The most fundamental theorist in this area is Jean Piaget. Additionally, Piaget demonstrated one of the first scientific movements in the filed, with the utilization of direct observation as the best tool for understanding. (Piaget, 1962, p. 107) Piaget also believes, and his theories reflect that children play a very active and dynamic role in development through interaction with their environment and active role imitation. (Piaget, 1962, p. 159) Sensory-motor intelligence is, in our view, the development of an assimilating activity which tends to incorporate external objects in its schemas while at the same time accommodating the schemas to the external world. A stable equilibrium between assimilation and accommodation results in properly intelligent adaptation. But if the subject's schemas of action are modified by the external world without his utilising this external world, i.e., if there is primacy of accommodation over assimilation, the activity tends to become imitation. Imitation is thus seen to be merely a continuation of the effort at accommodation, closely connected with the act of intelligence, of which it is one differentiated aspect, a temporarily detached part. (Piaget, 1962, p. 5)

Piaget's stage theory consists of four stages, heavily weighted by the act of imitation for assimilation and then moving forward to formal thought and independent abstract thought:

Sensorimotor stage, from birth to two years, consists of active involvement by the child in watching imitating and then eventually manipulating the environment through imitation of sound and physical actions, the goal of the stage is to develop object permanence, where the child is aware that an object or person exists even when they are not in direct view at any given moment. The Preoperational stage, from two to seven years where the child cannot conceptualize abstracts but instead needs direct physical examples and activities to further development. The Concrete operations stage, seven to eleven years, where physical experiences accumulate and allow the child to conceptualize and create logical structure to explain the environment. Some abstract problem solving is also possible at this stage. The final stage is formal operations stage, ages eleven to fifteen where a child's cognitive skills have developed enough to make them capable of many abstract thoughts and where their thoughts are most like adults and can include reasoning. Each of these four stages is also broken down into more stages creating one of the most comprehensive of stage theories. (Piaget, 1962)

Stage theories have also been applied to physical as well as psychological development and physical development is often a basis of judgment regarding overall development, as it has been postulated and correlated frequently that those who develop abnormally or slowly physically often also have developmental delays in other areas. (Ulijasjek, Johnson, Preece, 1998, p. 195) Physical development delays can be associated with genetics, nutritional deficiencies, disease or even environmental exposure but they often occur in conjunction with psychological, behavioral or sociological delays in development.

Behavioral Theories:

Behavioral theories are also indicative of observation, possibly more so that the observations conducted by Piaget, and conducted in less anecdotal and more scientific a manner. Watson, Pavlov and Skinner are the main theorists of the behavioral models, which conclude that behavior is determined by a set of repeatable factors, such as instinct that drive reactions. According to the behavioral theorists behavior is driven by rewards, punishments, stimuli and reinforcers and can be developed through conditioning. The two main types of conditioning are classical and operant. Watson, Pavlov and Skinner among others are owned significant credit for furthering his new view on development, as it stepped outside the introspective nature of the previous forms of psychology and stressed observation and objectivity, in the place of assumption in some ways.

Todd & Morris, 1995, p. xv) These behaviorists as they came to be known spent a great deal of time observing animal behavior but also applied their findings and research to humans, when it was allowable. The behaviorists, on the whole argued that individuals reacted to their environment based on a concrete set of rules having to do with reward and sanction and that they would continue to do so based on innate sets of biological rules of survival and the desire to meet certain higher and lower needs. Additionally, the child according to a behaviorist can and will only have unwanted behavior changed if it is replaced by more acceptable behaviors that are conditioned in the same manner as those that are more problematic. New behaviors can be associated with previously neutral stimulus...

Operant conditioning is on the other hand often associated with short or long-term rewards or sanctions and behavior is modified to gain reward or avoid punishment. (Catania, 1995, p. 192)
Social Development Theories:

The group of social development theories demonstrate the foundational idea that children do not develop in a vacuum but are constantly in an social environment, interacting with others. Bandura, Erikson, Freud and others are said to have been social development theorists as their work acknowledged the social aspect of human development and stressed emotional development as a result of socialization, either positive or negative. Bandura stresses the functioning that modeling or imitation plays in the development of emotional social skills. Though Bandura's work develops over a large spectrum of research and theory one of the most significant stresses of the researcher was that parenting behaviors serve as a model for child behavior and that punishment and aggression serve as a standard for the child to model through similar aggression. (Rubin & Burgess, 2002, p. 398) While Freud contends that aggression is a result of inappropriate social development, in the overall relationship between the mother and son and the father and daughter, Oedipus and Electra complexes, that develop the idea that at different times the child either seeks to emulate or replace the parent, in a gender role manner. If these challenges are not met, Freud contends that the individual will likely feel a sense of aggression toward the gender which they had the most problem with. (Guntrip, 1971, p. 40) (Zillmann, 1979, p. 52) Erikson as has been noted previously believed that each stage of human development had a certain conflict associated with it and if such a conflict was not met with the opposite resolution then aggression may result.

Erikson (1962) contributed the next major milestone in crisis intervention theory with the 1950 publication of Childhood and Society. Erikson's theory revolves around the notion of specific crises characterizing each developmental stage of an individual's life. His contribution was the notion of crisis as a normal developmental phenomenon and that intervention which leads to a balanced resolution at the time of a crisis would prevent later problems in emotional development and maturation. (Sandoval, 2002, p. 4)

Social development theory is inclusive of the need to understand the standards by which individuals resolve social and emotional conflict. Social theorists contend that conflict is a natural part of the development and learning process and that the resolution of such conflict will determine later socialization skills.

Infancy and Early Childhood:

As more and more attention has been placed upon the very early stages of development, and events and occurrences that can effect later behavior many theorists have developed rather specific ideals relating to the dynamic development of infancy and early childhood. Therefore many theorists, in all the above and below categories can also be grouped among those who paid particular attention to infancy and early childhood development. John Bowbly is one of these theorists, as he is known as a social theorist an attachment theorist but he paid very close attention to early childhood development. His work demonstrates a mark in the development of a greater understanding of how very early socialization determines later behavior. Bowbly stresses the importance of healthy parenting as a dynamic resolution to potential problems individuals may face as adults.

At some time of their lives, I believe, most human beings desire to have children and desire also that their children should grow up to be healthy, happy, and self-reliant. For those who succeed the rewards are great; but for those who have children but fail to rear them to be healthy, happy, and self-reliant the penalties in anxiety, frustration, friction, and perhaps shame or guilt, may be severe. Engaging in parenthood therefore is playing for high stakes. Furthermore, because successful parenting is a principal key to the mental health of the next generation, we need to know all we can both about its nature and about the manifold social and psychological conditions that influence its development for better or worse. (Bowlby, 1988, p. 1)

Bowlby was especially interested in attachment, between child and caregiver and seeking to understand the dynamic of how early attachment or lack there of can effect a child into adulthood. In fact in his foundational work, a Secure Base: Parent-Child Attachment and Healthy Human Development, he stresses the history of attachment theory as one that was driven by exclusive observations of the development of children who had been…