Abstract
Autism commonly co-occurs with attention-deficit/hyperactivity disorder (ADHD), but less is known regarding how ADHD symptoms impact the early presentation of autism. This study examined early behavioral characteristics of a community sample of toddlers later identified with autism diagnosis, ADHD symptoms, combined autism and ADHD symptoms, or neither condition. Participants were 506 toddlers who were part of a longitudinal study of children's behavioral development. Parents completed questionnaires about their children's behavior at two time points. Four groups were identified based on study measures or medical record: autism diagnosis (n = 45), elevated ADHD symptoms (n = 70), autism and ADHD symptoms (n = 30), or neurotypical development (n = 361). Relationships between early parent report of autism- and ADHD-related behaviors, social-emotional and behavioral functioning, and caregiver experience and subsequent group designation were evaluated with adjusted linear regression models controlling for sex. Significant group differences were found in measures of autism-related behaviors, ADHD-related behaviors, externalizing and internalizing behaviors, and parent support needs (p < .0001). Pairwise comparisons indicated toddlers later identified with combined autism diagnosis and ADHD symptoms had higher levels of autism-related behaviors, externalizing and internalizing behaviors, and autism-related parent support needs compared to the other groups. Toddlers with subsequent elevated ADHD symptoms or combined autism diagnosis and ADHD symptoms exhibited similar levels of ADHD-related behaviors, while both groups displayed more ADHD-related behaviors than toddlers subsequently identified with autism or those with neither condition. In this community sample, toddlers for whomcombined autism diagnosis and ADHD symptoms were subsequently identified showed a distinct presentationcharacterized by higher early autism-related behaviors, broader behavioral concerns, and higherparent support needs. Presence of ADHD symptoms (alone or in combination with autism) was associated with higher parent-reported ADHD-related behaviors during toddlerhood. Results indicate that ADHD-related behaviors are manifest by toddlerhood, supporting screening for both autism and ADHD during early childhood.
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