Piaget's Theory and the Three Mountains Task
Why Do Children Think Differently?
Have you ever tried to explain something straightforward to a young child, only to find they seem to be reasoning from a completely different set of rules? The Swiss psychologist Jean Piaget argued this was not because children know less — it is because children think differently. His Theory of Cognitive Development proposed that intelligence develops through four stages, each qualitatively different from the last, with every child passing through them in the same fixed order.
Piaget’s approach was distinctive because he investigated how children reasoned, not just what they knew. By observing his own children and running systematic tasks — including the Three Mountains Task you will study later in this lesson — he built a picture of development he believed applied universally, regardless of culture or background.
Schemas, Assimilation, Accommodation and Equilibrium
Central to Piaget’s theory is the idea that we build knowledge through mental frameworks called schemas. A young child might have a simple schema for ‘dog’ — a four-legged, furry animal. When they meet a new dog, they recognise it using this existing schema. Fitting new experience into an existing schema without changing it is called assimilation.
But what happens when the child meets a cat? It is also four-legged, but clearly not a dog. Their existing schema does not fit, so they must change it or build a new one. Changing or creating a schema to account for new information is called accommodation.
Piaget called the balance between these two processes equilibrium — a comfortable state in which a child’s schemas can explain what they experience. When new information does not fit, the child is motivated to accommodate their thinking until balance is restored. This constant cycle of assimilation, accommodation and equilibrium is what drives cognitive development forward.
Assimilation = fitting new information into an existing schema. Accommodation = changing or creating a schema to fit new information. Equilibrium = the balanced state a child seeks to restore whenever new experience does not fit an existing schema.
The Four Stages of Development
Piaget proposed that every child passes through four fixed stages in the same order. The age ranges are approximate, but a child cannot skip a stage or reach a later one before completing the earlier ones.
In the sensorimotor stage (roughly 0 to 2 years), infants explore the world through their senses and physical movement, before they have language or symbolic thought. A key achievement of this stage is object permanence — understanding that objects continue to exist even when out of sight. Before this develops, an infant who watches a toy being hidden simply stops looking for it. Once object permanence is established, they will actively search for it.
In the pre-operational stage (2 to 7 years), children gain language and can use symbols, but their thinking has two important limits. They are egocentric — assuming everyone else shares their own viewpoint. They also 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 (7 to 11 years), children develop logical thought, but only when applied to real, tangible objects and situations. They master conservation and become less egocentric, beginning to appreciate that others can hold different views. Purely abstract reasoning still causes them difficulty.
In the formal operational stage (11 years onwards), adolescents develop the capacity for abstract thought — reasoning about hypothetical situations, planning ahead and considering possibilities rather than only what is directly in front of them. Piaget believed this was the highest stage of cognitive development, and that not every adult fully reaches it.
Piaget and Inhelder’s Three Mountains Task (1956)
Piaget and Inhelder (1956) designed the Three Mountains Task to find out whether children in the pre-operational stage were egocentric, as Piaget’s stage theory predicted.
Children sat in front of a table model showing three differently shaped mountains, each with distinguishing features such as a house or snow on top. A doll was placed at a different position around the model, giving it a different view from the child’s own. Children were then asked to choose, from a set of pictures, which image showed the view the doll could see.
Children younger than about seven typically chose the picture that matched their own view of the mountains rather than the doll’s view. Older children, closer to the concrete operational stage, increasingly selected the picture matching the doll’s actual viewpoint.
Piaget and Inhelder concluded that younger, pre-operational children are egocentric — unable to appreciate that another person has a different visual perspective — supporting the idea that this ability develops later, as children move towards the concrete operational stage.
The Three Mountains Task (Piaget and Inhelder, 1956) found that children under about seven usually picked their own view of the model, not the doll’s view, showing egocentrism. This supported Piaget’s claim that egocentrism fades as children move towards the concrete operational stage.
A strength of the task is that it used a controlled, standardised procedure, letting Piaget and Inhelder compare children of different ages fairly. A weakness is that the model and doll were unfamiliar and abstract for young children, so the task may have underestimated their true ability to take another’s perspective — later researchers such as Hughes used more relatable tasks (for example, hiding a boy doll from policeman dolls) and found even three- and four-year-olds could show less egocentrism than Piaget’s task suggested.
Evaluating Piaget’s Theory Overall
Piaget’s stage theory has been hugely influential in education, shaping how schools sequence learning to match children’s readiness. However, critics argue his methods, including the Three Mountains Task, sometimes underestimated children’s abilities because tasks were unfamiliar or confusingly worded. Later researchers such as McGarrigle and Donaldson found that when tasks were made more child-friendly, younger children could demonstrate abilities Piaget thought were beyond them. Even so, Piaget’s central insight — that children’s thinking develops through qualitatively different stages, not just by accumulating more facts — remains widely accepted.
Piaget’s four stages in order: sensorimotor (0–2, object permanence), pre-operational (2–7, egocentric, lacks conservation), concrete operational (7–11, logical with real objects, gains conservation), formal operational (11+, abstract reasoning). The sequence is fixed; the ages are approximate.