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PART III: Infancy and Toddlerhood: The First Two Years

chapter 4 Physical Development in Infancy and Toddlerhood

chapter 5 Cognitive Development in Infancy and Toddlerhood

chapter 6 Emotional and Social Development in Infancy and Toddlerhood

chapter 4 Physical Development in Infancy and Toddlerhood

Infants acquire new motor skills by building on previously acquired capacities. Eager to explore her world, this baby practices the art of crawling. Once she can fully move on her own, she will make dramatic strides in understanding her surroundings.

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chapter outline

Body Growth

Changes in Body Size and Muscle–Fat Makeup

Individual and Group Differences

Changes in Body Proportions

Brain Development

Development of Neurons

Neurobiological Methods

Development of the Cerebral Cortex

Sensitive Periods in Brain Development

Changing States of Arousal

■ BIOLOGY AND ENVIRONMENT Brain Plasticity: Insights from Research on Brain-Damaged

Children and Adults

■ CULTURAL INFLUENCES Cultural Variation in Infant Sleeping Arrangements

Influences on Early Physical Growth

Heredity

Nutrition

Malnutrition

Learning Capacities

Classical Conditioning

Operant Conditioning

Habituation

Imitation

Motor Development

The Sequence of Motor Development

Motor Skills as Dynamic Systems

Fine-Motor Development: Reaching and Grasping

Perceptual Development

Hearing

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Vision

Intermodal Perception

Understanding Perceptual Development

■ BIOLOGY AND ENVIRONMENT “Tuning In” to Familiar Speech, Faces, and Music: A

Sensitive Period for Culture-Specific Learning

On a brilliant June morning, 16-month-old Caitlin emerged from her front door, ready for

the short drive to the child-care home where she spent her weekdays while her mother,

Carolyn, and her father, David, worked. Clutching a teddy bear in one hand and her

mother’s arm with the other, Caitlin descended the steps. “One! Two! Threeee!” Carolyn

counted as she helped Caitlin down. “How much she’s changed,” Carolyn thought to herself,

looking at the child who, not long ago, had been a newborn. With her first steps, Caitlin had

passed from infancy to toddlerhood—a period spanning the second year of life. At first,

Caitlin did, indeed, “toddle” with an awkward gait, tipping over frequently. But her face

reflected the thrill of conquering a new skill.

As they walked toward the car, Carolyn and Caitlin spotted 3-year-old Eli and his father,

Kevin, in the neighboring yard. Eli dashed toward them, waving a bright yellow envelope.

Carolyn bent down to open the envelope and took out a card. It read, “Announcing the

arrival of Grace Ann. Born: Cambodia. Age: 16 months.” Carolyn turned to Kevin and Eli.

“That’s wonderful news! When can we see her?”

“Let’s wait a few days,” Kevin suggested. “Monica’s taken Grace to the doctor this morning.

She’s underweight and malnourished.” Kevin described Monica’s first night with Grace in a

hotel room in Phnom Penh. Grace lay on the bed, withdrawn and fearful. Eventually she

fell asleep, gripping crackers in both hands.

Carolyn felt Caitlin’s impatient tug at her sleeve. Off they drove to child care, where

Vanessa had just dropped off her 18-month-old son, Timmy. Within moments, Caitlin and

Timmy were in the sandbox, shoveling sand into plastic cups and buckets with the help of

their caregiver, Ginette.

A few weeks later, Grace joined Caitlin and Timmy at Ginette’s child-care home. Although

still tiny and unable to crawl or walk, she had grown taller and heavier, and her sad, vacant

gaze had given way to an alert expression, a ready smile, and an enthusiastic desire to

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imitate and explore. When Caitlin headed for the sandbox, Grace stretched out her arms,

asking Ginette to carry her there, too. Soon Grace was pulling herself up at every

opportunity. Finally, at age 18 months, she walked!

This chapter traces physical growth during the first two years—one of the most remarkable

and busiest times of development. We will see how rapid changes in the

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infant’s body and brain support learning, motor skills, and perceptual capacities. Caitlin,

Grace, and Timmy will join us along the way to illustrate individual differences and

environmental influences on physical development.

Body Growth

TAKE A MOMENT… The next time you’re walking in your neighborhood park or at the mall,

note the contrast between infants’ and toddlers’ physical capabilities. One reason for the

vast changes in what children can do over the first two years is that their bodies change

enormously—faster than at any other time after birth.

