Differences in rate of development between the two hemispheres suggest that they are continuing to lateralize (specialize in cognitive functions)

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For more than a decade, my fourth-floor office window overlooked the preschool and kindergarten play yard of our university laboratory school. On mild fall and spring mornings, the doors of the classrooms swung open, and sand table, easels, and large blocks spilled out into a small courtyard. Alongside the building was a grassy area with jungle gyms, swings, a playhouse, and a flower garden planted by the children. Beyond it lay a circular path lined with tricycles and wagons. Each day, the setting was alive with activity.

The years from 2 to 6 are often called “the play years,” since play blossoms during this time and supports every aspect of development. Our discussion opens with the physical attainments of early childhood—growth in body size and improvements in motor coordination. We look at genetic and environmental factors that support these changes and at their intimate connection with other domains of development.

Then we explore early childhood cognition, beginning with Piaget’s preoperational stage. Recent research, along with Vygotsky’s sociocultural theory and information processing, extends our understanding of preschoolers’ cognitive competencies. Next, we address factors that contribute to early childhood mental development—the home environment, the quality of preschool and child care, and the many hours young children spend watching television and using computers. We conclude with the dramatic expansion of language in early childhood.

PHYSICAL DEVELOPMENT

image1 A Changing Body and Brain

In early childhood, body growth tapers off from the rapid rate of the first two years. On average, children add 2 to 3 inches in height and about 5 pounds in weight each year. Boys continue to be slightly larger than girls. As “baby fat” drops off further, children gradually become thinner, although girls retain somewhat more body fat than boys, who are slightly more muscular. As  Figure 7.1  shows, by age 5 the top-heavy, bowlegged, potbellied toddler has become a more streamlined, flat-tummied, longer-legged child with body proportions similar to those of adults. Consequently, posture and balance improve—changes that support gains in motor coordination.

Individual differences in body size are even more apparent during early childhood than in infancy and toddlerhood. Speeding around the bike path in the play yard, 5-year-old Darryl—at 48 inches tall and 55 pounds—towered over his kindergarten classmates. (The average North American 5-year-old boy is 43 inches tall and weighs 42 pounds.) Priti, an Asian-Indian child, was unusually small because of genetic factors linked to her cultural ancestry. Hal, a Caucasian child from a poverty-stricken home, was well below average for reasons we will discuss shortly.

FIGURE 7.1 Body growth during early childhood.

During the preschool years, children grow more slowly than in infancy and toddlerhood. Chris and Mariel’s bodies became more streamlined, flat-tummied, and longer-legged. Boys continue to be slightly taller, heavier, and more muscular than girls. But generally, the two sexes are similar in body proportions and physical capacities.

Skeletal Growth

The skeletal changes of infancy continue throughout early childhood. Between ages 2 and 6, approximately 45 new epiphyses, or growth centers in which cartilage hardens into bone, emerge in various parts of the skeleton. X-rays of these growth centers enable doctors to estimate children’s skeletal age, or progress toward physical maturity (see  page 121  in  Chapter 4 )—information helpful in diagnosing growth disorders.

By the end of the preschool years, children start to lose their primary, or “baby,” teeth. Genetic factors heavily influence the age at which they do so. For example, girls, who are ahead of boys in physical development, lose teeth earlier. Environmental influences also matter: Prolonged malnutrition delays the appearance of permanent teeth, whereas overweight and obesity accelerate it (Hilgers et al.,  2006 ).

Diseased baby teeth can affect the health of permanent teeth, so preventing decay in primary teeth is essential—by brushing consistently, avoiding sugary foods, drinking fluoridated water, and getting topical fluoride treatments and sealants (plastic coatings that protect tooth surfaces). Another factor is exposure to tobacco smoke, which suppresses children’s immune system, including the ability to fight bacteria responsible for tooth decay. Young children in homes with regular smokers are at increased risk for decayed teeth (Hanioka et al.,  2011 ).

Unfortunately, an estimated 28 percent of U.S. preschoolers have tooth decay, a figure that rises to 50 percent in middle childhood and 60 percent by age 18. Causes include poor diet and inadequate health care—factors that are more likely to affect low-SES children. About 30 percent of U.S. children living in poverty have untreated dental caries (National Institutes of Health,  2011 ).

Brain Development

Between ages 2 and 6, the brain increases from 70 percent of its adult weight to 90 percent. At the same time, preschoolers improve in a wide variety of skills—physical coordination, perception, attention, memory, language, logical thinking, and imagination.

By age 4, many parts of the cerebral cortex have overproduced synapses. In some regions, such as the prefrontal cortex, the number of synapses is nearly double the adult value. Together, synaptic growth and myelination of neural fibers result in a high energy need. In fact, fMRI evidence reveals that energy metabolism in the cerebral cortex reaches a peak around this age (Huttenlocher,  2002 ; Nelson, Thomas, & de Haan,  2006 ). Synaptic pruning follows: Neurons that are seldom stimulated lose their connective fibers, and the number of synapses gradually declines. By age 8 to 10, energy consumption of most cortical regions diminishes to near-adult levels (Nelson,  2002 ). And cognitive capacities increasingly localize in distinct neural systems, reflecting a developmental shift toward a more fine-tuned, efficient neural organization (Tsujimoto,  2008 ).

EEG, NIRS, and fMRI measures of neural activity indicate especially rapid growth from early to middle childhood in areas of the prefrontal cortex devoted to various aspects of executive function. These include inhibition of impulses, attention, memory, and planning and organizing behavior—capacities that advance markedly over the preschool years (Bunge & Wright,  2007 ; Durston & Casey,  2006 ). Furthermore, for most children, the left cerebral hemisphere is especially active between 3 and 6 years and then levels off. In contrast, activity in the right hemisphere increases steadily throughout early and middle childhood (Thatcher, Walker, & Giudice,  1987 ; Thompson et al.,  2000 ). These findings fit nicely with what we know about several aspects of cognitive development. Language skills (typically housed in the left hemisphere) increase at an astonishing pace in early childhood, and they support children’s improved executive function. In contrast, spatial skills (usually located in the right hemisphere), such as giving directions, drawing pictures, and recognizing geometric shapes, develop gradually over childhood and adolescence.

Differences in rate of development between the two hemispheres suggest that they are continuing to lateralize (specialize in cognitive functions). Let’s take a closer look at brain lateralization in early childhood by focusing on handedness.

A 5-year-old illustrates gains in executive function, supported by rapid growth of the prefrontal cortex, as she engages in an activity that challenges her capacity to attend, remember, and plan.

 
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