The Impact of Birth Order on Language Development in Children with Autism Spectrum Disorder in Simplex Families

The impact of birth order on language development has gained significant traction over the years, with contradictory evidence suggesting that lower birth order may hinder language development in typically-developing children (Nafissi & Vosoughi, 2015). However, results also suggest that when considering measures of social communication, second-born typically-developing children demonstrate a significant advantage (Kheirkhah & Cekaite, 2018). These findings have interesting ramifications when considering autistic children, as language impairments are characteristic of the disorder. The current study investigated the impact of birth order, in particular having an older, typically-developing sibling, on vocabulary and social language development in autistic youth. Participants included 1338 first-borns and 1049 second-borns (M age = 9.03 years, SD = 3.57; 86.4% male) with diagnoses of Autistic disorder, Aspergers, or PDD-NOS from the Simons Simplex Collection (Fischbach & Lord, 2010). Results indicated no significant differences in vocabulary or social language between first-borns and secondborns. Hierarchical linear regressions indicated no significant main effect of birth order; however, significant 2-way interactions with birth order x income and birth order x age predicted expressive vocabulary and inappropriate speech. Post-hoc simple slopes suggested that birth order may have a greater impact on language in younger autistic children, and lower-income families. This is the first work to date to investigate birth order and contextual factors on expressive language outcomes in autistic youth. The Impact of Birth Order on Language Development in Children with Autism Spectrum Disorder in Simplex Families Tyler Christine McFayden GENERAL AUDIENCE ABSTRACT Families provide an important context for important developmental milestones, such as language development (e.g., first words, firs phrases). Many parents and previous literature suggest that occasionally, older siblings can “speak on behalf” of their younger siblings, which reduces the number of opportunities second-born children have to practice important language skills. Previous literature in the field suggests that firstborn siblings demonstrate stronger language skills when considering vocabulary, but that second-born siblings demonstrate stronger social language skills. The current dissertation evaluated these birth order findings in a clinical group of children and adolescents with Autism Spectrum Disorder (ASD), as language differences are key features of the disorder. Using a large, representative data set from the Simon Simplex Collection, nearly 3,000 youth with ASD were evaluated on vocabulary and social language skills, grouped by birth order status. Group comparisons suggested that there were no significant differences between first-borns and second-borns in vocabulary and language. When evaluating what factors predicted vocabulary and language, birth order was also not significant. However, some interaction effects emerged between birth order and income, suggesting that in lower-income families, birth order may make a meaningful difference in vocabulary and social language. This is the first work to evaluate the role of siblings on language in ASD, and has important implications for interventions, especially for lower-income families impacted by ASD.