Siblings have the potential to shape the developmental trajectories of individuals with autism spectrum disorder. Having siblings has been associated with better social communication skills, less severe non-verbal communication deficits, and improved theory of mind abilities in autism spectrum disorder. This study examined the influence of the presence of a sibling, participant's position in the birth order, gender of the closest-age sibling, and gender match of the sibling dyad on the rate of growth in adaptive skills from ages 9 to 26 years among individuals with autism spectrum disorder and individuals with a history of non-spectrum developmental disorders. While all participants showed consistent gains in adaptive skill development as a function of age, there was variability in rates of growth based on sibling constellation group membership. Participants with at least one sibling, regardless of other sibling constellation factors, experienced greater rates of growth in adaptive behavior trajectories from childhood to adulthood than participants with no siblings across both Black and White participants, although differences were magnified in Black participants. Furthermore, among participants with siblings, those with male closest-age siblings and those with gender-matched siblings, irrespective of birth order position, demonstrated the steepest growth patterns in adaptive functioning within their respective groups over the 17-year period. Results suggest that siblings may serve an important role in improving the adaptive functioning trajectories and overall outcomes of individuals with autism spectrum disorder.
Keywords: adaptive behavior; autism spectrum disorder; sibling constellation; siblings; trajectories.