Abstract
Heterogeneity in autistic development is usually attributed to variation in child-level characteristics. We argue that it is better understood as the output of a process. The child and family system continually calibrates the balance between environmental demands and available capacities, and those calibrations accumulate. When demand exceeds capacity, families lower demand, often through accommodation, or raise capacity through scaffolding and support. Each calibration changes what the system brings to the next encounter. Iterated across daily life and developmental transitions, this process pulls trajectories apart, producing what has been termed chronogeneity. The components of this account are established, but they have been invoked separately, and the field still largely models the child alone. This leaves basic questions unanswered, including why children who start in similar places diverge, why the same response helps in one family and constrains in another, and why divergence concentrates at developmental transitions. This paper is the first to specify a family-system mechanism that accounts for all three, and to establish that it cannot be described without three interlocking elements. Context defines the demands the system faces. Transactions are the operations through which calibration occurs. Development is how each calibration alters the terms of the next. The model identifies the fit between demands and capacities, rather than focusing on the child alone, as what assessment should measure, what intervention and policy should target, and what outcome measures should index, and it generates testable predictions about when and why trajectories diverge.