The period of early childhood begins with the final stages of infancy, between 18 and 24 months, and ends at about age five. During this period, rapid physical growth continues, although at a somewhat slower pace than in infancy. By five years of age, the average American boy weighs 43 pounds and is 44 inches tall. Girls tend to be slightly smaller. The correlation between height at age five and mature height is .70.
The preschool period is the time when childhood illnesses are at their peak, particularly colds and other respiratory diseases, which increase during the second year and drop off during the sixth year. Allergies and gastrointestinal complaints reach their highest point during the first year of life, and then decline sharply at the sixth year.
During the preschool years, the child shows increasing mastery over the skills acquired during infancy and picks up many new ones as well. Jumping, climbing, and tricycle riding occupy a considerable portion of the preschooler's time. Although some skills are learned, others are the result of maturation. For instance, children who were given supplementary practice in ball-throwing were no more skillful than children who depended purely on maturation and self-initiated practice to acquire the skill. The child passes through four general stages of motor development in learning a skill: (1) when no attempt is made to carry out the motor skill; (2) when the skill is in the process of formation; (3) when the basic movements have been achieved; and (4) when there is skillful execution with variation in use of the skill.
About the middle of the second year, the infant enters the early stages of what Piaget calls preoperational thought. Symbol manipulation, especially in the form of language, now becomes quite important. The ability to allow symbols to represent reality enables the preschooler to consider a broad sweep of events and objects and to think about them. Young children tend to ascribe life to inanimate objects--animism. According to Piaget, animism develops in stages, like other forms of cognitive activity. Piaget also maintains that preschoolers tend to distort their thinking by "centering," or attending to only one feature of an object. This tendency leads to confusions and interferes with conservation. Young children will say that water poured from one container into another container of a different shape will have changed its quantity. Research generally supports Piaget's contentions, although English and American children apparently enter and proceed through cognitive stages more rapidly than Piaget's reports would lead one to expect. There is also some indication that the child's ability to verbalize his observations has an effect on the experimenter's judgments about his cognitive status. Experience, too, is important in the child's ability to develop conservation. A study of Mexican potters' children indicated