Carnett, AmarieYee Juin, Ling2026-04-292026-04-292026https://hdl.handle.net/10289/18238While Functional Communication Training (FCT) has been widely validated in child’s populations, its effectiveness among adults with developmental disabilities (DD) remains underexplored in the literature. Modern FCT increasingly emphasises selecting an augmentative and alternative communication (AAC) modality that fits the individual and is preferred by them, as this can directly influence acquisition speed, maintenance of the functional communication response (FCR) and risk of problem behaviour relapse. This study evaluated a function-based approach to reducing problem behaviour and strengthening functional communication for an autistic adult with DDs and limited verbal communication. Building on prior work, the study aimed to (a) identify an individually preferred mand topography following differential training of two AAC modalities and (b) test the durability of that preferred modality when embedded within FCT, including delay and denial tolerance training (DDTT). Using a single-subject design, a 25-year-old adult participant with history of problem behaviours was systematically exposed to phases of FCT intervention including DDTT. The result of the study demonstrated a fast acquisition rate when the preferred AAC modality was used and the suppression of problem behaviour using competing stimuli during tolerance training. The findings underscore the clinical implications of embedding AAC modality evaluation and preference assessment within FCT and systematic DDTT to promote durable functional communication and behavioural stability in real-world adult service contexts.enAll items in Research Commons are provided for private study and research purposes and are protected by copyright with all rights reserved unless otherwise indicated.Functional Communication TrainingAdultDevelopmental DisabilitiesAugmentative and Alternative CommunicationProblem BehaviourDelay and Denial Tolerance TrainingThesis