Effortful control, the ability to suppress a dominant response to perform a subdominant response, was assessed in 106 children during early childhood (at 22, 33, and 45 months) using multitask behavioral batteries. By 45 months, effortful control was highly longitudinally stable and coherent across tasks and thus appeared to be a traitlike characteristic of children's personality. Children who had been less intense in terms of proneness to anger and joy, and those who had been more inhibited to the unfamiliar in the second year developed higher effortful control. Children with higher effortful control at 22-45 months developed stronger consciences at 56 months and displayed fewer externalizing problems at 73 months. Effortful control mediated the oft-reported relations between maternal power assertion and impaired conscience development in children, even when child management difficulty was controlled.