plate 15, Figure 22, following p. 96. Public domain.
The idea that no two people have identical fingerprints has been a staple of the 20th-century literature on fingerprint identification. In 1920s China, for example, a fingerprint identification trainee named Chen Ruming explained the individual character of fingerprints in the following way:
Since all countries of East and West have been using fingerprinting, it has been put to the test repeatedly and two people with identical patterning have never been discovered. Thus we can know without doubt that people’s fingerprints are all different [3].
In this passage, Chen asserted that fingerprints can be viewed as unique due to the apparent fact that two identical fingerprints have never been discovered. This kind of argument has been common in the history of fingerprinting, as have others that are meant to “prove” (or at least assert) such claims about the individuality of fingerprints [4]. For example, some have used the observation that even identical (monozygotic) twins do not have identical fingerprints to make the same point. Whether or not such claims hold up is in some ways beside the point. Simon A. Cole has used the notion of the “fingerprint examiner’s fallacy” to describe the mistaken idea that the individual uniqueness of fingerprints could ever be taken as a guarantee of the accuracy of fingerprint identification. As Cole explains,
When asked by courts for proof of the “reliability” of forensic fingerprint evidence, fingerprint examiners answered that all fingerprint patterns are unique. Courts failed to grasp the gap in logic between the two statements and uniqueness became enshrined as the foundation of the accuracy of forensic fingerprint identification… We continue to labor under this fallacy today [5].
Cole’s point is that the uniqueness of fingerprints and the accuracy of fingerprint identification are two completely different questions. The ability to match a fingermark discovered at a crime scene with a fingerprint taken from a suspect is a complex process relying upon multiple levels of observation, analysis, and interpretation. Whether or not fingerprints are unique to the individual (in a general sense) tells us nothing about how the identification process is carried out and whether or not it is done accurately and reliably. In fact, these questions about the scientific validity, accuracy, and reliability of fingerprint identification are precisely the ones that are being most closely examined by researchers today.
Sources
[1] University of Applied Science. A Study in Finger Prints: Their Uses and Classification. Third Edition. Chicago: University of Applied Science, 1920, 59.
[2] Expert Working Group on Human Factors in Latent Print Analysis. Latent Print Examination and Human Factors: Improving the Practice through a Systems Approach. U.S. Department of Commerce, National Institute of Standards and Technology, 2012, 2.
[3] Chen Ruming. “Zhiwen shengli zhi yanjiu” [Research on the physiology of fingerprints]. Zhiwen zazhi, no. 3 (1926): 59-64, quote from p. 62.
[4] Simon A. Cole. “Is Fingerprint Identification Valid? Rhetorics of Reliability in Fingerprint Proponents’ Discourse.” Law & Policy 28, no. 1 (2006): 109–35.