Piaget's stages of cognitive development, and the age later research moved

Piaget's stages of cognitive development are four periods in the way children think: sensorimotor (birth to about 2 years), preoperational (about 2 to 7), concrete operational (about 7 to 11 or 12) and formal operational (from about 11 or 12 onward). Piaget held that the order is the same in all cultures, although the age at which a stage begins can vary. The best-known age in the theory belongs to object permanence, the knowledge that an object still exists when you cannot see it. Piaget tied it to infants who search for a hidden toy, which starts at about 8 months. In 1987 Renée Baillargeon reported that infants about 4½ months old already looked longer at an impossible event than at a possible one.

The ages need more care than the names. Textbooks draw the boundaries in slightly different places, and the object permanence age is the one that later research moved. This article gives the stages, then the experiment, then what still holds up.

• The order of the four stages is fixed in Piaget's theory. The ages are approximate, and sources differ by a year at the boundaries.
• Piaget treated object permanence as gradual: about 8 months for retrieving a covered object, about 12 months for the A-not-B task and about 18 months for invisible displacement.
• Baillargeon (1987) found that 4½-month-olds, and 3½-month-olds who habituated fast, looked longer at an impossible event. She read this as an early understanding that objects keep existing.
• Other researchers argue that looking time can reflect familiarity with the display, so the reading of the 4-month result is still debated.

The four stages at a glance

Piaget's theory says that a child's thinking changes in kind, not only in amount. The APA Dictionary of Psychology states that each stage builds on the one before. It adds that the age of onset might vary because of cultural and historical factors, while the order is the same in all cultures.

The same entry names two processes that drive the change. Assimilation fits new information into an existing schema, and accommodation changes the schema. OpenStax describes schemata as mental models for categorising information.

Sensorimotor stage: birth to about 2 years

In the sensorimotor stage, infants learn through their senses and their own movements. The best-known idea of the stage is object permanence, which the APA Dictionary defines as knowledge of the continued existence of objects even when they are not directly perceived.

Piaget judged object permanence by search. Baillargeon's 1987 paper summarises his evidence: before about 9 months, infants do not search for an object they watched being hidden. Cover a toy with a cloth, and they make no attempt to lift the cloth and grasp the toy, although they can perform each of those actions. Piaget speculated that for young infants, objects stop existing when out of sight and begin anew when they return.

He treated the ability as gradual. The APA Dictionary lists three milestones: retrieving a covered object (about 8 months), retrieving an object from location B after it was hidden several times at location A, the A-not-B task (about 12 months), and finding an object after an invisible displacement (about 18 months).

Sources do not agree on one number. The APA Dictionary says about 8 months for the first search, Baillargeon's paper says about 9 months for Piaget's claim, and OpenStax says object permanence develops between 5 and 8 months. For an exam, use the figure in your own course text.

Preoperational stage: about 2 to 7 years

In the preoperational stage, children use symbols to stand for words, images and ideas. Pretend play is typical of this stage: a stick becomes a sword. OpenStax names two limits, egocentrism and the lack of conservation.

The name is easy to misread as "not logical at all". In Piaget's vocabulary, operational refers to the logical manipulation of information, and a preoperational child cannot yet do this mentally. OpenStax adds that the child's logic rests on personal knowledge of the world so far, not on conventional knowledge.

The standard test is conservation. The APA Dictionary defines conservation as the awareness that physical quantities do not change in amount when their appearance changes, as when water is poured from a wide, short beaker into a thin, tall one. A child who has not yet developed conservation thinks the amount has changed.

Egocentrism, in the Piagetian sense, is the tendency to perceive a situation from your own point of view and to believe that others see it the same way, according to the APA Dictionary. Piaget's Three-Mountain Task tests it: the child views a three-dimensional mountain scene and says what a person at another position would see.

Concrete operational stage: about 7 to 11 or 12 years

In the concrete operational stage, children think logically about real objects and events. They master conservation. They also understand reversibility: pour the water back, and you have the same amount again, apart from a few drops. The APA Dictionary adds that they become less egocentric. OpenStax states the limit of the stage: thinking is logical only about concrete events.

Formal operational stage: from about 11 or 12 years

In the formal operational stage, thinkers deal with abstract ideas and hypothetical situations. They can use abstract thinking to solve a problem, look at alternative solutions and test them, as OpenStax describes. The APA Dictionary names the skill hypothetico-deductive reasoning. OpenStax says Piaget placed this development between 11 and 20 years of age.

Ages are ranges, not cutoffs

The table uses the APA Dictionary's figures. OpenStax draws some lines differently. Its text starts the formal stage at about age 11, but its table says 12. Its table gives 2 to 6 for the preoperational stage, but its text says 2 to 7. The differences are small. They show what the numbers are: averages from the literature.

These ranges are not milestones to test a particular child against. A parent who is worried about a child's development should talk to a paediatrician or another health professional, not compare the child with a stage chart.

