Infant rhesus macaques were studies to test the effects of breastfeeding versus formula-feeding on cerebral cortex maturation. Through analysis of structural and diffusion MR images, brain volume over the first 6 months of life was not significantly altered by formula-feeding versus breastfeeding, or by formula lutein supplementation. However, cellular maturation within cerebral cortical gray matter differed between formula-fed and breastfed animals. Lower gray matter FA of breastfed infants is interpreted to reflect greater neuronal arborization. This difference may be related to the different social experience as well as the nutrient composition of breast milk.