How Often are Fingerprints Repeated in the Population? Expanding on Evidence from AI With the Birthday Paradox

Abstract: The assumption of fingerprint uniqueness is foundational in forensic science and central to criminal identification practices. However, empirical evidence supporting this assumption is limited, and recent findings from artificial intelligence challenge its validity. This paper uses a probabilistic approach to examine whether fingerprint patterns remain unique across large populations. We do this by drawing on Francis Galton's 1892 argument and applying the birthday paradox to estimate the probability of fingerprint repetition. Our findings indicate that there is a 50\% probability of coincidental fingerprint matches in populations of 14 million, rising to near certainty at 40 million, which contradicts the traditional view of fingerprints as unique identifiers. We introduce the concept of a Random Overlap Probability (ROP) to assess the likelihood of fingerprint repetition within specific population sizes. We recommend a shift toward probabilistic models for fingerprint comparisons that account for the likelihood of pattern repetition. This approach could strengthen the reliability and fairness of fingerprint comparisons in the criminal justice system.