Piaget's Theory of Cognitive Development
Why Do Children Think Differently?
Try explaining a straightforward idea to a young child and you may find they reason from a completely different set of rules. The Swiss psychologist Jean Piaget argued this was not a case of children knowing less — it was a case of children thinking differently. His Theory of Cognitive Development proposed that intelligence develops in stages, each one qualitatively different from the last.
Piaget did not just measure what children knew; he studied how they reasoned. By observing children closely and setting them small tasks, he identified a pattern of development he believed was universal, applying to all children regardless of background.
Schemas, Assimilation and Accommodation
Piaget believed we build knowledge through mental frameworks called schemas. A young child might have a simple schema for ‘dog’ — a four-legged animal. Fitting a new dog into this existing schema is called assimilation.
But a cat is also four-legged and clearly not a dog. The existing schema no longer fits, so the child must adjust or build a new one — this is called accommodation. Assimilation and accommodation work together, constantly reorganising a child’s thinking as they move through the four stages.
Assimilation = fitting new information into an existing schema. Accommodation = changing or creating a schema to fit new information. Both processes drive development through Piaget’s four stages.
The Four Invariant Stages of Development
Piaget proposed that all children pass through four invariant stages, always in the same order. The age ranges are approximate, but the sequence itself is fixed — no child can reach a later stage without passing through the earlier ones.
In the sensori-motor stage, infants explore the world through their senses and movements, with no language yet. A key achievement is object permanence — understanding that an object still exists once it is out of sight. Before this develops, a baby who watches a toy being hidden simply stops looking, as if it has vanished; once object permanence is in place, they will actively search for it.
In the pre-operational stage, children gain language and can use symbols, but their thinking has real limits. They show egocentrism — assuming everyone else sees, thinks and feels exactly as they do.
They also show animism — believing that objects which are not alive have feelings or intentions, such as insisting a teddy bear is sad because it fell off the bed.
Crucially, pre-operational children lack conservation. Pour water from a short wide glass into a tall thin one, and a pre-operational child will insist there is now more water, even though they watched the same amount being poured.
In the concrete-operational stage, children become capable of decentration — they can consider more than one feature of a problem at once instead of fixating on the most obvious change, such as the height of a glass. They also gain reversibility — understanding that a transformation can be mentally undone. Together, these allow them to conserve number, volume and mass, and to become less egocentric. They still struggle with pure abstract reasoning that has no physical objects to work with.
In the formal-operational stage, adolescents can reason about hypothetical situations, plan ahead, and think about possibilities rather than only actualities. Piaget believed this was the highest stage of cognitive development, and that not every adult necessarily reaches it fully.
Piaget’s four invariant stages: sensori-motor (0–2, object permanence), pre-operational (2–7, egocentrism, animism, no conservation), concrete-operational (7–11, decentration, reversibility, conservation gained), formal-operational (11+, abstract reasoning). The sequence is fixed; the ages are approximate.
The Core Study: Piaget (1952) and the Conservation of Number
Piaget wanted to find out at what point children truly grasp that a quantity stays the same even when its appearance changes. His conservation of number study, published in The Child’s Conception of Number (1952), is one of the two named core studies for this topic and should be revised as its own object: background, method, results, conclusions and criticisms.
Piaget used a clinical interview design with young children as his sample. The materials were two identical rows of counters laid out in one-to-one correspondence, so each counter in one row lined up directly opposite a counter in the other.
In the procedure, the child was first asked whether the two rows had the same number of counters, and confirmed that they did. While the child watched, the experimenter then spread out one row so it became longer, without adding or removing any counters, and asked the child again whether the two rows still had the same number.
Younger, pre-operational children said the longer, spread-out row now had more counters — they judged number by the length of the row rather than by logic, showing they had not yet decentred from the most visually obvious feature. Older, concrete-operational children correctly said the number had stayed the same, because none had been added or taken away.
Piaget concluded that younger children lack the logical operation of conservation because they cannot yet decentre or reverse the transformation mentally. This supported his claim that cognitive abilities appear in a fixed sequence, moving from pre-operational to concrete-operational thought as the child matures.
Evaluating Piaget’s Theory: The Reductionism/Holism Debate
One major criticism of Piaget’s conservation study relates to the reductionism/holism debate. A reductionist critique argues that Piaget’s task reduced a child’s complex understanding of number down to a single, narrow response to an artificial arrangement of counters, which may not reflect what the child truly understands about quantity in everyday life.
A more holistic view would say that a child’s grasp of number should be judged across many real-world situations — sharing sweets, counting toys, comparing groups of friends — rather than one clinical task. If the wording of the question or the unfamiliar setting confused the child, Piaget may have underestimated their true ability. Despite this, the theory remains hugely influential, particularly in shaping how education is structured around a child’s stage of readiness.
In Piaget’s (1952) study, two equal rows of counters were shown to children. When one row was spread out, pre-operational children said it now had more counters; concrete-operational children knew the number had not changed.