Objectives: The objectives of this study are to replicate previous studies by comparing cognitive abilities and symptom severity between oldest and younger siblings and examine the effects birth order has on each category.
Methods: Archival data of individuals was obtained from 69 individuals who received an ASD diagnosis from 2006-2008 at Sparks Clinics. Only children who received a cognitive evaluation and had siblings, in addition to the ASD diagnosis, were included in this study. The ADOS was used in the ASD diagnosis and scores from the WISC-IV, DAS, and DAS-II were obtained to measure cognitive ability.
Results: It was determined that birth order did not have a significant effect on either cognitive ability or symptom severity in children with autism. There was however an interaction between the oldest and younger group's verbal and non-verbal ability scores. Also, maternal age was found to be correlated with non-verbal and verbal abilities.
Conclusions: The current finding of no birth order effect on cognitive ability, as compared to the results of previous research demonstrating lower abilities in younger siblings, can be supported by several explanations. It is possible that there is a different mechanism of action underlying the expression of cognitive ability in multiplex versus simplex families, and the current sample included only families with one child diagnosed with an ASD. Another possibility is that the genetic and biological components of autism overlay any environmental factors, thus overpowering a child's social rank in the home. Although further examination will be necessary, the presence of older typically developing children could have a protective effect on children with ASD, being responsible for the younger children in the current sample not having lower cognitive ability than other children (eg. the younger siblings with ASD benefited from having a socially-typical role model in the home).