Changes in Body Size and Muscle–Fat Makeup

By the end of the first year, a typical infant’s height is about 32 inches—more than 50

percent greater than at birth. By 2 years, it is 75 percent greater (36 inches). Similarly, by 5

months of age, birth weight has doubled, to about 15 pounds. At 1 year it has tripled, to 22

pounds, and at 2 years it has quadrupled, to about 30 pounds.

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FIGURE 4.1 Body growth during the first two years.

These photos depict the dramatic changes in body size and proportions during infancy and toddlerhood in two individuals—a boy, Chris, and a girl, Mai. In the first year, the head is quite large in proportion to the rest of the body, and height and weight gain are especially rapid. During the second year, the lower portion of the body catches up. Notice, also, how both children added “baby fat” in the early months of life and then slimmed down, a trend that continues into middle childhood.

Figure 4.1 illustrates this dramatic increase in body size. But rather than making steady

gains, infants and toddlers grow in little spurts. In one study, children who were followed

over the first 21 months of life went for periods of 7 to 63 days with no growth, then added

as much as half an inch in a 24-hour period! Almost always, parents described their babies

as irritable and

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very hungry on the day before the spurt (Lampl, 1993; Lampl, Veldhuis, & Johnson, 1992).

One of the most obvious changes in infants’ appearance is their transformation into round,

plump babies by the middle of the first year. This early rise in “baby fat,” which peaks at

about 9 months, helps the small infant maintain a constant body temperature. In the second

year, most toddlers slim down, a trend that continues into middle childhood (Fomon &

Nelson, 2002). In contrast, muscle tissue increases very slowly during infancy and will not

reach a peak until adolescence. Babies are not very muscular; their strength and physical

coordination are limited.

Individual and Group Differences

In infancy, girls are slightly shorter and lighter than boys, with a higher ratio of fat to

muscle. These small sex differences persist throughout early and middle childhood and are

greatly magnified at adolescence. Ethnic differences in body size are apparent as well.

Grace was below the growth norms (height and weight averages for children her age). Early

malnutrition contributed, but even after substantial catch-up, Grace—as is typical for Asian

children—remained below North American norms. In contrast, Timmy is slightly above

average, as African-American children tend to be (Bogin, 2001).

Children of the same age also differ in rate of physical growth; some make faster progress

toward a mature body size than others. But current body size is not enough to tell us how

quickly a child’s physical growth is moving along. Although Timmy is larger and heavier

than Caitlin and Grace, he is not physically more mature. In a moment, you will see why.

The best estimate of a child’s physical maturity is skeletal age, a measure of bone

development. It is determined by X-raying the long bones of the body to see the extent to

which soft, pliable cartilage has hardened into bone, a gradual process that is completed in

adolescence. When skeletal ages are examined, African-American children tend to be

slightly ahead of Caucasian children at all ages, and girls are considerably ahead of boys. At

birth, the sexes differ by about 4 to 6 weeks, a gap that widens over infancy and childhood

(Tanner, Healy, & Cameron, 2001). This greater physical maturity may contribute to girls’

greater resistance to harmful environmental influences. As noted in Chapter 2, girls

experience fewer developmental problems than boys and have lower infant and childhood

mortality rates.

Changes in Body Proportions

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As the child’s overall size increases, different parts of the body grow at different rates. Two

growth patterns describe these changes. The first is the cephalocaudal trend—from the

Latin for “head to tail.” During the prenatal period, the head develops more rapidly than the

lower part of the body. At birth, the head takes up one-fourth of total body length, the legs

only one-third. Notice how, in Figure 4.1, the lower portion of the body catches up. By age 2,

the head accounts for only one-fifth and the legs for nearly one-half of total body length.

In the second pattern, the proximodistal trend, growth proceeds, literally, from “near to

far”—from the center of the body outward. In the prenatal period, the head, chest, and

trunk grow first, then the arms and legs, and finally the hands and feet. During infancy and

childhood, the arms and legs continue to grow somewhat ahead of the hands and feet.

Brain Development

At birth, the brain is nearer to its adult size than any other physical structure, and it

continues to develop at an astounding pace throughout infancy and toddlerhood. We can

best understand brain growth by looking at it from two vantage points: (1) the microscopic

level of individual brain cells and (2) the larger level of the cerebral cortex, the most

complex brain structure and the one responsible for the highly developed intelligence of

our species.

Development of Neurons

The human brain has 100 to 200 billion neurons, or nerve cells that store and transmit

information, many of which have thousands of direct connections with other neurons.

Unlike other body cells, neurons are not tightly packed together. Between them are tiny

gaps, or synapses, where fibers from different neurons come close together but do not touch

(see Figure 4.2). Neurons send messages to one another by releasing chemicals called

neurotransmitters, which cross the synapse.