What later research moved: object permanence at about 4 months

Baillargeon's drawbridge experiment

Baillargeon's 1987 paper, published in Developmental Psychology, built on a study with 5½-month-olds by Baillargeon, Spelke and Wasserman (1985). The method does not need the infant to search. An infant who cannot yet reach and search can still look, and surprise should lengthen the look.

The infants watched a solid screen rotate back and forth through a 180° arc, like a drawbridge, until their interest dropped. Then a box was placed behind the screen. Two test events followed:

• In the possible event, the screen rotated until it reached the box and stopped at 112°.
• In the impossible event, the screen rotated the full 180°, as if the box were no longer there. A hidden platform lowered the box out of the way.

In Experiment 1, 24 infants aged from 4 months 2 days to 5 months 2 days took part. Half saw the events with the box, and half were in a control condition without the box. With the box, infants looked at the impossible event for a mean of 29.2 seconds and at the possible event for 17.7 seconds. In the control condition the means were 15.1 and 16.2 seconds. The control matters: the infants did not simply find the longer rotation more interesting.

Experiments 2 and 3 tested infants about 3½ months old. As a group they showed no reliable preference. The fast habituators did: these were the infants who reached the habituation criterion in six or seven trials. Experiment 3 replicated this with a toy, Mr. Potato Head, in place of the box. The slow habituators looked about equally at both events.

What she concluded, and what she did not

Baillargeon concluded that the infants understood two things: the box continued to exist after the screen hid it, and the screen could not rotate through the space the box occupied. Her abstract says the results call into serious question Piaget's claims about the age at which object permanence emerges and about the processes behind it.

She also stated the limits. The finding does not mean that a 3½-month-old's idea of hidden objects is as sophisticated as an older infant's. And she offered a reason why Piaget's task gave a later age. Searching requires coordinated actions on separate objects, which begin at about 9 months, so young infants could fail the search task for that motor reason alone. Why infants find such action sequences hard, she added, "remains somewhat of a mystery".

She left the split between fast and slow habituators open: slow habituators may have been less alert, or fast ones may have formed their expectations earlier. Only longitudinal data could decide, she said.

In her reading, looking time showed an understanding that search could not yet show. The study did not show that 4-month-olds search for hidden objects.

How firm the finding is

Looking time is an indirect measure. The researcher infers surprise from a longer look, and that step has been challenged.

Thomas Schilling used the rotating screen design in 2000. After seven familiarisation trials with the 180° screen, 4-month-olds looked longer at the 180° test events than at the 112° test events. After twelve trials they looked longer at the 112° events. Six-month-olds with seven trials showed no preference. Schilling concluded that looking in the test trials depended on the familiarisation and on age, "not necessarily an inferred violation of physics".

Schöner and Thelen (2006) built a computer model of infant habituation and ran it on Baillargeon's drawbridge experiments of 1986 and 1987. They report that it explains the results "without invoking infant object knowledge", and that effects attributed to conceptual knowledge "may be explained by the dynamics of habituation".

Neither study shows that infants lack object knowledge, and one is a simulation. Both show that looking time alone does not settle what infants know. The textbooks also state the other side. The APA Dictionary says research with nonreaching tasks suggests that infants show some knowledge of object permanence earlier than Piaget proposed.

A careful summary: Piaget's ages describe when infants search for hidden objects. They do not settle when infants first expect objects to keep existing, and the studies disagree about how early that expectation appears.

What still holds up

OpenStax still teaches the four stages with object permanence, egocentrism and conservation, and it puts the criticisms in the same section. In its account, the criticisms take three forms:

• Development looks more continuous than a set of discrete stages (Courage and Howe, 2002; Siegler, 2005, 2006).
• Many children reach cognitive milestones earlier than Piaget described (Baillargeon, 2004; de Hevia and Spelke, 2010).
• Many developmental psychologists suggest a fifth, postformal stage in adulthood (Basseches, 1984; Commons and Bresette, 2006; Sinnott, 1998).

Baillargeon's result challenges the timing and the explanation, not the existence of a sensorimotor period. A fair exam answer: Piaget described a sequence of qualitative changes in thinking, later research suggests that children reach milestones earlier, and it questions whether the changes come in sharp steps.

Learning the four stages and their terms

Our guide to the method of loci builds a memory-palace route for exactly these four names. It also shows where the method stops: the images keep the names, not the meanings. Terms that look alike are easier to hold apart when you learn them as a group.

The vocabulary of this topic is a small set of related terms: sensorimotor stage, object permanence, A-not-B task, egocentrism, conservation, reversibility and habituation. In Psychology World, you look up a term and save it to one of your own lists. Then you drill the list as flashcards, by typing the answer, or as multiple-choice questions once the list has four or more terms. The four steps of the learning loop and what each study mode does are on the home page.

Piaget describes how children think. Attachment theory describes how infants bond with a caregiver, which is a different part of developmental psychology. The four attachment styles explains where those names came from.