Fluctuating asymmetry in dental and mandibular nonmetric traits as evidence for childcare sex bias in 19th/20th century Portugal

Fluctuating asymmetry, often considered a measure of developmental instability, was studied in the dental morphological traits of 600 individuals from among the poorest sectors of society in 19th-20th century Portugal. The aims are to identify and interpret any differences between: (1) males and females, and (2) patterns of distribution among teeth with different odontogenic timings, to assess if any sex bias existed in childcare. Dental and mandibular morphological traits were recorded using the Arizona State University Dental Anthropology System. z-Ratios were used to compare summed absolute fluctuating asymmetry frequencies between sexes and age groups. Results from rank correlation coefficients ruled out directional asymmetry and antisymmetry, based on positive (>0.2) bilateral association of traits in larger samples. Sex differences were significant (z-ratio=3.128; p=0.0018), while age differences were not (z-ratio=-0.644; p=0.5196). Teeth forming after infancy tended to be more asymmetric in females. Potential reasons for the sex difference include: (1) greater female susceptibility to developmental instability, (2) greater male childhood mortality that yields lower fluctuating asymmetry in surviving males, and/or (3) cultural bias favoring male access to resources. Results suggest the latter hypothesis is most likely, as fluctuating asymmetry is enhanced during childhood, perhaps coinciding with gender role definitions. There seems to be no association between asymmetry and early mortality in males. A lack of parallels in prior research renders differential sex reaction to environmental stress dubious. This population may have favored male children in their access to appropriate conditions for development.