Examining the Relationship Between Birth Order and Birth Interval and the Emotional and Behavioral Adjustment of Siblings of Children with Autism

Background:  Sibling relationships are known to have a significant impact on children’s social and emotional development (Dunn, 1988). Studies have focused on the adjustment of typically developing (TD) siblings of children who have an autism spectrum disorder (ASD) have had mixed results; some showing positive effects for TD siblings (Macks et al., 2007), others demonstrating negative effects (Dempsey, 2012). Several variables appear to moderate the effect of having a sibling with ASD (Campe et al., 2013). Previous studies have examined the impact of birth order (Warren et al., 2012), yet the impact of birth interval has received little research attention in the ASD population (Martin & Horriat, 2012). Such studies have suggested that TD siblings who are younger and closer in age to their sibling with ASD tend to have more difficulties. However, these studies have been limited by small samples. 

Objectives:  This study sought to examine how birth order and birth interval impact the emotional and behavioral adjustment of TD siblings of children with ASD using a large heterogeneous sample, within group comparisons, and non-parametric statistics.

Methods:  A sample of 926 children with ASD and their TD siblings (463 sibling pairs) were drawn from the Boston cohort of the Simons Simplex Collection and the Boston Autism Consortium. Participants were between the ages of 25-216 months (mean=105, SD=46). 232 TD siblings were older than their sibling with ASD, 319 TD siblings were younger, and there were 12 sets of twins (TD+ASD) with in the sample. The Child Behavior Checklist (CBCL) was completed for all TD siblings.  For birth interval analyses, TD siblings were classified into groups based on the difference in age from their sibling with ASD, in 24-month intervals (i.e. 48-72 months younger, 24-48 months younger, <24 months younger, twin, <24 months older, 24-48 months older, 48-72 months older). Non-parametric statistics including Kruskal-Wallis tests with Mann-Whitney comparisons were conducted to examine the impact of birth order and birth interval on the behavior profiles of TD siblings as measured by the Child Behavior Checklist.

Results:  Analyses revealed a trend of higher rates of externalizing problems in younger TD siblings of children with ASD (p<.1) which is commensurate with findings from previous research. Birth interval analyses revealed that high rates of externalizing symptoms were consistent across birth intervals in younger siblings of children with ASD. Among older siblings, those who were 24-48 months older had the lowest rates of externalizing behaviors, including significantly lower rates of aggression. Analyses of Internalizing symptoms indicated similar rates in younger versus older TD siblings. Among the older TD siblings, however, those who were 48-72 months older had significantly higher rates of anxious/depressed symptoms.

Conclusions:  Consistent with previous research, younger siblings of children with ASD appear to have a higher rate of behavioral difficulties overall. Protective and deleterious effects of birth interval were found for older siblings of children with ASD only.