A Swiss-born theorist, Jean Piaget, was the first developmental psychologist to use careful observation of toddlers and school age children to establish an integrative theory outlining the cognitive advances that children make as they experiment with the world around them.
His model is divided into four stages, the passage through which he theorized all healthy children progressed at a more or less uniform rate.
Sensorimotor Stage and Schemas
The first stage described by Piaget is the sensorimotor stage, occurring during the first two years of life. At this stage Children, "think," with their senses developing consistent schemas about the world and expectations about how their interaction with it will alter it. A child's understanding about the world is being formed as they experiment upon it by touching, tasting, seeing, and hearing what surrounds them. The development of these schemas he called, "adaptation."
Adaption is realized through the interplay of, "assimilation," and, "accommodation." Assimilation is the interpretation of interaction with the outside world and accommodation is the creation of new schemas and the integration of instances of exception from predictable schemas.
At this age, for example, children often drop things simply to see what will happen. As they come to appreciate a consistent result in the act of dropping objects they begin to become more creative in their object manipulation, now engaging in throwing both softly and forcefully, bouncing toys off walls or throwing them down stairs. In these new experiments we see accommodation. If a child let's go of a helium balloon and, contrary to their schema, watches rise instead of fall this is also an example of accommodation.
A healthy balance of assimilation and accommodation is important. Expected schemas being verified through play and exceptions to those schemas occurring at a frequency that the child is capable of processing results in cognitive equilibrium. If the occurrence of accommodation greatly out numbers instances of assimilation a child may experience what Piaget called, "disequilibrium." This is a kind of cognitive confusion and anxiety.
This is a very complex and formative stage and is best fostered by a variety of interactive objects and toys through which accurate and reliable schemas can develop.
Chance behaviors that result in interesting and pleasing outcomes, come to be repeated and circularly reinforced thus leading to goal directed behavior. The organization of separate schemas into larger explanatory schemas is a process Piaget termed, "organization."
Mental Representations of objects that are not immediately present begin to emerge toward the end of this stage lending the child an understanding of object permanence, Object permanence is the understanding that just because they cannot see an object hidden once it is hidden within another object the object has not ceased to be. They are beginning to keep people and objects in their mind as an image. They also begin to organize objects and information into categories or, "concepts," making them much more efficient thinkers who can assimilate a variety of experiences into a coherent and meaningful meta-schema.
During this stage, self-awareness, begins to emerge as children are able to recognize reflections of themselves as a representation of self rather than simply as sensory data that is separate from them. They begin to use short two word phrases and play simple games,
The Preoperational Stage
This stage spans the early childhood years of about 2 through 7. The ability for representation and an understanding of symbols greatly increases during this stage as does the propensity for make-believe play. A child under two years of age will often not use an object in play and pretend it is another object. After two years of age child will use whatever is handy to stand in for what ever object their make-believe play requires. For instance to a child under 2 a toy phone is a toy phone and nothing else while children over the age of 2 might pretend a toy truck is a toy phone. There is a great expanse of imagination and adaptability in play. This make-believe is very important in the process of ossifying schemas and in organizing a vast array of information drawn from observation and experience.
At age 2 1/2 to about 3 children can begin to appreciate pictures, maps, and models as objects that stand for something else. This is called dual representation in that the child can both recognize the object as something distinct in it's own right but also as a representation of something else.
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This stage is also defined by what children are not yet able to do. Egocentrism is a hallmark of this stage. Children are often unable to appreciate the view-points of others. They also show a tendency toward animistic thinking, that is believing that inanimate objects have thoughts, intentions, and wishes.
Piaget also concluded that they could not understand Conservation. The experiment he ran to verify this involved a tall slim glass filled with water and a round shorter class that the water is poured into. When asked if their was more or less water after the water was poured from the tall to the short vessel preoperational children often said that there was less water in the stout glass. They assumed this because the water level was lower in the stout glass than in the slim glass.
Children at early points in this stage have trouble with the concept of reversibility. They can accomplish a task in one direction but have trouble undoing the task through reversing the steps they took.
The Concrete Operational Stage
This stage last from about 7-11 and accompanies a great cognitive leap forward for children. The cognitive processes become markedly more flexible and logical than in the Preoperational Stage. Children now pass conservation tests with ease. They demonstrate an understanding of reversibility and can perform more complex tasks both forward and backward.
Children at this stage also develop complex cognitive hierarchies grouping objects and concepts with similar characteristics together. They are able to understand, for instance, that a number of different sports balls belong in a category together but they are also able to break this grouping into a more specific grouping perhaps by color.
The ability to order objects by quantitative dimensions (i.e. length, volume) is called seriation and also emerges during this stage. Spatial reasoning, including an understanding of buildings, neighborhoods, and how to navigate through them is greatly improved.
The Formal Operational Stage
This stage is reached typically by adolescence and is accompanied by cognitive abilities such as abstract and systematic thought.
Children at this stage are able to form and test hypothesis and then make deductions based on their observations. This is where the cognitive abilities required for scientific thought become honed.
"Propositional Thought," is characteristic of the formal operational stage. Children can now evaluate logical propositions. An experiment that illustrates this was done with poker chips. When the statements, "The chip in my hand is either green or it is not green" and, "The chip is my hand is green and it is red," children in the concrete operational stage would generally assent to both propositions provided the chip was hidden in the experiment's hand. Formal operational stage children on the other hand recognize the impossibility of the second statement. This shows the beginning of formal logic at work. Children at this stage also develop their ability for abstract thought by studying Algebra and Literature replete with metaphor, simile, and personification.
Limitations of Piaget's Model
This model has been mainly criticized for it's rigid step-wise model of progression. Many researchers envision cognitive development as a continuous progression rather than as a series of plateaus.
Further the limitations Piaget placed upon children's cognitive abilities at various stages have proven not to be an absolute inability to perform certain tasks at certain ages but more of a generalization with many exception. With a little added help and encouragement children are able to learn to perform tasks from more advance stages than this model allows.
While the model is far from perfect it does contain some very useful observation and generalizations about what can reasonably be expected of children at different ages. Understanding where a child is cognitively prevents adult proscription of unrealistic undertakings from burdening a child's developing sense of self-efficacy and self-esteem.