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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.

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.

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 …