What's luck got to do with it? A generative model for examining the role of stochasticity in age-at-death, with implications for bioarchaeology.

INTRODUCTION: The role of "luck" in determining individual exposure to health insults is a critical component of the processes that shape age-at-death distributions in mortality samples but is difficult to address using traditional bioarcheological analysis of skeletal materials. The present study introduces a computer simulation approach to modeling stochasticity's contribution to the mortality schedule of a simulated cohort. METHODS: The present study employs an agent-based model of 15,100 individuals across a 120 year period to examine the predictive value of birth frailty on age-at-death when varying the likelihood of exposure to health insults. RESULTS: Birth frailty, when accounting for varying exposure likelihood scenarios, was found to account for 18.7% of the observed variation in individual age-at-death. Analysis stratified by exposure likelihood demonstrated that birth frailty alone explains 10.2%-12.1% of the variation observed across exposure likelihood scenarios, with the stochasticity associated with exposure to health insults (i.e., severity of health insult) and mortality likelihood driving the majority of variation observed. CONCLUSIONS: Stochasticity of stressor exposure and intrinsic stressor severity are underappreciated but powerful drivers of mortality in this simulation. This study demonstrates the potential value of simulation modeling for bioarchaeological research.

Assessing the association of skeletal indicators of stress with mean age-at-death in sub-adults.

OBJECTIVES: The present study investigated the association of skeletal indicator of stress presence with mean age-at-death as a means of understanding whether commonly studied indicators are indeed indicative of increased frailty. MATERIALS AND METHODS: Using a medieval Gaelic population from Ballyhanna (Co. Donegal), the present study assessed the association between skeletal indicators of stress and mean age-at-death using the Kaplan-Meier survival function with log rank test to determine whether these indicators were associated with younger age-at-death, and therefore increased frailty, in sub-adults only (0 to 18 years, N = 139) and through comparison to an all-ages cohort (N = 318). RESULTS: Only linear enamel hypoplasia was found to be associated with significantly decreased survivorship across the all-ages cohort but, conversely, was associated with increased survivorship when analysis was restricted to sub-adults. All other indicators assessed were associated with increased age-at-death for both all-age cohorts and sub-adult cohorts (cribra orbitalia), increased age-at-death when assessing all ages only (porotic hyperostosis and healed periosteal lesions); or were sufficiently rare in adults to prevent comparative analysis (stunting and micronutrient deficiency). Increased survivorship in individuals with higher numbers of co-morbid skeletal indicators was observed for both sub-adults alone and all age cohort. DISCUSSION: These findings suggest that these commonly recorded skeletal indicators may be more accurately viewed simply as records of stressor exposure and subsequent survival only, rather than providing evidence that these sub-adults are frailer than their similarly aged-at-death peers. Thus, the demographic and sociocultural context is essential to the interpretation of observed skeletal indicators of stress.