At all ages, a child who can sustain attention, regulate emotions, inhibit impulsive behaviour, and relate appropriately to adults and peers is able to take advantage of learning opportunities in the classroom, and beyond. This study assesses differences in children's socio-emotional development according to family background, and whether early gaps in these skills widen across childhood in a contemporary UK cohort. Applying models replicating a much-discussed investigation of cognitive trajectories, it finds that large gaps in early childhood persist through school, and that children with similar initial skills diverge over time according to family background. The transition to secondary school is identified as a crucial time when disadvantaged children fall behind, and a priority for future intervention.