Early developmental trajectories of restricted and repetitive behaviors in infants and toddlers with and without autism

Restricted and repetitive behaviors (RRBs) are an early-emerging and defining feature of autism spectrum disorder (ASD). This longitudinal study examined developmental trajectories of RRBs as measured by the Repetitive Behavior Scale-Revised (RBS-R) in 12-36-month-old infants (n = 207), including those with elevated likelihood of ASD due to familial history (ELSib) or early social communication concerns (ELScreen), and low-likelihood infants (LL), with observations at 12, 18, 24, and 36 months. Across these two years, the development of RRBs was non-linear, with a peak in RBS-R Overall Scores at 27 months in typically developing infants (those with no developmental concerns). The ASD group (those who received an ASD diagnosis) showed significantly faster increases in RRBs than all groups of infants not diagnosed with ASD, regardless of initial likelihood status. Among participants diagnosed with ASD, RRB trajectories were similar between ELSib-ASD and ELScreen-ASD, suggesting findings are generalizable across simplex and multiplex families. ASD infants demonstrated faster growth in RRBs than infants with non-ASD developmental delays (e.g., speech or global delays). Significant group differences in levels of RRBs did not emerge until 24-months of age, suggesting RRBs may lack specificity earlier in development. These results underscore the potential of RRBs as an early marker of ASD, as elevations compared to typically developing infants are evident by 12-months of age and increase in the following two years. However, clinicians and parents should be aware that smaller transient increases in RRBs are normative, and that early (<2 years) RRB elevations may not be specific to ASD.