What to focus on
For Piaget, prioritise the four stages and what children can and cannot do at each one — especially the preoperational limitations (egocentrism, centration, irreversibility) and how conservation failure demonstrates them. Understand the core mechanism: schemas, assimilation, accommodation, and equilibration. Don't just list the stages — know why development moves from one to the next.
For Vygotsky, the zone of proximal development (ZPD) is the centrepiece. Understand what it is, why instruction should target it, and how scaffolding (Wood, Bruner & Ross, 1976) operationalises it. Know the story of private speech: why Piaget saw it as immaturity, and why Vygotsky saw it as a cognitive tool being internalised.
For Baillargeon, focus on what the violation-of-expectation (VOE) paradigm actually shows — why looking longer at an "impossible" event is taken as evidence of knowledge — and what the competence–performance distinction means for interpreting infant research. Know the landmark study (Baillargeon, Spelke & Wasserman, 1985) and the age claims it makes.
Key terms to memorise
Piaget: schema · assimilation · accommodation · equilibration · object permanence · A-not-B error · conservation · egocentrism · centration · irreversibility · decentration · reversibility
Vygotsky: zone of proximal development (ZPD) · scaffolding · more knowledgeable other (MKO) · private speech · inner speech · internalisation · cultural tools
Baillargeon: violation-of-expectation (VOE) · habituation · looking time · competence–performance distinction · core knowledge · continuity · solidity · gravity
Going deeper: apply, compare, and critique
Apply to examples: Given a child behaviour, be able to say what each theorist would predict. A child who fails a conservation task: Piaget says they lack decentration (preoperational stage); Vygotsky would ask what happens with scaffolding from an adult; Baillargeon would ask whether a motor-free version of the task reveals hidden competence.
Compare the theories: The most productive comparisons are — what drives development (equilibration vs social interaction vs core knowledge); the role of instruction (Piaget: readiness precedes learning; Vygotsky: instruction leads development); and timing (Baillargeon consistently finds competence far earlier than Piaget predicts).
Critique each: Piaget underestimates children — Donaldson (1978) showed his tasks are alien to children, not that children lack the competence. Vygotsky's ZPD is hard to operationalise — there's no agreed measure. Baillargeon's VOE paradigm may tap novelty or attention rather than conceptual knowledge; replication across labs has been inconsistent.
Why it still matters
These frameworks directly shape educational practice. Vygotsky's ZPD underlies differentiated teaching, peer learning, and scaffolded instruction used in classrooms worldwide. Piaget's stages influenced the timing of curriculum content — though his age estimates are now treated as upper bounds rather than fixed milestones. Baillargeon's core knowledge work has shifted developmental science toward a picture in which much knowledge is available far earlier than traditional accounts assumed, with implications for early childhood policy and intervention.
Common exam pitfalls
Confusing assimilation and accommodation: assimilation is fitting new experience into an existing schema; accommodation is changing the schema when new experience doesn't fit. Students often reverse them.
Getting Vygotsky's instruction claim backwards: Vygotsky said instruction *leads* development — it shouldn't wait for the child to be ready. Piaget said the opposite. This reversal appears in many exam answers.
Describing VOE without explaining the logic: writing "babies looked longer at the impossible event" without explaining why longer looking implies surprise (and therefore that an expectation was violated) will cost marks.
Treating the three theories as wholly incompatible: a sophisticated answer acknowledges they're partly complementary. Modern developmental science draws on all three — Piaget on logical structure, Vygotsky on social scaffolding, Baillargeon on initial knowledge systems.