Prevalence of Rocker Jaw in Prehistoric Thai Populations: A Review of Knowledge Status and the Issue of Inconsistent Scoring Criteria

Pollawat Sawangsri1*, Peerasak Klubket2, Patara Rattanachet1, Nirinrat Poolsiri3, and Robert W. Mann4

Rocker jaw is a mandibular morphological trait defined by a curved inferior border that prevents the bone from resting steadily on a flat surface, causing it to “rock". While frequently reported in Polynesian populations, this trait has received limited attention in Thailand, where it is typically documented as a secondary non-metric cranial feature. This study investigated the prevalence of rocker jaw and its biological implications among prehistoric populations in Thailand, identifying an overall frequency of 48.48% from 99 samples. Notably, the highest prevalence was observed at the Non Nok Tha archaeological site, accounting for 22.22% of the analyzed 99 samples. Statistical analyses were employed to examine associations between rocker jaw and variables including shovel-shaped incisors, sex, and geographical distribution. Results indicate a statistically significant spatial difference in trait distribution among the sites, suggesting that these communities predominantly practiced endogamy with limited gene flow from external groups. The distinct frequency observed at Non Nok Tha supports the hypothesis that rocker jaw is a genetically heritable trait. However, contrary to previous literature, this study found no significant correlation between rocker jaw and shovel-shaped incisors. A critical challenge identified through this review is the absence of standardized methodologies and inconsistent recording practices in existing studies. Such variability hinders comparative analysis between archaeological sites and introduces significant observer bias due to subjective criteria. Consequently, this study emphasizes the need to establish standardized scoring systems and precise morphological definitions for rocker jaw to ensure the reliability, comparability, and interpretative accuracy of future bioarchaeological research.