Definition
Developmental stages are ordered phases of psychological growth, each defined by a characteristic set of capacities, conflicts, or modes of thinking. Stage theories assume that progression is sequential — a later stage cannot stably appear until the earlier ones are in place — and that the transitions are qualitative rather than merely quantitative. The child does not just know more than the infant; the child reasons differently. Piaget's cognitive stages, Erikson's psychosocial stages, and Kohlberg's moral stages are the canonical examples, each carving the lifespan into a different sequence of distinctive challenges.
Why it matters
How it works
Each stage theory specifies a set of structures or themes that organise behaviour during a given window, and a developmental task whose resolution carries the person into the next window. In Piaget the structures are cognitive operations: sensorimotor schemes, then concrete operations on tangible objects, then formal operations on abstract symbols. The mechanism of transition is equilibration — the child encounters experiences the current scheme cannot assimilate, accommodates by reorganising, and stabilises a richer structure.
Erikson's psychosocial stages run on the same scaffolding but with relational themes — trust versus mistrust, identity versus role confusion, generativity versus stagnation — and the resolution at each step (favourable or unfavourable) becomes a layer in the personality. Stage transitions are not strictly age-locked: cultural conditions, individual temperament, and earlier resolutions all shift timing. The shared claim across these theories is that there is an underlying logic to the order — not just a checklist that could be reshuffled.