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Developmental Psychology — the Three Streams

Of psychology’s many branches, developmental psychology is the one that studies the acquiring itself: how knowledge and understanding are built up through experience, watched longitudinally as it happens. To the question at its centre — how does the acquiring work? — the field has given three rival answers, and the dispute among them found its arena in language acquisition: the one capability every child acquires, on schedule, from wildly varying input. Each stream is treated in full on its own pages; here each appears as its answer to the question.

Shaping from outside

Behaviourism answered that the acquiring is done to the child. Skinner extended the framework to its most ambitious case in Verbal Behavior (1957): language as behaviour like any other, shaped by conditioning and differential reinforcement, with no inner process worth studying — the organism’s history of consequences carries the whole explanation. Chomsky’s 1959 review of that book became the hinge on which the field turned: the argument that shaping from outside cannot account for what children actually produce — novel sentences never uttered before, errors never modelled for them — marked the limit of the answer and opened the door to the study of internal process.

The endowment unfolding

Nativism answered that the acquiring is largely an unfolding of what is already there. Chomsky’s poverty-of-stimulus argument holds that the input a child receives is too thin and too noisy to determine the grammar every child nonetheless converges on; what bridges the gap is an innate endowment — in its strongest form a universal grammar, a built-in structure that experience triggers and tunes rather than builds. On this answer, experience selects among possibilities the child brought along; the deep structure of the capability was never acquired at all.

The builder in the encounter

The third stream answered that the acquiring is the child’s own construction, worked in the encounter with world and others — and it carries the substance of the field’s positive account.

Piaget found the acquiring constructive. Knowledge is not received but built: the child assimilates what experience brings into the structures of understanding it already has, and accommodates those structures when the new will not fit — and out of that two-way work the structures themselves grow, stage by stage. The process builds its own capability as it runs: what has been acquired shapes what can be acquired next.

Vygotsky found the acquiring socially fed. The capabilities that end up running privately — inner speech, deliberate memory, reasoning — arrive first as shared activity: carried in language, in tools, in interaction with others who can already do what the child cannot yet do alone. What becomes the individual’s own was internalised from the shared.

Tomasello brought the stream’s answer to the arena itself. Language is learned from use: the child reads what others mean — joint attention, intention-reading — and builds grammar up out of concrete instances, pattern by pattern, with no innate grammar required. The usage-based account meets nativism on its home ground, offering a constructive answer to the very case the poverty-of-stimulus argument was built on.

What the three streams bring to cognition, taken together, is the question itself made empirical: whether the acquiring is imposed, unfolded, or built is not settled by definition but watched, in children, longitudinally. And what the watching keeps returning is activity — at every point where the streams have met the evidence, the child has turned out to be doing something: extracting, constructing, reading intentions, reworking structure. The acquiring has an owner.

See also: Piaget · Vygotsky · Tomasello · Chomsky · Skinner · Nativism · Behaviourism