Abstract The shortened developmental period and similar brain organization make nonhuman primates an ideal model for studying the exaggerated brain growth hypothesis of autism spectrum disorder (ASD). Specifically, research indicates that the brain development trajectory is significantly steeper and different in children with ASD compared to neurotypical controls, and this overgrowth is likely driven by several genes associated with neurogenesis and synaptic pruning. One such gene is DUF1220, which has been shown to affect brain growth, but remains largely understudied in the context of intra-individual variability in nonhuman primate brain development. In adult humans, increased copy number variants in DUF1220 are positively correlated with individual variation in brain size and brain size pathology. Further, in adult humans, increases in DUF1220 copy number is associated with higher scores on (1) general intelligence, (2) math ability, and (3) increases in ASD symptom severity. Here, we propose utilizing a nonhuman primate model (rhesus monkeys) to test whether intra-species increases in DUF1220 copy number is associated with whole and regional brain growth as well as increased severity of ASD-like behavioral phenotypes. In aim 1, we will quantify copy number variants in DUF1220 in the 1q21 region (from blood) and acquire in vivo MRI and DTI scans longitudinally in a sample of 42 monkeys at 6, 12, 18, and 24 months of age. we will perform analyses of variance (ANOVAs) with DUF1220 copy number in each clade (low/high) and sex (female/male) as the independent variables and the brain size measures as the dependent variables (total volume, gray and white matter, surface area, cortical thickness, gyrification, and connectivity). We will also examine the effect of DUF1220 copy number on the slope of change for our brain measures (across 6, 12, 18, 24 months). In addition, we will use a region-of- interest approach to quantify gray and white matter within brain regions comprising the social brain network and test for their association with DUF1220 copy number variants. In aim 2, we plan to assess social cognition every six months using three cognitive tests selected for their relevance to ASD, including mutual eye gaze and gaze following, and a social motivation test. We will use ANOVAs to determine the differences in social motivation, mutual eye gaze, and gaze following between those with low and high DUF1220 copy numbers in any of the six clades at each time point (6, 12, 18, and 24 months). We will also perform partial correlations to examine the relationship between gray matter and white matter volumes in the social brain network and scores on the social cognition tests, with DUF1220 copy number as a covariate. Together these findings will: (1) provide additional evidence of the utility of nonhuman primate models of ASD, and (2) help elucidate the developmental time frame for changes in brain size and social cognition, in relation to DUF1220, as ...