ADHD as a disorder of operational capacity: a buffered-state framework for sustained engagement instability.

Attention-deficit/hyperactivity disorder (ADHD) is commonly conceptualized as a disorder of executive dysfunction or impaired motivation. In clinical practice, however, many individuals with ADHD describe a different core difficulty: they can initiate tasks appropriately, understand goals, and exert effort, yet struggle to remain engaged as time-on-task and internal strain accumulate. These failures are often abrupt, ego-dystonic, and strongly context-dependent, posing challenges for models based solely on static deficits or gradual depletion. Building on this phenomenology, we propose a constraint-level framework that conceptualizes ADHD as a disorder of operational capacity-the finite tolerance for sustaining stable mental engagement under cumulative cognitive and emotional demand. Within this framework, sustained engagement is modeled as a buffered state that remains stable while cumulative load is within tolerance but becomes increasingly fragile near saturation and may undergo nonlinear collapse into disengagement. We formalize this governing constraint as Operational Economy (OE)-a regulatory, not energetic, constraint principle-and describe a pre-volitional Global Switching Framework (GSF) that mediates rapid state transitions once engagement stability is exceeded. This formulation integrates and extends cognitive-energetic and state-regulation accounts while explicitly differentiating the OE/GSF framework from these predecessors by its emphasis on threshold-dependent dynamics, bounded tolerance, and asymmetric re-engagement. This framework offers a coherent account of clinical features such as intact initial performance with late-stage disengagement, pronounced time-on-task effects, and context-dependent hyperfocus. The framework generates falsifiable predictions regarding sensitivity to cumulative demand, stress-related acceleration of disengagement, pharmacological effects that preferentially extend engagement endurance rather than normalize peak executive performance, and individual differences across ADHD subtypes and comorbid presentations. These predictions are consistent with existing empirical findings on time-dependent variability in ADHD and provide a basis for future experimental validation.