¶ … theorist discussing the cognitive development throughout the lifespan is Jean Piaget. The cognitive development of children is of interest to this writer as a budding pediatric psychologist. Piaget's model, while criticized, has stood the test of time and remains the basic model of cognitive development presented in developmental psychology textbooks as well as a being a general framework for research and theory development in the field (Egan, 2012). Piaget's overall stage theory is briefly discussed in this paper. Piaget's initial research subjects were children from birth to seven (he also studied people through the age of adulthood). In actuality, Piaget's early subjects were his own children; later he studied children of different ages under laboratory situations (Burman, 2008). In essence Piaget's methods were correlational and consisted of mostly a combination of observation and interview. He relied primarily of presenting the child with a task and observing the child solve the task and then asking the children questions on how they solved tasks. When studying play the child might be given a toy and observed or put in a room full of toys with other children and be observed. For other cognitive principles children were presented with different stimuli, for example a tall thin glass and a shorter stout one (both that held equal amounts of liquid) and then decide which had more water in it (conservation; Berk, 2011; Burman, 2008).

The results of Piaget's research was a stage model of cognitive development including four major stages (Berk, 2011): The Sensorimotor stage (birth to two years of age); The Preoperational stage (two to about seven years of age); The Concrete Operational Stage (seven to about eleven years of age); and The Formal Operational stage (eleven years and beyond). Each of the stages is marked by a particular style of thinking and reasoning. In addition, some stages are further broken down into sub-stages. A brief explanation of each stage follows (Burk, 2011; Egan, 2012):

Sensorimotor Stage: Birth to Approximately Two Years of Age

Many striking changes in behavior...

In this stage the child is focused in the here and now of its existence because it cannot consistently for mental representations of things or project into the future. So objects exist only when the child can actually sense them and when they are absent from the child's immediate experience to the child they are nonexistent. According to Piaget during the sensorimotor stage the child experiences the world through their senses and through movement. Children are extremely egocentric in this stage, meaning that they are unable to experience the world through perspectives other than their own viewpoint. The sensorimotor stage is divided into six sub-stages:
1. Simple reflexes (birth - one Month). Characterized by reactions to the environment with simple reflexes such as rooting and sucking.

2. Primary circular reactions (one -- four months). A circular reaction occurs when the infant attempts to repeat activities that create a desirable action. Primary circular reactions referred to the infant's tendency to focus on repeating bodily actions in this matter. Typically the action first occurs by accident in the child attempts to repeat it due to its desired effect. An example here would be thumb-sucking.

3. Secondary circular reactions (four - eight months). Children develop an awareness of things outside their own body and begin to focus on these "objects." For example, a child accidentally shaking arousal and then repeated the shaking the rattle to hear the noise is an example.

4. Coordination of secondary circular reactions (eight-twelve months). At this stage children begin to use their knowledge to produce and affect (intentionality) as opposed accidentally discovering things. An example here would be a child using a stick to reach foreign object.

5. Tertiary circular reactions (twelve-eighteen months). In the stage children begin to explore new possibilities. For example, a child may pick up a spoon, drop on the floor, and wait to hear what happens. The child may start banging various toys, foods, etc. To listen for the…