The Impact of Birth Order on Language Development in Autistic Children from Simplex Families.

The current study investigated the impact of birth order on vocabulary and social language development in 1338 first-born and 1049 s-born autistic youth (M age = 9.03 years, SD = 3.57; 86.4% male) from the Simons Simplex Collection. Frequentist and Bayesian analyses revealed mixed findings in language development. There were no differences in vocabulary or social language between first-born and second-born children. However, birth order and income together predicted expressive vocabulary and inappropriate speech such that birth order had a greater impact on language in lower-income families. This is the first study to investigate the impact of birth order on language outcomes in autistic youth and has implications for early intervention in lower-resourced communities.