Mean ridge breadth and ridge density tell the same story for ancient fingerprints: A critique of the “Age-Sex Identification Matrix” method of demographic reconstruction Akiva Sanders a , Andrew Burchill b a Faculty of Humanities, Tel Aviv University, Gilman Building room 159, Ramat Aviv, 6994801 Tel Aviv, Israel b School of Life Sciences, Arizona State University, Tempe, Arizona, USA ABSTRACT In recent years, the field of archaeological dermatoglyphics has sought to use the ridges of ancient fingerprint impressions to infer demographic information about their creators. Although such a pursuit is inherently challenging, a recent method used in publications (the “Age-Sex Identification Matrix’’ approach) relies on flawed as- sumptions that produce unacceptably inaccurate results. Among other issues, basic mathematics, the literature cited as ostensibly supporting this method, and even the authors’ own published data contradict fundamental assumptions underlying their approach. The two types of fingerprint measurements used in the Age-Sex Identifi- cation Matrix approach as proxies for age and sex respectively—mean ridge breadth (distance per ridge) and ridge density (ridges per distance)—are reciprocal values that are not independent. Additionally, in opposition to the methodological thesis of this approach, the previous studies cited in support of this method demonstrate, on the contrary, that both age and sex vary with both mean ridge breadth and ridge density. The published data used in this method also highlight the non-independent nature of the two measures. We caution other researchers to disregard the results of these studies until the data are re-analyzed with a more accurate methodology. Finally, we offer a methodology from the field of Bayesian statistics as an alternative that is able to disentangle the effects of age and sex on fingerprint ridge measurements. 1. The growth of the hand over life Over the course of a person’s life, the number of ridges on the palm and fingers and their general arrangement do not change; they are set before birth. As a person grows, ridges are not added nor do new pat- terns appear. However, as the individual grows, these ridges grow wider and further apart from one another, expanding over the space of the individual’s growing hands. Therefore, fingerprints left during the pro- duction of archaeologically recovered objects allow us to make indirect measurements of the bodies of the individuals who formed these objects. If analyzed correctly, these measurements can be used to learn about the demographics of ceramic production and the life courses of those who participated in the industry. The possibility of systematically linking fingerprint ridge spacing to both age and sex has been noted over the last century (e.g. Hecht 1924; Ohler and Cummins 1942; David 1981). Research into applying these observations to archaeologically recovered clay objects began several decades ago with the work of Kamp (2001) and (Kamp et al., 1999) and Kralik and Novotny (2003) by comparing the spacing of fingerprint ridges on prints made by modern subjects and those of archaeological fingerprints. In her application of these comparisons to a dataset of archaeologically recovered Sinagua ceramics and figurines, Kamp was satisfied by finding a difference in ridge spacing values between the two datasets and concluding that the figurines were made by a younger group of individuals than the ceramics, without specifying either sex designations or precise age ranges (Kamp 2001: 442–444). This field was not intensively pursued until the publication of Sanders’ 2015 study of fingerprints on ceramics from Tell Leilan. This study, along with the one carried out by Kantner et al., 2019 on ancestral Pueblo ceramics a few years later, interpreted datasets of ridge spacing data as implying that fingerprints had been impressed by both adult men and women, based on the distributions’ bimodal shape, without making any claims about the precise ages of these adult potters. All the ancient fingerprint studies that had been undertaken until 2019 were thus carried out with non-replicable analytical methodologies that were specific to the dataset being studied and had limited interpretive potential. These were espe- cially limited in terms of analyzing sex and age dimensions simulta- neously and identifying precise age ranges. 2. The contributions of the “Age-Sex Identification Matrix’’ approach to the study of ancient fingerprints 2.1. A new methodology There are two widely-used methods for measuring the spacing of fingerprint ridges resulting from the growth of the hand over an in- dividual’s life span (Fig. 1). Mean ridge breadth (MRB) is calculated by measuring the perpendicular distance across any number of sequential E-mail address: sandersa@tauex.tau.ac.il (A. Sanders). Contents lists available at ScienceDirect Journal of Archaeological Science journal homepage: www.elsevier.com/locate/jas https://doi.org/10.1016/j.jas.2024.106036 Received 28 April 2024; Received in revised form 10 July 2024; Accepted 19 July 2024 Journal of Archaeological Science 169 (2024) 106036 Available online 2 August 2024 0305-4403/© 2024 Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.