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The Swiss researcher Jean Piaget (pea-ah-ZHAY) was one of the most influential figures in developmental psychology. Piaget died in 1980 at the age of 84. He published his first article in 1904 at the age of 8.

Kaye (1980) pointed out revealing passages in a long-neglected novel published by Piaget in 1918. They show that by the age of 20, Piaget already saw himself carrying out something like a divine mission to study the origins of human knowledge.

Who was Piaget? How did Piaget describe himself and his concerns?

Ironically, considering his impact on psychology, Piaget did not consider himself a psychologist. He started his career as a biologist, and by late in his career preferred to call himself an epistemologist.

Epistemology is the study of know­ledge. It is much older than psychology, and it is part of philosophy, not an experimental science. Piaget said he was a genetic epistemologist, meaning he was interested in the origins of knowledge.

sketch of Piaget
Jean Piaget

Piaget did not do experimental research in the modern sense, using control groups and statistical analyses. He employed what he called the clinical method, impro­vising conver­sations with children to understand their unique mental worlds.

Piaget's experiments are what most psychologists would call demonstrations. A child is asked a few questions or given a simple task to perform. Despite their informal nature, many of these demon­strations are quite revealing.

The Sensory-Motor Period (0-18 months)

Piaget labeled the first stage of cognitive development, that of a newborn, the sensory-motor period. It lasts from birth to about 18 months of age.

At first (Piaget believed) the infant is unable to differentiate itself from the environment. In the first months of life it comes to recognize the existence of people and things separate from itself.

Babies developed intentionality (goal-directed behavior) during the sensory-motor stage. They learn to execute simple goal-directed plans, such as reaching out and grasping a pacifier.

What are characteristics of the sensory-motor period, in Piaget's scheme?

Piaget pointed out a phenomenon that surprised people at first. Infants act as though a hidden object ceases to exist. To very young children—younger than about 6 months—out of sight is not only out of mind but also out of existence.

If a toy eagerly pursued by a 4 month old slips under a blanket, it is no longer pursued...even if it makes an obvious bump. A three month old baby loses interest in a toy once it is hidden behind a curtain. Piaget said such a child lacks object constancy: the awareness that a hidden object continues to exist.

What is object constancy?

By the age of one, a baby will brush a blanket aside and attempt to locate a toy hidden there. This shows the baby has developed object constancy. The baby now has a mental representation of the object: a mental structure or schema.

This developmental sequence is basic and primitive, and there is no reason to expect it would be limited to humans. Wise, Wise and Zimmermann (1974) showed that an infant rhesus monkey goes through the same process.

The Conservation Experiments

During the pre-operational stage defined by Piaget, a child fails to pass a series of tests called conservation experiments. Piaget is perhaps better known for these demonstrations than anything else.

To conserve in Piaget's terminology is to preserve something internally, namely, a representation of an abstract concept. The conservation experiments all require a child to demonstrate possession of some idea or concept about reality that develops around the ages of 5 to 7.

A classic conservation experiments involves liquid quantity. A child is first shown two short, fat beakers. They are filled to identical levels with colored water, as the child watches.

The child is asked to say when the two beakers have the same amount of water in them. If necessary, the experimenter pours a bit of liquid from one to the other until the child agrees the level of colored water is the same in both beakers.

Next the adult takes a tall, thin beaker and pours colored water from one of the short, fat beakers into it. The child is asked to compare the tall thin beaker to the short fat one.

The experimenter asks, "Which contains more, or do they contain the same amount?" Most children under the age of 6 will point to the tall beaker and say, "This one has more in it." The child is swayed by the perceptual cue of height.

Such a child lacks a conception of the quantity of a liquid. Not having this notion, the child has no way to realize that something stays the same when liquid is poured from one vessel to another.

What stays the same? Adults call it liquid quantity. We know what liquid quantity is; we measure it in quarts or liters, and we take it for granted. But a child does not yet have this concept or schema.

What does it mean to say a child can conserve something, in Piaget's terminology? What is the classic conservation experiment involving liquid quantity?

Some students find it hard to believe that a child could be fooled so easily by appearances at the age of 4 or 5. They go home and try the experiment with a little brother or sister, only to verify Piaget's basic finding. Little children are swayed by external appearances.

What were some other conservation experiments by Piaget?

Piaget described several conservation experiments. For example, in his conservation of mass experiment, a bit of clay (which Piaget called plasticene) was rolled into a ball. A second ball of clay the same size is shown to the child, who agreed they were equal.

Then the adult rolls one of the balls into a sausage shape. "Now does one have more clay in it than the other, or do they still have the same amount of clay in them?" The small child typically points to the sausage shape, which is longer, and claims that it has more clay in it.

Conservation of area is tested by asking the child whether more ground is covered by the blocks that are spread out or the blocks that are close together. Non-conserving children tend to assume the blocks cover more area when spread out, even though this opens up areas between them.

Placing coins such as pennies on a table tests conservation of number. Seven or eight coins are placed in a row, and a matching series of coins is placed directly below the first one. With the two rows of coins lined up, one to one, the child agrees there is an equal number in both rows.

