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 and Vosoughi, 2015). However, results also suggest that when considering measures of social communication, second-born typically-developing children demonstrate a significant advantage (Kheirkhah and 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 and Lord, 2010). Results indicated no significant differences in vocabulary or social language between first-borns and second-borns. 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.