Abstract This study investigated the influence of Cognitive Behavioural Therapy (CBT) on aggressive behaviour among out-of-school adolescents in Ondo West Local Government Area of Ondo State, Nigeria. The study aimed to (a) determine the extent of aggressive behaviour among out-of-school adolescents, (b) examine the effect of CBT on aggression reduction, and (c) explore the influence of gender on the effectiveness of CBT. A descriptive survey and quasi-experimental research design was adopted. A total of 150 out-of-school adolescents were selected through simple random sampling from communities within the study area. Data were collected using a structured questionnaire and analysed using descriptive statistics, Pearson Product Moment Correlation (PPMC), analysis of variance (ANOVA), multiple regression, and paired-samples t-tests at a 0.05 level of significance. Findings revealed that aggressive behaviour was moderate among participants, with verbal and emotional forms being most prevalent. CBT significantly reduced aggressive behaviour (Cohen's d = 0.85) and enhanced cognitive functioning (Cohen's d = 0.94) post-intervention. CBT group membership was the strongest predictor of reduced aggression (β = −0.567, p < .001), accounting for 21.5% of variance in post-intervention aggression scores. Gender did not moderate CBT outcomes (p = .854), indicating equal effectiveness across male and female participants. The study concludes that CBT is a practical, evidence-based, and culturally adaptable intervention for managing aggression among socially vulnerable adolescents in low-resource Nigerian contexts. Keywords: Out-of-school adolescents, Aggressive behavior, Cognitive Behavioural Therapy, Anger management,
Imoh et al. (Fri,) studied this question.