Now the experimenter spreads out one of the rows so the coins are farther apart. The child is asked whether the two rows of coins still have the same number of coins in them. A small child who does not yet know how to count will typically claim that the spread-out row has more coins in it.

All the conservation experiments are variations on a theme. The word conser­vation means preserving something in the face of change. To come up with the correct answer in a conservation exper­iment, the child must preserve something in his or her head. That something is an awareness of quantity, mass, number, area, or other abstract characteristic of reality.

That was Piaget's point. He said he was studying the construction of reality in the child, which was the title of one of his books.

Appearance vs. Reality

John H. Flavell of Stanford University agreed with Piaget that children go through a progression of changes as they learn to distinguish between superficial appearances and a deeper reality.

Here is an example from Flavell (1986). An adult takes a red car and shows it to a child, who can see it is red. Now the adult places the car behind a plastic filter that makes it appear black.

If asked, "What color is this car? Red or black?" a 3-year-old will say "Black" but a 6-year-old will say "Red." This occurs even if the adult carefully phrases the question in a way that emphasizes the appearance/reality distinction. For example:

"I don't want you to tell me what it looks like to your eyes right now; I want you to tell me what the color really and truly is."

The 3-year-old will nevertheless say, "Black." To Flavell, this shows that the 3-year-old lacks the distinction between how things presently appear to the senses and how or what they really and enduringly are: the distinction between appearance and reality.

How did Flavell study the "appearance/reality distinction"?

"The six-year-old possesses some knowledge about this distinction and quickly senses what the task is about. The three-year-old, who is much less knowledgeable about the distinction, does not." (Flavell, 1986)

Flavell argues that learning the distinction between appearances and reality is "probably a universal developmental outcome in our species," essential for basic coping in the world. We distinguish fantasy, play, satire, and all forms of acting from "the real thing."

The distinction between appearance and reality also seems to be the core issue underlying Piaget's conservation exper­iments. The conservation experiments deceive three- and four-year-olds precisely because children of that age are swayed by appearances. They do not grasp the idea of a reality that exists apart from appearances.

The Concrete Operational Period (5-12)

Suddenly, around the age of 5 to 7, the average child catches on to the conser­vation experiments. This marks the transition to a new stage of development that Piaget called the concrete operational period. Now the child can understand simple operations performed on concrete reality, such as pouring a beaker of water from one vessel to another.

What happens at the beginning of the concrete operational period?

Now the child easily passes the same tests he or she failed a year earlier. Piaget felt the child had jumped up to a new stage of development. The child has mental representations or schemas (schemata) for liquid quantity, mass, number, and other such concepts.

Some conservation problems are easier than others, but within a year or two they are all mastered. In fact, the child now is likely to wonder why an adult is asking such silly questions. "Of course the two beakers have the same amount of liquid in them..."

What explanations did Piaget accept as indicating true understanding?

Piaget and his co-workers did not call a child a conserver until the child could explain the conservation experiment. This guarded against children learning to say, "Yes, they are equal" to please an adult, or based on some subtle cue, without really understanding.

Piaget identified three types of explan­ation that he considered good evidence of true understanding by the child:

1. Reversibility ("You can put it back like it was.")

2. Compensation ("It's taller but it's also skinnier.")

3. Identity ("You didn't change it; it's still the same.")

All these explanations showed the child now had the certainty of something remaining constant, even as appear­ances changed in superficial ways.

The Formal Operational Stage (12-adulthood)

Around the age of 12, Piaget said, children go into a new, higher stage of cognition. The adolescent's mind becomes able to manipulate complex mental representations.

Young adults become able to think in terms of abstractions (using ideas instead of concrete objects) in ways that formerly eluded their grasp. They can reason about hypothetical situations with precision and realism.

Assessing Piaget

American developmental psychologists became intensely interested in Piaget's theory during the 1960s. Piaget influenced educational psychology, especially. He made it clear that children must develop logical concepts at their own pace, when they are mentally ready.

One implication is that there is no need to rush things by trying to teach a child abstractions the child cannot yet appreciate. Children have natural limits on what they can learn at early ages.

What is an influence Piaget continues to have upon educational psychology? What are some criticisms of Piaget?

Betsy reading a Piaget textbook at less than one year old
Piaget showed that educators need not attempt to introduce high level concepts to small children.

Perhaps the most severe criticism of Piaget is that he did not perform well-controlled scientific experiments. His conservation experiments were not experiments in the modern sense. He did not gather statistics or analyze his data like modern researchers.

Piaget ignored or rebuffed these criti­cisms. By the time he drew the attention of American psychologists, Piaget was already a highly respected authority figure in Europe. He did not feel obligated to live up to the standards of American psychologists with their obsessive concern for scientific controls.

Piaget may not have been a great practitioner of scientific method, but he was a good observer and original thinker, and he had a big impact on devel­opmental and educational psychology.


Flavell, J. H. (1986) The development of children's knowledge about the appearance-reality distinction. American Psychologist, 41, 418-425.

Kaye, K. (1980, November). Piaget's forgotten novel. Psychology Today, p.102.

Wise, K. L., Wise, L. A., & Zimmermann, R. R. (1974). Piagetian object permanence in the infant rhesus monkey. Developmental Psychology, 10, 429-437.

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