Developmental Trajectories of Autistic Traits in Infants With Autism and/or ADHD Family History: Predictors and Mid-Childhood Outcomes.
PURPOSE Autism is both a developmental and heterogeneous condition, with individual variation in the timing and shape of autistic symptom trajectories over time, as recognised by the concept of 'chronogeneity'. Prospective infant autism family history studies are well-suited to examining developmental trajectory variability, a key aspect of autism trait heterogeneity. METHODS We examined autistic trait trajectories in a large sample of infants with autism and/or ADHD family history (N = 384) followed from infancy to mid-childhood. Using group-based multi-trajectory modelling, we identified classes with distinct trajectories across three autism trait measures taken at six, five, and four timepoints, respectively, between 5 months and 9 years. RESULTS A 5-class solution provided the best fit. Three classes included 50-75% children meeting autism diagnostic criteria at 3 years or mid-childhood, with distinct differences in timing, onset, and developmental patterns of autistic traits. Two classes were predominantly typically developing, with one characterised as 'slow starters'. Background characteristics including autism/ADHD family history, sex, maternal education, and household income (but not ethnicity) predicted class membership. Mid-childhood outcomes-IQ, adaptive function, autism and ADHD traits, anxiety and behavioural difficulties-differed across trajectory classes. CONCLUSION The timing and developmental course of autistic traits-independent of trait level-relate meaningfully to infant and family characteristics, diagnostic timing, and mid-childhood outcomes. These findings support chronogeneity as a valuable framework for understanding autism heterogeneity in family history studies. The trajectory classes identified here are descriptive; future studies should examine their replicability, and explanatory and mechanistic value.