Early Motor Development in ADHD: Monogenic and Polygenic Trajectories Using NF1 As a Model

ADHD is often accompanied by motor difficulties in childhood, yet little is known about how these traits emerge in early life or the genetic pathways that contribute to their development. Studying infants with an elevated familial likelihood of ADHD (EL-ADHD) alongside those with a monogenic condition associated with ADHD, such as Neurofibromatosis type 1 (NF1), provides a unique opportunity to compare polygenic and monogenic influences on early motor development and its relationship to later ADHD traits. Accordingly, this study examined motor profiles in EL-ADHD, NF1, and typically developing (TD) infants, and investigated how early motor behaviour relates to emerging ADHD characteristics. The motor behaviour of infants with EL-ADHD, NF1, and TD were assessed at 10 and 14 months using a multimodal approach that included standardized motor assessments, parent reports of activity levels, behavioural coding of movements during free play, and accelerometery. Early ADHD traits were subsequently evaluated at follow-up visits at ages 2 and 3 years using the Child Behaviour Checklist (CBCL). Across infancy, EL-ADHD infants exhibited higher gross motor scores on standardised assessments compared to TD and NF1 infants, although they did not differ from either group in their fine motor scores. In contrast, NF1 infants showed both lower gross and fine motor scores relative to TD and EL-ADHD peers. During free play, EL-ADHD infants displayed more mature motor behaviours and marginially higher mean acceleration levels compared to TD and NF1 infants, whereas only minimal free play differences were observed between TD and NF1. Additionally, group-specific relationships emerged between motor behaviour and later ADHD traits. In the EL-ADHD group only, 14-month gross motor skills showed a trend-level positive association with ADHD traits at 36 months, and 14-month parent-reported activity levels significantly predicted ADHD traits at both 24 and 36 months. This study is the first to examine the relationship between motor development and ADHD traits in both polygenic and monogenic contexts. Our findings show that motor difficulties emerge early in infants with NF1, whereas such difficulties are not evident in those at elevated likelihood of ADHD in the general population. Moreover, early motor behaviour predicts later ADHD traits in ways that are specific to both developmental context and population. These results suggest that ADHD phenotypes can arise through distinct neurodevelopmental trajectories across different atypically developing populations.