Infancy

Compared to the young of other mammals, human infants are precocious in some ways—notably sensory development—and relatively helpless in others, such as physical strength and mobility. At birth, the average American infant weighs approximately 7.5 pounds (3.37 kg), although a baby born 28 weeks after conception may weigh as little as two pounds (0.9 kg). The average length of an American newborn is about 21 inches (53 cm).

Infants are born with several reflexes that are activated by particular stimuli, such as the grasping reflex when a finger is placed in the palm of a baby's hand. Other reflexes include rooting (turning the mouth toward the breast or bottle) and sucking. Many early reflexes—such as reaching and performing a step-like motion—disappear, only to reappear later. While the most important senses in human adults are vision and hearing, infants acquire much of their information about the world through touch. At birth, a baby's eyes and the pathways between the eyes and the brain are not fully developed; the eyesight of a newborn is estimated at 20/600 (an object viewed from 20 feet [609 cm] away appears as a distance of 600 feet [182 m] by an adult with 20/20 vision). The senses of newborns are particularly well adapted for bonding with their caregivers. Infants can see large objects close up and are especially interested in faces. Their hearing is most acute in the range of human speech.

In the first year, the shape and proportion of an infant's body are better suited to crawling on all fours than to walking erect. During the first three months of life, infants also lack the lower body strength and muscular control to support their weight sitting up unaided or standing upright. The urge to stand and walk upright is very strong, however, and babies work hard to accomplish this task. By seven to eight months, infants can usually stand holding on to a playpen or other object; at 10 or 11 months they can walk with assistance, and by 13 months, they can usually take a few steps unaided.

As infants are developing physically, they are also developing cognitively in their ability to perform such mental processes as thinking, knowing, and remembering. The theory of childhood cognitive development developed by the Swiss psychology Jean Piaget describes four stages of increasingly complex and abstract thought that occur between birth and adolescence, each qualitatively different from but dependent upon the stages before it. The first, or sensorimotor, stage, (birth to approximately two years), is a time of nonverbal, experimental basic learning when infants gradually gain mastery of their own bodies and external objects. By sucking, shaking, banging, hitting, and other physical acts, children at this age learn about the properties of objects and how to manipulate them. The main goal at this stage is to achieve what Piaget termed “object constancy,” or permanence: the sense that objects exist even when they are not visible and that they are independent of the infant's own actions. This sense forms the basis for the perception of a stable universe. The sensorimotor stage is followed by the preoperational stage (ages two to six), which involves the association of objects with words.

Infants are born with different temperaments. There are “easy babies,” who are cheerful and seldom fuss; difficult babies, who are often irritable; and timid babies, who are wary when approaching new situations. Most people believe that temperament is inborn, although there is little hard evidence to prove it. Temperament's interaction with a variety of environmental factors, including parental expectations, determines the course of an individual's development. The most important aspect of an infant's socialization is forming secure attachments, primarily to parents or other principal caregivers. Attachment problems may have a negative effect on a child's normal development. Initially, infants will respond positively to all contact with adults, even though they recognize familiar faces and prefer their primary caregiver. By the age of three months, babies will begin to smile in response to outside stimuli, maintain eye contact, and vocalize, as distinguished from crying. Eventually, they will advance to what Piaget called the “secondary level” of concentration, at which they are aware of social changes in addition to objects and events. During this period, infants enjoy social contact and will fuss when left alone. They are able to distinguish their parents from other people, will smile and vocalize at familiar people, and will cry when those individuals are absent. At the age of six or seven months, when infants begin to develop a conception of object permanence, an especially strong bond begins to form with the primary caregiver. This is accompanied by separation anxiety (distress at being separated from the primary caregiver) and stranger anxiety (shyness or fear in the presence of strangers). Such behaviors are an integral part of normal cognitive development and displays a healthy attachment to the primary caregiver.