The incremental theory of personality interventions (ITPI) teaches adolescents that people can change. Researchers have found that these interventions can reduce the perpetration of bullying and cyberbullying. Moreover, there is reciprocity between perpetrating bullying behaviors and being a victim of them. The objective of this study was to examine whether the ITPI reduces the reciprocity between victimization and perpetration of bullying and cyberbullying. A sample of 858 high school students (52% boys) aged 12 to 17 at pretest (M = 14.56, SD = 0.97) participated in a double-blind randomized controlled trial (452 participants were assigned to the experimental condition and 406 to the control condition). Measures of bullying and cyberbullying were taken at baseline, six-month, and 12-month follow-ups. The results indicated that victimization was a strong predictor of perpetration for bullying and cyberbullying over time. Perpetration was not a predictor of victimization. Consistently, for both forms of aggressive behavior, the intervention reduced the intensity of the association between victimization and perpetration. This effect was not moderated by the age or sex of the participants. Finally, the effectiveness of the ITPI was moderated by age. Specifically, among the youngest (< 14.48 years), those who received the ITPI showed a slight tendency to reduce aggressive behavior that contrasted with the growing trend in the control group. Among the oldest participants (> 14.48), the trajectories were similar in the two groups. Our findings show that influencing adolescents' reactions to peer aggression victimization is one of the mechanisms that could explain the beneficial effects of the ITPI and other preventive interventions.