Investigating associations between birth order and autism diagnostic phenotypes

Background Birth order effects have been linked to variability in intelligence, educational attainment and sexual orientation. First‐ and later‐born children have been linked to an increased likelihood of an Autism Spectrum Disorder (ASD) diagnosis, with a smaller body of evidence implicating decreases in cognitive functioning with increased birth order. The present study investigated the potential association between birth order and ASD diagnostic phenotypes in a large and representative population sample. Methods Data were obtained from an ongoing prospective diagnostic registry, collected between 1999 and 2017, including children (1–18 years of age, n = 5,404) diagnosed with ASD in the state of Western Australia. Children with ASD were ranked relative to sibling’s birth to establish birth order within families at time of ASD diagnosis. Information reported to the registry by health professionals at the time of diagnostic evaluation included demographic and family characteristics, functional abilities and intellectual capacity. Results Adaptive functioning and intelligence scores decreased with increasing birth order, with later‐born children more likely to have an intellectual disability. Compared to first‐born children with siblings, first‐born children without siblings at the time of diagnosis also exhibited decreased cognitive functioning. Conclusions These findings demonstrate for the first time an association between increasing birth order and variability in ASD clinical phenotypes at diagnosis, with potential evidence of reproductive curtailment in children without siblings. Taken together, these findings have significant implications for advancing understanding about the potential mechanisms that contribute to heterogeneity in ASD clinical presentations as a function of birth order and family size.