Objectives: The present study aimed to investigate for the first time the association between autistic-like traits and ADHD behaviors in early childhood, in a community sample of 2-year-olds. By employing a classic twin design, the study aimed to determine the degree to which autistic-like traits and ADHD behaviors covary due to genetic and environmental influences.
Methods: Twins from the Boston University Twin Project (N = 312 pairs) were assessed on their second birthday by their parents on autistic-like traits and ADHD behaviors using the Childhood Behavior Checklist. Structural equation model-fitting was carried out.
Results: Phenotypic analyses showed that after controlling for general cognitive ability and socioeconomic status, autistic-like traits (total scale as well as social and nonsocial subscales) correlated positively with ADHD behaviors (r = .23-.26). Structural equation model-fitting revealed that both ADHD- and autistic traits were moderately heritable in early childhood and that there were modest shared genetic influences between them (genetic correlation = .27). Common environmental influences also explained part of the covariation between these behaviors.
Conclusions: These findings show that there is a significant positive association between autistic-like traits and ADHD behaviors in 2-year-olds, which concurs with findings from clinical studies. This association is caused by both shared genetic and shared environmental influences. Compared to studies of older children and young adults, the relationship between autistic-like traits and ADHD behaviors appears to be weaker, both phenotypically and genetically, suggesting that the covariance of these behaviors, and the degree of shared genetic influences, may increase with age. This evidence for overlap between ASD and ADHD behaviors both clinically and when assessed as quantitative traits should be used as a springboard for evaluating diagnostic rules concerning dual diagnoses of ASD and ADHD, as well as for developing hypotheses for molecular genetic research.