The Evaluation and Refinement of Nonmetric Sex and Ancestry Assessment Methods in Modern Japanese and Thai Individuals

Effective biological profiles in forensic anthropology and bioarchaeology depend on the development, validation, and refinement of population-specific methods. However, most methods were developed in North America on individuals of African and European descent, and it is unlikely that such methods can generate accurate biological profiles for Asian individuals. Moreover, Native Americans have served as biological proxies for Asians due to their distantly shared genetic history, resulting in largely untested assumptions that Native Americans and Asians are homogenous and share nonmetric sexually dimorphic skeletal features and a unique suite of cranial traits that can be used in ancestry assessment.

This study explores nonmetric sexual dimorphism and cranial nonmetric variability in 1,397 modern Japanese and Thai individuals 17 to 96 years of age. The first objective tests and refines the methods based on 15 traits used to predict sex from the cranium, pelvis, clavicle, and humerus that were developed on non-Asian populations. The second objective establishes 37 cranial and mandibular trait frequencies to determine if the Japanese and Thai differ from each other and from Native Americans in trait expressions. Further, the affects of sex, age, population, inter-trait correlations, intraobserver error, and secular change on the traits are assessed.

The results indicate that population-specific sex assessment methods perform better in classifying the Japanese and Thai compared to those developed on non-Asian populations, producing correct classification rates of 66-98%. Additionally, the majority of cranial and mandibular traits used in ancestry assessment significantly differ in frequency between the Japanese and Thai, resulting in correct classification rates of 60-90%. Further, the Japanese and Thai are different from Native Americans in the expression of nonmetric traits. However, sex, age, population, intraobserver error, and secular change affect many nonmetric traits, thereby complicating their use in sex and ancestry assessment.

This study demonstrates that the Japanese, Thai, and Native Americans are not skeletally homogenous, as they exhibit differences in sexual dimorphism and in the expression of cranial trait frequencies due to unique population histories. Moreover, the findings of this research underscore the importance of developing population-specific biological profile methods for diverse Asian populations, such as those provided here.