This study addresses the effects of separation-individuation processes and fanaticism levels of young adults on the emergence of aggressive behavior. The sample group consists of 377 young adults between 18 and 22 of age. The participants are asked to respond to the Separation-Individuation Test of Adolescence (SITA), Fanaticism Scale for Football Fans (FSFFF), and the Buss-Perry Aggression Scale (BPAS). Chi-square, Pearson Product-Moment correlation analysis, One-way ANOVA, and moderation analysis were used for the statistical analysis. The physical and verbal aggression scores of the males are higher than the female participants. The separation-individuation scores of female participants are higher than that of the male participants. The fanaticism levels and physical-verbal aggression, and anger of the participants are correlated positively. Finally, the separation-individuation level is observed to have moderating effect on the correlation between aggression and fanaticism. The findings show that the fanatical supporters carry their self-needs to the team-self due to the problems they experience in the process of separation from their parents while engaging in aggressive behaviors. Therefore, considering the separation processes of individuals with their parents in studies aimed at preventing aggressive behaviors due to fanaticism will increase the success of the studies.