Investigating associations between birth order and autism diagnostic phenotypes

Area of Science:

• Developmental Psychology
• Neurodevelopmental Disorders
• Pediatric Psychiatry

Background:

• Birth order is associated with variations in cognitive abilities and developmental outcomes.
• Previous research suggests a link between birth order and Autism Spectrum Disorder (ASD) diagnosis.
• Limited evidence indicates reduced cognitive functioning with higher birth order.

Purpose of the Study:

• To investigate the association between birth order and Autism Spectrum Disorder (ASD) phenotypes.
• To examine how birth order influences cognitive and adaptive functioning in children with ASD.

Main Methods:

• Analysis of a large prospective diagnostic registry of 5,404 children (1-18 years) diagnosed with ASD.
• Children with ASD were categorized by birth order within their families.
• Data included demographic, family, functional, and intellectual characteristics at diagnosis.

Main Results:

• Adaptive functioning and intelligence scores declined with increasing birth order.
• Later-born children with ASD were more likely to have intellectual disabilities.
• First-born children without siblings at diagnosis also showed reduced cognitive functioning.

Conclusions:

• Increasing birth order is associated with greater variability in ASD clinical phenotypes.
• Findings suggest potential reproductive curtailment in children without siblings.
• Results advance understanding of ASD heterogeneity related to birth order and family size.