Sibling Competition & Growth Tradeoffs. Biological vs. Statistical Significance

Abstract

Early childhood growth has many downstream effects on future health and reproduction and is an important measure of offspring quality. While a tradeoff between family size and child growth outcomes is theoretically predicted in high-fertility societies, empirical evidence is mixed. This is often attributed to phenotypic variation in parental condition. However, inconsistent study results may also arise because family size confounds the potentially differential effects that older and younger siblings can have on young children’s growth. Additionally, inconsistent results might reflect that the biological significance associated with different growth trajectories is poorly understood. This paper addresses these concerns by tracking children’s monthly gains in height and weight from weaning to age five in a high fertility Maya community. We predict that: 1) as an aggregate measure family size will not have a major impact on child growth during the post weaning period; 2) competition from young siblings will negatively impact child growth during the post weaning period; 3) however because of their economic value, older siblings will have a negligible effect on young children’s growth. Accounting for parental condition, we use linear mixed models to evaluate the effects that family size, younger and older siblings have on children’s growth. Congruent with our expectations, it is younger siblings who have the most detrimental effect on children’s growth. While we find statistical evidence of a quantity/quality tradeoff effect, the biological significance of these results is negligible in early childhood. Our findings help to resolve why quantity/quality studies have had inconsistent results by showing that sibling competition varies with sibling age composition, not just family size, and that biological significance is distinct from statistical significance.

Childhood body mass is positively associated with cesarean birth in Yucatec Maya subsistence farmers

“…Children are generally weaned by two and a half years of age (median age = 2.58, n = 74, 95% CI = 2.46–2.69). Though extremely short in stature, Maya children and adults are generally adequately nourished, and child mortality and morbidity are low in the community (Kramer et al, ).…”

“…Both WHO and Maya population‐specific Z ‐scores were calculated. The justification for using population‐specific Z ‐scores, and methods used to compute them, are described in detail in Kramer et al (). Here, we use Maya population‐specific Z ‐scores for intrapopulation comparisons and WHO Z ‐scores for cross‐cultural reference purposes only.…”

Biological Anthropology in 2018: Grounded in Theory, Questioning Contexts, Embracing Innovation

“…However, because our data are often observational and collected under dynamic, unique conditions in time and place, reproducible methods should be a greater concern for anthropologists than replicable results per se (Beheim ). Problems of both replicability and reproducibility, however, stem from flawed or dubious practices (e.g., insufficient sample size and power, “p‐hacking,” file drawer effects, post hoc hypothesizing) that in turn arise from an emphasis on statistically significant versus biologically relevant findings (Kramer, Veile, and Otárola‐Castillo ) and the academic incentives that reward them (Smaldino and McElreath ). The extent to which such practices affect the veracity and quality of anthropological research is unknown but is the subject of an increasingly public conversation.…”

Economic and demographic predictors of dietary variation and nutritional indicators in Nicaragua

“…Standardized height‐for‐age (HAZ) and weight‐for‐age (WAZ) z ‐scores calculated using means from international growth charts (WHO or CDC) reveal the growth of individual children against reference populations (Helfrecht & Meehan, ). These standardized scores are the basis for characterizing child health and determining if children are failing to thrive (Kramer, Veile, & Otárola‐Castillo, ). Across different populations, variation in these anthropometric measures have been linked to sex (Christiaensen & Alderman, ; Sakisaka et al, ), age (Helfrecht & Meehan, ; Kramer et al, ), education (Cochrane, Leslie, & O'Hara, ), and wealth (Barros et al, ; Godoy et al, ).…”

“…Overall, larger households with few productive adults may experience greater stress given the anticipated limits on available resources (Hagen et al, ; Helfrecht & Meehan, ). Access to resources can also depend on birth order and age, as that can influence when children are weaned, and when they are expected to begin contributing labor to household production (Helfrecht & Meehan, ; Kramer et al, ). A number of studies have found that the presence of younger siblings can be detrimental to a child's growth, while the impact of older siblings is negligible (Helfrecht & Meehan, ; Kramer et al, ).…”