Around 1920, a young Swiss researcher named Jean Piaget was scoring children's answers on reasoning tests in Paris. He grew more interested in the wrong answers than the right ones: children of similar ages kept making the same mistakes. Piaget concluded that children aren't just less informed than adults. They think in a different way. He spent the next half century, including close observation of his own three children, mapping how that thinking changes.
Schemas, assimilation and accommodation
Piaget described knowledge as built from schemas, mental frameworks for making sense of things.
- Assimilation: fitting something new into a schema you already have. A toddler who knows dogs calls a cow a "doggy".
- Accommodation: changing the schema when the new thing won't fit. The toddler learns cows are their own category.
- Equilibration: the push to keep the two in balance. Being confused is what drives the change.
The key idea: children actively build their understanding. They aren't empty containers waiting to be filled.
The four stages
Piaget gave the ages as rough guides, not deadlines.
- Sensorimotor (about birth to 2). Babies learn through senses and actions. Object permanence, knowing things exist out of sight, develops gradually. Babies of about 8 to 12 months make the A-not-B error: they search where a toy was hidden before, not where they just saw it go.
- Preoperational (about 2 to 7). Language and pretend play take off. Thinking is egocentric: in Piaget and Inhelder's three mountains task, young children often picked their own view when asked what a doll could see. Children also fail conservation: they say the taller glass holds more water, because they focus on height alone (centration).
- Concrete operational (about 7 to 11). Children conserve, classify and order things logically, as long as the problem involves real, concrete objects.
- Formal operational (about 11 and up). Thinking becomes abstract and hypothetical. Teenagers can test ideas systematically, like changing one variable at a time to find out what controls a pendulum's swing.
Where the research is shaky
Piaget's broad picture, that children's thinking changes in quality and that children build their own understanding, is still influential. Several specifics haven't held up as he described them.
- Babies know more, earlier. Piaget tested object permanence by whether babies searched for hidden toys. Renée Baillargeon and colleagues measured where babies looked. In a 1985 study, 5-month-olds looked longer when a screen seemed to pass through a hidden box, as if surprised. How much looking-time studies reveal is still debated, but most researchers now think Piaget underestimated infants.
- The tasks made children look worse. In McGarrigle and Donaldson's 1974 "naughty teddy" study, more young children conserved number when a teddy messed up the counters "by accident" than when an adult moved them on purpose. McGarrigle and Donaldson argued that an adult's deliberate change signals that something important has changed, so children may answer about the change rather than the number.
- Stages are less tidy. Piaget himself noted that a child can conserve number before weight. Later researchers, such as Robert Siegler, argued that children use several strategies at once that rise and fall gradually, rather than switching in steps.
- Not every adult reasons formally. Research suggests many adults don't use formal operational reasoning consistently, especially outside areas they know well.
Vygotsky: the main contrast
Lev Vygotsky, working in the Soviet Union around the same time, saw development as more social. Children learn by working alongside more skilled people, and language is a central tool for thinking. Where Piaget saw young children's self-talk as egocentric, Vygotsky saw private speech as children guiding themselves. Many researchers today combine ideas from both.
Common mix-ups
- Egocentric doesn't mean selfish. It means not yet realizing others see things differently.
- Assimilation vs accommodation. Assimilation keeps the schema; accommodation changes it.
- The ages are approximate. Piaget cared about the order of stages more than exact birthdays.
Frequently asked questions
What are Piaget's 4 stages of development?
Sensorimotor (about birth to 2), preoperational (about 2 to 7), concrete operational (about 7 to 11) and formal operational (about 11 and up).
What is the difference between assimilation and accommodation?
Assimilation fits new information into an existing schema. Accommodation changes the schema because the information doesn't fit.
Is Piaget's theory still accepted?
Partly. His idea that children actively build understanding is still influential, but research suggests infants are more capable than he thought and that stages are less fixed.
How is Vygotsky different from Piaget?
Vygotsky put more weight on social interaction and language, while Piaget focused on the child exploring and reasoning on their own.
Try it
Pour the same amount of water into two identical glasses, then pour one into a taller, thinner glass. Before you think it through, notice which one looks like more. Your eyes still fall for it; your reasoning just overrides them.