The current study aimed to identify heterogeneous groups of adolescents differing on their levels of conduct problems (CP) and callous-unemotional (CU) traits, and compare them on startle reactivity to emotional videos (violent/erotic) during young adulthood. A Latent Profile Analysis, conducted among a longitudinal sample of 2306 adolescents, provided evidence for the existence of heterogeneous CP and CU subgroups (i.e., CP-only, CU-only, and CP+CU groups). Three years later, 82 young adults (Mage = 19.95), randomly selected from the identified groups, participated in an experiment assessing their startle eye-blink responses during violent, erotic and neutral video scenes. Self-report ratings of fear, valence and arousal to affective scenes were also collected. Findings suggested that adolescents high on CP and low on CU traits showed high startle potentiation when viewing violent scenes during young adulthood, while those high on both CP and CU traits showed diminished startle reactivity to violent stimuli. Individuals high on CU traits without CP showed similar startle reactivity to controls. Further, the findings indicate that startle potentiation to violent stimuli can be a reliable physiological marker to distinguish antisocial individuals with and without CU traits. The finding that the CU only group was not differentiated from the control group on startle reactivity when viewing violent stimuli might explain their lower likelihood of engaging in antisocial behavior.