To date, differences in craniofacial robusticity among modern and fossil humans have been primarily addressed by analyzing adult individuals; thus, the developmental basis of such differentiation remains poorly understood. This article aims to analyze the ontogenetic development of craniofacial robusticity in human populations from South America. Geometric morphometric methods were used to describe cranial traits in lateral view by using landmarks and semilandmarks. We compare the patterns of variation among populations obtained with subadults and adults to determine whether population-specific differences are evident at early postnatal ontogeny, compare ontogenetic allometric trajectories to ascertain whether changes in the ontogeny of shape contribute to the differentiation of adult morphologies, and estimate the amount of size change that occurs during growth along each population-specific trajectory. The results obtained indicate that the pattern of interpopulation variation in shape and size is already established at the age of 5 years, meaning that processes acting early during ontogeny contribute to the adult variation. The ontogenetic allometric trajectories are not parallel among all samples, suggesting the divergence in the size-related shape changes. Finally, the extension of ontogenetic trajectories also seems to contribute to shape variation observed among adults. Am J Phys Anthropol 2010. © 2009 Wiley-Liss, Inc.