Background: Life history theory and resource dilution models of the family suggest that siblings may present a threat to healthy development because they compete for resources that parents have available to invest in individual offspring. Using data from a large cohort study of contemporary British families (ALSPAC), we test this hypothesis using childhood growth trajectories as a biomarker for health status.
Methods: Incorporating time-varying measures of changing family structure and socio-economic environment, this study represents the first true longitudinal analysis of family configuration effects on human growth. Using separate multi-variate multi-level models we estimate the effect of sibling number and sibling age and sex on height from birth to 10 years.
Results: Adjusting for family level socio-economic factors, the presence of siblings is associated with deficits in height across the study period. At the largest comparison, we estimate that compared with only children, children with four siblings have a reduced birth length by -8.7 mm (95% confidence interval (CI): -14.8 to -2.6) and a reduced rate of growth by -2.3 mm per year (95% CI: -3.8 to -0.8), leading to a deficit of 31.5 mm by age 10. Older siblings are associated with larger lasting negative consequences on height than younger siblings. We find no difference in the height of children in relation to the sex of siblings.
Conclusions: Even in the relatively wealthy, well-nourished conditions of modern Western society, children are not buffered from the health costs of reduced parental investment. Later-born children appear worst affected by within family resource division.