Objective: To investigate the longitudinal development of callous-unemotional traits (CU) in middle childhood using developmental trajectory analyses in a large twin dataset and examine the degree to which genetic and environmental influences contributed to the CU trajectory-group membership in children. Method: The study included 9,462 youths from the Twins Early Development Study, a population-based sample of twins from the United Kingdom. Developmental trajectories were described using teachers' ratings of CU at 7, 9, and 12 years old. Results: We identified four trajectories of CU through general growth mixture modeling: stable high, increasing, decreasing, and stable low. In most cases, the trajectory-group membership was largely driven by genetic and to a lesser extent by nonshared environmental influences for boys and girls. The most notable exception was a strong contribution of shared environment for the girls in the stable-high trajectory group. Conclusions: Our findings suggest distinct developmental trajectories of CU from childhood to early adolescence, which are in most cases influenced by genetic factors and, to a lesser degree, by nonshared environmental factors. Highest heritability was observed for boys on a stable-high CU trajectory. Interestingly, the trajectory-group membership for girls on a stable-high CU trajectory appeared to be almost entirely driven by shared environmental influences. These differences in the etiology of stable-high CU in boys and girls have potential implications for clinical practice and studies attempting to identify genetic and environmental risk factors for high CU. (Contains 1 figure and 4 tables.)