• We aimed to investigate separate and combined biological and social determination of young children’s cognitive performance. • We used path analysis of longitudinal data from birth cohort studies in four differing low-and-middle countries. • We distinguished biological and social factors, as well as household resources from parental capacity. • Biological factors determine birthweight, 2-year height depends on social factors, and the latter drive child cognitive performance. • No single domain intervention provides both necessary and sufficient support for young children’s unfolding development.