Evaluating dentofacial asymmetry in a modern South African sample

Abstract

Forensic and biological anthropologists rely on skeletal analysis to estimate biological parameters and assist in the identification of unknown individuals. A deeper understanding of factors that influence skeletal variation, such as fluctuating asymmetry (FA) and directional asymmetry (DA), is therefore essential. More specifically, FA represents random deviations from perfect bilateral symmetry and serves as an indicator of developmental instability and environmental stress, whereas DA reflects consistent side dominance related to functional or behavioural lateralisation and can mask the effects of FA. Although FA has been documented in dentition and mandibles across global populations, and DA has been reported primarily in the mandible, little is known about these asymmetries in modern South Africans. This study assessed FA and DA in both the mandibular dentition and mandibles of male and female black and white South Africans. The dental sample consisted of 98 individuals, with mesiodistal and buccolingual dimensions collected from left and right mandibular lateral incisors, canines, and first premolars. The mandibular sample comprised 101 individuals, from whom inter-landmark distances (ILDs) and shape variables were obtained from micro-XCT scans. Mandibular analyses focused on both the mandible as well as individual mandibular regions, including the condyles, gonial regions, and mandibular rami. Intra- and interobserver testing showed high repeatability for dental measurements and for nearly all cranial landmarks, except for two condylar landmarks. Two-way ANOVA models demonstrated that nearly all dental variables and all ILDs contributed significantly to overall asymmetry. As DA did not significantly influence FA for most dental measurements, FA indices were computed accordingly. Dentition showed significant FA only, whereas the mandible displayed both FA and DA. Sex and population comparisons revealed meaningful differences. Females tended to exhibit greater FA in dental dimensions, particularly mesiodistal measurements, and white South Africans showed higher dental FA levels than black South Africans. Among ILDs, condylar breadth showed the highest FA overall, with significant sex differences in mandibular length. White South Africans presented the greatest FA in condylar breadth, and differences between black South African males and females were also significant for the mandibular ILDs. Shape analyses indicated significant FA across all mandibular regions. DA was present in overall mandibular shape and gonial shape, particularly in males and white South Africans. Shape FA was generally higher in males and black South Africans. Age trends revealed that individuals aged 41–60 exhibited the highest FA and significant DA, whereas other age groups showed only FA. Overall, the findings emphasise the importance of a multi-modal approach to asymmetry analysis, highlighting how environmental, socioeconomic, developmental, and functional factors shape skeletal variation. The study provides valuable baseline data for interpreting FA and DA in modern South Africans and offers important considerations for forensic anthropological practice.