Twin fingerprints are much beloved by scientists, who see them as a classic arena for the old nature-versus-nurture debate: What made you what you are today, your genes or your environment? Twin fingerprints clearly show that it’s a little of both. If you compare palm prints and fingerprints of the Dionne quintuplets (born in 1934, they were the first quints of which all five survived), you find that the broad-brush pattern of lines, whorls, loops, etc., as well as what researchers call “ridge count,” were quite similar for the whole crew. Nonetheless each kid had unique prints due to differences in detail. “There is as yet no evidence that the arrangement of the minutiae (ending ridges, bifurcating ridges, etc.) is in any way genetically influenced,” writes fingerprint expert James Cowger. Presumably these minor but crucial differences arise from random local events during fetal development…
But as I understand, the key points in fingerprint ID are the points where lines, meet, or dead end, and their relation to each other. What form the pattern takes is relevant in establishing “yes, the are both whorls” or such, but the main thing is where those particular line point characteristics are.
So therefore, a set of 9 prints is not likely to have any details on whether the missing finger would match a crime scene, except maybe to predict the pattern - which is what, 1 in 3?
So if the fingerprints of identical twins are not identical, but are often similar…
Q: Are they sufficiently similar that a forensic scientist looking at the fingerprints of innocent twin A (in custody as a suspect) and finding them different-from-but-similar-to the crime scene evidence, might see sufficient similarity so as to posit the existence of evil twin B, who needs to be apprehended for questioning?
(That is, without prior knowledge that there even exists a twin B)
1.) Here’s an article addressing precisely this topic:
Tao, Xunqiang, Xinjian Chen, Xin Yang, and Jie Tian. “Fingerprint recognition with identical twin fingerprints.” PloS one 7, no. 4 (2012): e35704.
Abstract
Fingerprint recognition with identical twins is a challenging task due to the closest genetics-based relationship existing in the identical twins. Several pioneers have analyzed the similarity between twins’ fingerprints. In this work we continue to investigate the topic of the similarity of identical twin fingerprints. Our study was tested based on a large identical twin fingerprint database that contains 83 twin pairs, 4 fingers per individual and six impressions per finger: 3984 (83*2*4*6) images. Compared to the previous work, our contributions are summarized as follows: (1) Two state-of-the-art fingerprint identification methods: P071 and VeriFinger 6.1 were used, rather than one fingerprint identification method in previous studies. (2) Six impressions per finger were captured, rather than just one impression, which makes the genuine distribution of matching scores more realistic. (3) A larger sample (83 pairs) was collected. (4) A novel statistical analysis, which aims at showing the probability distribution of the fingerprint types for the corresponding fingers of identical twins which have same fingerprint type, has been conducted. (5) A novel analysis, which aims at showing which finger from identical twins has higher probability of having same fingerprint type, has been conducted. Our results showed that: (a) A state-of-the-art automatic fingerprint verification system can distinguish identical twins without drastic degradation in performance. (b) The chance that the fingerprints have the same type from identical twins is 0.7440, comparing to 0.3215 from non-identical twins. (c) For the corresponding fingers of identical twins which have same fingerprint type, the probability distribution of five major fingerprint types is similar to the probability distribution for all the fingers’ fingerprint type. (d) For each of four fingers of identical twins, the probability of having same fingerprint type is similar.
2.) One famous case showing the superiority of fingerprint identification vs. the earlier Bertillon system of identification based on appearance is the case of Will and William West in 1903:
The West Case (1903)
The Incident: When Will West arrived at the U.S. Penitentiary at Leavenworth, Kansas, in 1903, he was told by staff that he was already in custody.
The Mix-Up: A man named William West was already imprisoned there, and the two men had nearly identical faces, builds, and names, but were not related.
System Failure: The Bertillon system (based on body measurements) failed to distinguish between them, demonstrating its fallibility.
Fingerprint Triumph: Fingerprints, which were then being introduced, proved they were different people.
The kicker – according to forensic researcher Joe Nickell it’s highly likely that Will and William West weren’t two strangers with similar names who just happened to look alike, but were probably cousins. That they were distinguished by different fingerprints shows that ther was enough of a difference for the legal system