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Key Points Question Are changes in the maturation of the social brain in infants associated with vulnerability to neurodevelopmental conditions such as autism spectrum disorder? Findings In this cohort study of 36 neonates with and without a family history of autism spectrum disorder, newborns with a family history of autism spectrum disorder had significantly higher neural activity in the right fusiform and left parietal cortex. In addition, the pattern of age-related changes in spontaneous activity in the cingulate and insula was disrupted in infants with a family history of autism spectrum disorder. Meaning Atypical development of functional activity patterns in key regions responsible for social processing may be a vulnerability mechanism for autism., This cohort study compares functional magnetic resonance imaging of the brains of neonates with a family history of autism spectrum disorder (ASD) vs those of neonates with no family history of ASD., Importance What is inherited or acquired in neurodevelopmental conditions such as autism spectrum disorder (ASD) is not a fixed outcome, but instead is a vulnerability to a spectrum of traits, especially social difficulties. Identifying the biological mechanisms associated with vulnerability requires looking as early in life as possible, before the brain is shaped by postnatal mechanisms and/or the experiences of living with these traits. Animal studies suggest that susceptibility to neurodevelopmental disorders arises when genetic and/or environmental risks for these conditions alter patterns of synchronous brain activity in the perinatal period, but this has never been examined in human neonates. Objective To assess whether alternation of functional maturation of social brain circuits is associated with a family history of ASD in newborns. Design, Setting, and Participants In this cohort study of 36 neonates with and without a family history of ASD, neonates underwent magnetic resonance imaging at St Thomas Hospital in London, England, using a dedicated neonatal brain imaging system between June 23, 2015, and August 1, 2018. Neonates with a first-degree relative with ASD (R+) and therefore vulnerable to autistic traits and neonates without a family history (R−) were recruited for the study. Synchronous neural activity in brain regions linked to social function was compared. Main Outcomes and Measures Regions responsible for social function were selected with reference to a published meta-analysis and the level of synchronous activity within each region was used as a measure of local functional connectivity in a regional homogeneity analysis. Group differences, controlling for sex, age at birth, age at scan, and group × age interactions, were examined. Results The final data set consisted of 18 R+ infants (13 male; median [range] postmenstrual age at scan, 42.93 [40.00-44.86] weeks) and 18 R− infants (13 male; median [range] postmenstrual age at scan, 42.50 [39.29-44.58] weeks). Neonates who were R+ had significantly higher levels of synchronous activity in the right posterior fusiform (t = 2.48; P = .04) and left parietal cortices (t = 3.96; P = .04). In addition, there was a significant group × age interaction within the anterior segment of the left insula (t = 3.03; P = .04) and cingulate cortices (right anterior: t = 3.00; P = .03; left anterior: t = 2.81; P = .03; right posterior: t = 2.77; P = .03; left posterior: t = 2.55; P = .03). In R+ infants, levels of synchronous activity decreased over 39 to 45 weeks’ postmenstrual age, whereas synchronous activity levels increased in R− infants over the same period. Conclusions and Relevance Synchronous activity is required during maturation of functionally connected networks. This study found that in newborn humans, having a first-degree relative with ASD was associated with higher levels of local functional connectivity and dysmaturation of interconnected regions responsible for processing higher-order social information.