A previous finding from our group indicated that teacher-rated antisocial behaviour (AB) among 7-year-olds is particularly heritable in the presence of callous-unemotional (CU) traits. Using a sample of 1865 same-sex twin pairs, we employed DeFries-Fulker extremes analysis to investigate whether teacher-rated AB with/without CU traits also shows aetiological differences among 9-year-olds. Furthermore, we assessed whether the differences in the magnitude of heritability would be evident even when hyperactive symptoms were controlled for in the statistical analysis. AB among 9-year-olds was more heritable with than without concomitant CU. The heritability difference was even more pronounced in magnitude when hyperactive symptoms were controlled. CU traits thus appear to index one valid way of sub-typing children with early-onset AB.