Piaget's Stages of Cognitive Development

Jean Piaget was born in Neuchâtel, Switzerland, on August 9, 1896, and died in Geneva on September 16, 1980. He trained as a zoologist, earning his PhD from the University of Neuchâtel in 1918, before turning to philosophy at the suggestion of his godfather, Samuel Cornut. As a boy of ten, he published his first scientific paper, an observation of an albino sparrow, in the Journal of Natural History of Neuchâtel.

To study how children reason, Piaget developed what he called the “clinical method,” a technique combining interview and experiment that he applied first to his own three children, Jacqueline, Lucienne and Laurent, observed from their earliest months. Those observations led to works such as The Language and Thought of the Child (1923), Judgment and Reasoning in the Child (1924) and The Origins of Intelligence in Children (1936).

Schema, assimilation and accommodation

At the center of Piaget’s theory sits the concept of schema: a mental structure that organizes and interprets information about the world, a kind of mental category where a child files what they encounter. Knowledge grows through two movements. In assimilation, new information is folded into a schema that already exists, without changing it. In accommodation, the opposite happens: the schema itself is revised, because the new information doesn’t fit as it is.

Practical example: a child who only knows dogs calls the first cat they see a “dog” too, assimilating it into the schema they already have. Once someone explains it’s a different animal, the child builds or adjusts a schema just for cats: that’s accommodation.

According to Social Sci LibreTexts, for Piaget learning proceeds through this alternation: fitting new experiences into concepts already held, and fitting those concepts to new experiences. OpenStax frames it in a similar way, describing schemata as mental models used to categorize and interpret information.

The four stages of cognitive development

Piaget describes cognitive development as a path through four stages, each with a different way of reasoning about the world.

The sensorimotor stage

In the first two years of life, according to Simply Psychology, children learn about the world entirely through their senses and physical actions. It’s during this period, around 8 months, that object permanence develops: the understanding that an object continues to exist even once it’s out of sight. Social Sci LibreTexts describes the same milestone as the belief that objects exist whether or not they are actually present.

The preoperational stage

Between ages 2 and 7, language and the use of symbols appear, but thinking still struggles with logic. Social Sci LibreTexts notes dramatic play in this period, the improvised make-believe typical of preschool children. The best-known trait of the stage is egocentrism: difficulty seeing things from someone else’s point of view, demonstrated by Piaget’s Three Mountains experiment, in which a child facing a model landscape keeps describing it as they themselves see it, even when asked how it would look to someone sitting elsewhere in the room.

The concrete operational stage

Between ages 7 and 11, children begin to grasp conservation: a quantity stays the same even if its appearance changes, as with a liquid poured into a different-shaped glass. Not every form of conservation arrives at once: according to Simply Psychology, number is typically the first quantity to be conserved, already around age 6, before the child is fully into the concrete operational stage. Social Sci LibreTexts adds two further abilities to this stage: reversibility, thinking through the steps of a process in any order, and decentering, the capacity to weigh more than one feature of a problem at a time.

The formal operational stage

From age 11-12 onward, thought becomes capable of working on abstract concepts, not just concrete objects. Simply Psychology describes the emergence of hypothetical and deductive reasoning, which lets adolescents tackle complex ideas like ethics, science or mathematics. Social Sci LibreTexts defines the formal operational stage as the period when a person can “operate” on abstract representations, not only on things in front of them.

The ages aren’t a fixed calendar

Sources describing Piaget’s theory agree on the first three stages but vary slightly on the last boundary: Simply Psychology places the start of the formal operational stage at age 12, while Social Sci LibreTexts moves it to 11. Piaget himself treated his ages as indicative, and the theory sits alongside other currents in developmental psychology that have tackled how thinking forms, from behaviorism, which looks mainly at external stimuli, to cognitivism, which describes the mind as an information processor. Piaget’s idea that children actively build their own knowledge underlies the school of thought known as constructivism, one of the references of modern psychological science applied to development.