Effects of sibling quantity and birth order on problem behaviors in children with autism spectrum disorder and their typically-developing siblings

Although systems theory assumes all siblings necessarily impact each other's development, less is known about the nature and extent of these effects, particularly when one sibling has autism ("proband" sibling). Existing research has yielded divergent conclusions about the impact of an autism diagnosis on typically-developing (TD) sibling development. This study examined effects of family composition (birth order and sibling quantity) on problem behaviors in TD children and their proband siblings. Problem behaviors were measured by the internalizing and externalizing behavior composites yielded by the Child Behavior Checklist (CBCL). Data were previously collected by the Simons Simplex Collection (n = 2,645 families). Results indicate that increased sibling quantity is related to lower levels of internalizing and externalizing problems for TD children. No relationship was observed between sibling quantity and problem behaviors for proband children. Regarding birth order, TD and proband siblings both experienced fewer problem behaviors when the TD sibling was older than the proband. When the proband was older than the TD sibling, the proband experienced higher internalizing problems, while the TD sibling experienced higher externalizing problems. Gender was not found to impact any of the findings. Implications for families and clinicians are discussed.