Aggressive challenging behaviour in adults with intellectual disabilities (ID) severely impacts social inclusion, physical safety, and quality of life, with prevalence rates of 10-15% in clinical settings. Traditional management relied on reactive, punitive, or pharmacological interventions that suppressed symptoms but failed to address root causes, often exacerbating long-term dependence on restraints. This paper evaluates Positive Behaviour Support (PBS), a person-centered, evidence-based framework that integrates Applied Behaviour Analysis (ABA) with human rights principles to reframe aggression as unmet communication (e.g., escape, attention, tangibles, sensory needs). At PBS's core is Functional Behavioural Assessment (FBA), using multi-method tools like ABC charts and Motivation Assessment Scale to hypothesize functions via data triangulation. Interventions follow a tiered structure: primary (environmental modifications, visual schedules), secondary (early de-escalation via precursor detection), and tertiary (non-aversive safety responses). Functional Communication Training (FCT) empowers skill mastery, replacing aggression with adaptive alternatives like "break" cards. Longitudinal simulations from ID cohorts demonstrate PBS efficacy: aggressive incidents dropped 92% (52 to 4 over 6 months), restrictive practices reached 0, and FCT proficiency rose from 12% to 90%, with linear correlations (r=-0.98, p<0.01) validating skill-behaviour inverses. Success requires staff competency training, consistent data monitoring, and organizational shifts toward autonomy. By prioritizing environments, skills, and dignity over control, PBS achieves sustainable outcomes reduced harm, enhanced self-determination, and policy alignment. This synthesis addresses gaps in integrated ABA-social-rights models, guiding clinicians toward humane, scalable support.