<p><strong>Background:</strong> Developmental neuroscience research suggests that relative differences in emerging social skills between very young children with ASD may influence the trajectories of multiple important developmental domains, such as language ability. Such “cascade” associations between developmental trajectories may contribute in important ways to the substantial heterogeneity in symptoms and functioning seen in children with ASD. However, longitudinal research has yet to test such “cascade” models of change in children diagnosed with ASD.</p> <p><strong>Objectives:</strong> In this dissertation I aimed to model cascade patterns of association between social competence and language ability pathways in the year after diagnosis of ASD in preschool-aged children. Data for 365 participants aged 2-4 years old who had been recently diagnosed with ASD and followed prospectively were obtained from the “Pathways To Better Outcomes in ASD Study”.</p> <p><strong>Methods:</strong> Study 1 aimed to determine whether social competence and structural language ability could be measured as distinct constructs that were invariant over time and between clinically relevant groups of young children with ASD. Study 2 modeled longitudinal reciprocal associations between these domains. Study 3 addressed the issue of variability and change within this sample, and tested whether baseline levels and rate of change in one domain were associated with subsequent growth in the other.</p> <p><strong>Results:</strong> Social competence and language ability constructs were measurably distinct and invariant in a young sample comprising verbal and non-verbal children with ASD. Only small reciprocal cascade effects were evident between social and language pathways. Levels of social competence at time of diagnosis were significantly associated with subsequent language growth.</p> <p><strong>Conclusions:</strong> In preschoolers with ASD, advantages in social competence as measured at time of diagnosis appear to “spill over” in a feed-forward cascade model to influence progress in language ability. Social competence and language ability then appear to develop along more specialized, less interrelated pathways.</p> / Doctor of Philosophy (PhD)