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Infancy and early childhood development are times of great change in a living organism, and they are perhaps the most formative years of development. Developmental progress may be measured in the following domains: physical, cognitive, emotional, and social. For example, certain common physical milestones often mark a child's physical and motor development. Psychology theorists like Jean Piaget, Lawrence Kohlberg, and Erik Erikson proposed stage theories for other aspects of development.
A child’s physical development and subsequent motor capabilities represent some of the most documented milestones in the child’s young life. Particular averages vary greatly, but in general certain major events mark the physical progression of a child. In early infancy, a child is mainly guided by instinctive reflexes. The body is small and vulnerable, so movement is limited.
The head and upper body are first to develop on a child, perhaps in order to accommodate cognitive development. As such, some of the first major physical progressions occur when the child can lift its head and sit up unsupported. This latter step occurs at roughly six months of age.
Once the infant’s lower body begins to more fully develop, motor functioning progresses to stages of increased mobility. The infant can generally roll over at about three months, with tentative crawling occurring in the latter part of the first year. By about 18 months, many infants have the ability to walk, with running occurring sometimes in the two year landmark.
During infancy and early childhood development, sensory abilities are also developing. While touch is consistently more highly developed than other senses, at around three months, most senses — especially vision — have significantly improved. Depth perception is a particularly important sensory milestone for an infant.
Emotional and moral development can also occur in stages for infancy and early childhood development. Lawrence Kohlberg's theory of moral development placed children in early childhood in stages one or two. As such, the emotions and decisions of children in these times are mainly guided by either fear of punishment or personal satisfaction.
One of the most prominent and studied cognitive development theories derives from Jean Piaget, which includes four primary stages of development: sensorimotor, preoperational, concrete operational, and formal operational. These stages refer to how a child begins to think and mentally process the world, and the first two stages make up infancy and early childhood development. According to this theory, the earliest stage, the sensorimotor stage, lasts until about the age of two, during which the child moves from simple reflex perceptions to being able to form mental images and the understanding that physical objects are real. The second stage of early childhood is called the preoperational stage, and a child remains in this stage until the beginning of middle childhood at about age six or seven. This stage is characterized by development of the child's ability to think in symbolic terms, although most of the child's thinking still revolves around the child and his or her needs.
Psychologist Erik Erikson marked a child's social development in eight stages, with the first three stages concerning early childhood. For the first year-and-a-half of the child's life, developing attachment — particularly to the mother — is important. The main social outcome the child will develop in stage one is either trust or mistrust. During stage two, the toddler will either develop independence or shame as he or she tests newly developed physical and mental capabilities. Starting at around age three to about five years of age, the child begins to define social roles through play-acting and taking greater imaginative initiative in activities.