Behavioral characteristics of toddlers later identified with an autism diagnosis, ADHD symptoms, or combined autism and ADHD symptoms

Background 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. Methods 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. Results 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. Conclusions In this community sample, toddlers for whom combined autism diagnosis and ADHD symptoms were subsequently identified showed a distinct presentation characterized by higher early autism‐related behaviors, broader behavioral concerns, and higher parent 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.