Prior to the work of psychologist Jean Piaget, the predominant view of cognitive development in children was based on the idea that children's mental processes are roughly the same as those of adults. Empiricists postulated that the mental machinery in adults and children is exactly the same, but that children simply have fewer associations. Nativists believed that certain concepts, such as time, space, number, and causality are a priori knowledge; that is, infants are born with these basic categories already firmly in place. Piaget was the first to propose that children aren't simply miniature adults; their cognitive development occurs in a series of fixed stages. Just as the human fetus goes through dramatic structural changes during gestation, the human mind goes through dramatic changes during childhood.
According to Piaget, all cognitive development can be explained through assimilation and accommodation. An infant is born with certain limited mental schemas. Throughout his or her development, the child interprets new phenomena using existing schemas by assimilation, and changes existing schemas as a result of experience by the process of accommodation. These general processes of assimilation and accommodation are further grouped into specific developmental stages: the sensory-motor intelligence stage, the preoperational stage, concrete operational thinking and formal operational thinking.
The sensory-motor (or sensorimotor) intelligence stage lasts from birth through approximately 24 months of age. During this period, schemas consist primarily of sensory impressions and motor actions. The attainment of several important milestones mark this stage, namely, object permanence and overcoming the "A-not-B" effect. A newborn has little sense of object permanence, according to Piaget. Out of sight is literally out of mind. If the infant, for example, drops a toy out of sight, he or she will continue as if the toy never existed. By nine months of age, however, object permanence has been established, and an infant will search for lost toys. This searching has some limitations however, due to the A-not-B effect (sometimes called the A-not-B error). If a toy is hidden in location A in full view of an infant, the infant will automatically reach for the hiding place and discover the toy. This process is repeated several times. Afterwards, if the toy is hidden in a different place, location B, again in full view of the infant, he or she will still reach for the first hiding place. The infant has not yet established that the object exists in its own right, regardless of placement. He or she assumes that location is an integral characteristic of the object. Once the infant has mastered this A-not-B effect, he or she enters the preoperational stage.
The preoperational stage (two to five years of age) marks the emergence of pretend-play and egocentric perception. Though children in this stage will happily concoct fantasies about themselves, they are incapable of imagining the viewpoint of another individual. They assume that everyone sees the world exactly as they do. The most notable limitation of this stage is that preschoolers are unable to understand conservation of quantity or number. A preschooler who is shown two glasses of water, equal in size and shape, with equal amounts of fluid, can easily state that the amount of water in the glasses is the same. However, if, in front of the child, one of the glasses of fluid is poured into a glass that is narrower and taller, the preschooler will erroneously conclude that the new glass has more fluid in it, because the level of the water is higher.
Concrete operational thinking (seven to eleven years of age) includes the ability to appreciate conservation of quantity and number. However, children in this stage have extreme difficulty understanding abstract concepts. They can, for example, manipulate numbers using simple arithmetic, but cannot understand abstract patterns or rules behind this process. A child has no problem adding 4 + 1 or 2 + 1, but cannot grasp the notion that one added to an even number will always produce an odd number. This sort of abstract reasoning comes with the final stage, formal operational thinking, at around eleven years of age.
Piaget revolutionized the concept of cognitive development with his theory of stages, and he supported it with extensive experimentation. However, his theory has come under much criticism in recent times, due to several inconsistencies. One of the major criticisms of Piaget asserts that he severely underestimated an infant's native endowments. Several experiments have suggested that infants do indeed have some sense of space and objects at birth. One experiment studied the principle of occlusion in infants through the habituation procedure. An infant is shown a rod moving back and forth behind a block. This display continues until the infant is bored and looks away. The block is removed, and the infant is then shown either an unbroken rod that moves back and forth, or two broken rod pieces that move back and forth in alignment. The hypothesis was that if the infant could perceive occlusion (in other words, the assumption that the rod is actually whole, even though part of it was hidden behind the block), they would pay more attention to the broken rod (the more surprising situation), than the unbroken one. This, in fact, was the case, discounting Piaget's assertion that infants know nothing of objects in the external world and must learn these principles.
If infants do have a sense of objects in the external world, there must be some alternative explanation for Piaget's observations of the A-not-B effect. In fact, some researchers now think that this phenomenon has more to do with the infant's inability to coordinate his movements with his thoughts than an inability to perceive the location of the object. When, in an A-not-B effect experiment, the object is hidden in location B, the infant looks at location B while the object is placed there, and often continues to look at location B even while reaching for location A. This evidence casts suspicion on Piaget's explanation of the A-not-B effect, suggesting instead that the infant is indeed aware the object has moved from location A to location B, but is simply unable to override the motor response that is currently dominant, having already been prompted to reach for location A several times.
Finally, Piaget's precise definition of the cognitive stages as a result of the scientific method has come under intense criticism. Though cognitive development obviously occurs in some sort of sequence, it does not necessarily occur in the fixed stages that Piaget proposed. He set up distinct milestones that marked the end of each stage. If one uses the exact same tests that Piaget used, then his theory holds true. However, different tests that are more sensitive to an infant's responses or are specially geared and manipulated towards a certain age group prove that some of Piaget's theories are incorrect.
Yay for noding homework. Unfortunately, this assignment didn't require in-text citations, but much of it is paraphrased from:
Gleitman, J. Psychology. New York: Norton, 1995.