Advances and challenges of the delayed brain development hypothesis for ADHD: Evidence from Behavior, the Brain, and...

Attention deficit hyperactivity disorder (ADHD) is hypothesized to be associated with delayed brain maturation rather than atypical brain development. While this developmental delay hypothesis has gained widespread recognition, research progress has stagnated, and the field lacks a consolidated overview of existing findings and challenges. This narrative review integrates multi-dimensional findings across behavioral manifestations, electroencephalography (EEG), structural,diffusion, and functional magnetic resonance imaging (MRI), and genetics, supporting a regionally specific and multi-trajectory heterogeneous delay model. Based on a critical discussion of the literature and recent advancements, four critical unresolved challenges are identified: (1) incomplete explanatory power of delay phenomena; (2) whether temporal delays lead to reduced peak amplitudes in developmental trajectories; (3) whether delayed trajectories can achieve developmental "catch-up" prior to brain maturation; and (4) heterogeneity in delayed developmental pathways. To address these challenges, we propose four strategies: (1) establishing longitudinal high-density tracking cohorts; (2) constructing multimodal brain-age prediction models; (3) identifying neurodevelopmental delay subtypes through heterogeneity modeling; and (4) quantifying ADHD-related developmental delays via disease progression modeling. These approaches aim to lay a foundation for precise diagnosis, treatment, and early intervention in ADHD. This review integrates existing evidence on the developmental delay hypothesis to help focus future efforts, while highlighting current challenges and potential solutions to inspire innovative directions for research into the neurodevelopmental mechanisms underlying ADHD.