This study tested if persistent externalizing behaviour and symptoms of Attention Deficit Hyperactivity Disorder (ADHD) in childhood are associated with personality and behavioural aspects of the psychopathic personality constellation in adolescence. The target sample consisted of all 1,480 twin pairs born in Sweden between 1985 and 1986. Parent-reported externalizing behaviour and ADHD symptoms were obtained when the participants were 8–9 and 13–14 years old. Personality and behavioural aspects of the psychopathic personality constellation were measured with self-report questionnaires when the participants were 16–17 years old. Persistent externalizing behaviour in childhood was associated with both psychopathic personality and antisocial behaviour in adolescence. However, within twin-pair analyses showed that genetic factors explained the association between persistent externalizing behaviour and psychopathic personality, while shared environmental factors explained the association between persistent externalizing and antisocial behaviours. We conclude that persistent externalizing behaviour is a genetically influenced precursor of psychopathic personality among boys.