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FIGURE 4.2 Neurons and their connective fibers.

This photograph of several neurons, taken with the aid of a powerful microscope, shows the elaborate synaptic connections that form with neighboring cells.

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FIGURE 4.3 Major milestones of brain development.

Formation of synapses is rapid during the first two years, especially in the auditory, visual, and language areas of the cerebral cortex. The prefrontal cortex undergoes more extended synaptic growth. In each area, overproduction of synapses is followed by synaptic pruning. The prefrontal cortex is among the last regions to attain adult levels of synaptic connections—in mid-to late adolescence. Myelination occurs at a dramatic pace during the first two years, more slowly through childhood, followed by an acceleration at adolescence and then a reduced pace in early adulthood. The multiple yellow lines indicate that the timing of myelination varies among different brain areas. For example, neural fibers myelinate over a longer period in the language areas, and especially in the prefrontal cortex, than in the visual and auditory areas.

(Adapted from Thompson & Nelson, 2001.)

The basic story of brain growth concerns how neurons develop and form this elaborate

communication system. Figure 4.3 summarizes major milestones of brain development. In

the prenatal period, neurons are produced in the embryo’s primitive neural tube. From

there, they migrate to form the major parts of the brain (see Chapter 3, page 82). Once

neurons are in place, they differentiate, establishing their unique functions by extending

their fibers to form synaptic connections with neighboring cells. During the first two years,

neural fibers and synapses increase at an astounding pace (Huttenlocher, 2002; Moore,

Persaud, & Torchia, 2013). A surprising aspect of brain growth is programmed cell death,

which makes space for these connective structures: As synapses form, many surrounding

neurons die—20 to 80 percent, depending on the brain region (de Haan & Johnson, 2003;

Stiles, 2008). Fortunately, during the prenatal period, the neural tube produces far more

neurons than the brain will ever need.

As neurons form connections, stimulation becomes vital to their survival. Neurons that are

stimulated by input from the surrounding environment continue to establish synapses,

forming increasingly elaborate systems of communication that support more complex

abilities. At first, stimulation results in a massive overabundance of synapses, many of

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which serve identical functions, thereby ensuring that the child will acquire the motor,

cognitive, and social skills that our species needs to survive. Neurons that are seldom

stimulated soon lose their synapses, in a process called synaptic pruning that returns

neurons not needed at the moment to an uncommitted state so they can support future

development. In all, about 40 percent of synapses are pruned during childhood and

adolescence to reach the adult level (Webb, Monk, & Nelson, 2001). For this process to

advance, appropriate stimulation of the child’s brain is vital during periods in which the

formation of synapses is at its peak (Bryk & Fisher, 2012).

If few new neurons are produced after the prenatal period, what causes the dramatic

increase in brain size during the first two years? About half the brain’s volume is made up

of glial cells, which are responsible for myelination, the coating of neural fibers with an

insulating fatty sheath (called myelin) that improves the efficiency of message transfer. Glial

cells multiply rapidly from the fourth month of pregnancy through the second year of life—

a process that continues at a slower pace through middle childhood and accelerates again in

adolescence. Gains in neural fibers and myelination are responsible for the extraordinary

gain in overall size of the brain—from nearly 30 percent of its adult weight at birth to 70

percent by age 2 (Johnson, 2011; Knickmeyer et al., 2008).

Brain development can be compared to molding a “living sculpture.” First, neurons and

synapses are overproduced. Then, cell death and synaptic pruning sculpt away excess

building material to form the mature brain—a process jointly influenced by genetically

programmed events and the child’s experiences. The resulting “sculpture” is a set of

interconnected regions, each with specific functions—much like countries on a globe that

communicate with one another (Johnston et al., 2001). This “geography” of the brain permits

researchers to study its developing organization and the activity of its regions using

neurobiological methods.

Neurobiological Methods

Table 4.1 describes major measures of brain functioning. The first two methods detect

changes in electrical activity in the cerebral cortex. In an electroencephalogram (EEG),

researchers examine brain-wave patterns for stability and organization—signs of mature

functioning of the cortex. And as the person

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processes a particular stimulus, event-related potentials (ERPs) detect the general location of

brain-wave activity—a technique often used to study preverbal infants’ responsiveness to

various stimuli, the impact of experience on specialization of specific brain regions, and

atypical brain functioning in individuals with learning and emotional problems (DeBoer,

Scott, & Nelson, 2007; deRegnier, 2005).

