Jean Piaget was a Swiss psychologist who studied the development of cognitive processes from infancy through adulthood. Piaget often spoke about the relationship between cognitive development and language skills, but he was never exclusively focused on childhood language development. Piaget's theories have been extremely influential on psychologists studying early childhood.
The Sensorimotor Stage
According to Piaget's theory, all children develop cognitive abilities such as language in four stages. In the sensorimotor stage, which lasts until the child is around 2 years old, the emphasis is on movement and physical reactions. Small babies don't realize they can control their own bodies, so much of their play is initially based on figuring out how to perform basic motor activities like opening the fingers or waving the legs followed by more complex tasks like crawling and finally walking. At this early stage in cognitive development, Piaget saw language skills as basically physical. The baby experiments with what her mouth can do just as she experiments with what her hands can do. In the process she learns how to imitate some of the sounds she hears her parents making and in what context those sounds should be made.
The Preoperational Stage
The preoperational stage begins at around 2 years and lasts until the child is 6 or 7. The defining feature of this stage, in Piaget's view, is egocentricity. The child seems to talk constantly, but much of what he says does not need to be said out loud. For instance, the child might describe what he is doing even though others can easily see what he is doing. He shows no awareness of the possibility that others have a viewpoint of their own. Piaget sees little distinction at this stage of development between talking with others and thinking aloud.
The Concrete Operational Stage
The concrete operational stage begins around age 7 and lasts until at least age 11 or 12. At this stage, the child is capable of using logic and of solving problems in the form of stories as long as the story deals only with facts rather than abstract ideas. Language at this stage is used to refer to specific and concrete facts, not mental concepts. Piaget believed that some people remain in this stage for the remainder of their lives, even though a child in this stage has not yet reached full cognitive maturity.
The Formal Operational Stage
The formal operational stage begins at age 11 or 12 at the earliest. At this stage, the child can start to use abstract reason and to make a mental distinction between her self and an idea she is considering. Children who have reached this stage can use language to express and debate abstract theoretical concepts such as those found in mathematics, philosophy or logic. Piaget believed that these four stages of cognitive and linguistic development were universal and that no children ever skipped over one of the four steps.