Abstract Research has established the importance of early socioeconomic advantage and disadvantage for understanding later life outcomes, but less is known about change in the relationship between socioeconomic status (SES) and child development within the period of early childhood. Competing hypotheses drawn from the literature posited: (1) a stable SES-development relationship, (2) a stronger relationship in infancy than at older ages, and (3) a stronger relationship at school entry than at younger ages. Using the nationally representative Early Childhood Longitudinal Study-Birth Cohort (2001–2007), we followed 8600 children from infancy through kindergarten entry to model change over time in the relationship between socioeconomic status and cognitive and behavioral development. The unexpected main finding was that the relationships between three socioeconomic measures (household income, assets, and maternal educational attainment) strengthened from infancy through age 4 or 4½, then weakened slightly until the start of kindergarten. Indirect evidence suggested preschool education as one possible explanation. We argue for researchers to expand the school transition concept to include the now widespread prekindergarten year, as well as for attention to psychological and physiological developmental factors that may shape the relationship between SES and cognitive and behavioral development throughout early childhood.