Abstract
Autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD) can be traced back to specific early childhood temperament patterns. However, no unique pattern has been identified for their co-occurrence. Given that children with both traits often require more clinical attention, this study aimed to discover such patterns by examining three temperament domains measured during early childhood-Surgency/Extraversion (SE), Negative Affectivity (NA), and Effortful Control (EC)-and their association with group membership defined as being above the cut-off points for either ASD- or ADHD-trait scores or their co-occurrence at school age. We enrolled 814 children from a birth cohort, assessing temperament at 18 months using the Early Childhood Behavior Questionnaire, and ASD- and ADHD-trait scores at ages 8-9 using the Social Responsiveness Scale-2 and ADHD-Rating Scale. Group membership was determined by clinically significant symptoms, defined as + 1 SD after standardizing scores by age and sex. Multinomial regression analyses examined associations between temperament domain scores and group membership (ASD-dominant, ADHD-dominant, co-occurring, neither-ASD-nor-ADHD). The co-occurring group showed a unique temperament profile, with higher scores in both NA and EC (OR in NA = 1.48, 95% confidence interval (CI): 1.11 to 1.96 and OR in EC = 1.61, 95% CI: 1.18 to 2.20), distinct from the patterns shown by the ASD-dominant and ADHD-dominant groups. The combination of high NA and EC scores uniquely characterizes the co-occurring group, highlighting the need for early temperament assessments to identify children potentially requiring clinical attention for both ASD and ADHD traits.