Objective: To examine how gradients in socioeconomic status (SES) impact US children's reading and math ability at kindergarten entry and determine the contributions of family background, health, home learning, parenting, and early education factors to those gradients. Methods: Analysis of 6600 children with cognitive assessments at kindergarten entry from the US Early Childhood Longitudinal Birth Cohort Study. A composite SES measure based on parent's occupation, education, and income was divided into quintiles. Wald F tests assessed bivariate associations between SES and child's cognitive ability and candidate explanatory variables. A decomposition methodology examined mediators of early cognitive gradients. Results: Average reading percentile rankings increased from 34 to 67 across SES quintiles and math from 33 to 70. Children in lower SES quintiles had younger mothers, less frequent parent reading, less home computer use (27%-84%), and fewer books at home (26-114). Parent's supportive interactions, expectations for their child to earn a college degree (57%-96%), and child's preschool attendance (64%-89%) increased across quintiles. Candidate explanatory factors explained just over half the gradients, with family background factors explaining 8% to 13%, health factors 4% to 6%, home learning environment 18%, parenting style/beliefs 14% to 15%, and early education 6% to 7% of the gaps between the lowest versus highest quintiles in reading and math. Conclusions: Steep social gradients in cognitive outcomes at kindergarten are due to many factors. Findings suggest policies targeting levels of socioeconomic inequality and a range of early childhood interventions are needed to address these disparities. (author abstract)