Neuroimaging techniques, which yield detailed, three-dimensional computerized pictures of

the entire brain and its active areas, provide the most precise information about which

brain regions are specialized for certain capacities and about abnormalities in brain

functioning. The most promising of these methods is functional magnetic resonance

imaging (fMRI). Unlike positron emission tomography (PET), fMRI does not depend on X-ray

photography, which requires injection of a radioactive substance. Rather, when an

individual is exposed to a stimulus, fMRI detects changes in blood flow and oxygen

metabolism throughout the brain magnetically, yielding a colorful, moving picture of parts

of the brain used to perform a given activity (see Figure 4.4a, b, and c).

TABLE 4.1 Methods for Measuring Brain Functioning

METHOD DESCRIPTION

Electroencephalogram (EEG)

Electrodes embedded in a head cap record electrical brain- wave activity in the brain’s outer layers—the cerebral cortex. Today, researchers use an advanced tool called a geodesic sensor net (GSN) to hold interconnected electrodes (up to 128 for infants and 256 for children and adults) in place through a cap that adjusts to each person’s head shape, yielding improved brain-wave detection.

Event-related potentials (ERPs)

Using the EEG, the frequency and amplitude of brain waves in response to particular stimuli (such as a picture, music, or speech) are recorded in multiple areas of the cerebral cortex. Enables identification of general regions of stimulus-induced activity.

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METHOD DESCRIPTION

Functional magnetic resonance imaging (fMRI)

While the person lies inside a tunnel-shaped apparatus that creates a magnetic field, a scanner magnetically detects increased blood flow and oxygen metabolism in areas of the brain as the individual processes particular stimuli. The scanner typically records images every 1 to 4 seconds; these are combined into a computerized moving picture of activity anywhere in the brain (not just its outer layers). Not appropriate for children younger than age 5 to 6, who cannot remain still during testing.

Positron emission tomography (PET)

After injection or inhalation of a radioactive substance, the person lies on an apparatus with a scanner that emits fine streams of X-rays, which detect increased blood flow and oxygen metabolism in areas of the brain as the person processes particular stimuli. As with fMRI, the result is a computerized image of “online” activity anywhere in the brain. Not appropriate for children younger than age 5 to 6.

Near-infrared spectroscopy (NIRS)

Using thin, flexible optical fibers attached to the scalp through a head cap, infrared (invisible) light is beamed at the brain; its absorption by areas of the cerebral cortex varies with changes in blood flow and oxygen metabolism as the individual processes particular stimuli. The result is a computerized moving picture of active areas in the cerebral cortex. Unlike fMRI and PET, NIRS is appropriate for infants and young children, who can move within limited range.

FIGURE 4.4 Functional magnetic resonance imaging (fMRI) and near-infrared spectroscopy (NIRS).

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(a) This 6-year-old is part of a study that uses fMRI to find out how his brain processes light and motion. (b) The fMRI image shows which areas of the child’s brain are active while he views changing visual stimuli. (c) Here, NIRS is used to investigate a 2-month-old’s response to a visual stimulus. During testing, the baby can move freely within a limited range.

(Photo (c) from G. Taga, K. Asakawa, A. Maki, Y. Konishi, & H. Koisumi, 2003, “Brain Imaging in Awake Infants by Near-Infrared Optical Topography,” Proceedings of the National Academy of Sciences, 100, p. 10723. Reprinted by permission.)

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Because PET and fMRI require that the participant lie as motionless as possible for an

extended time, they are not suitable for infants and young children (Nelson, Thomas, & de

Haan, 2006). A neuroimaging technique that works well in infancy and early childhood is

near-infrared spectroscopy (NIRS), in which infrared (invisible) light is beamed at regions of

the cerebral cortex to measure blood flow and oxygen metabolism while the child attends to

a stimulus (refer again to Table 4.1). Because the apparatus consists only of thin, flexible

optical fibers attached to the scalp using a head cap, a baby can sit on the parent’s lap and

move during testing—as Figure 4.4c illustrates (Hespos et al., 2010). But unlike PET and

fMRI, which map activity changes throughout the brain, NIRS examines only the functioning

of the cerebral cortex.

Development of the Cerebral Cortex

The cerebral cortex surrounds the rest of the brain, resembling half of a shelled walnut. It is

the largest brain structure, accounting for 85 percent of the brain’s weight and containing

the greatest number of neurons and synapses. Because the cerebral cortex is the last part of

the brain to stop growing, it is sensitive to environmental influences for a much longer

period than any other part of the brain.

Regions of the Cerebral Cortex.

Figure 4.5 shows specific functions of regions of the cerebral cortex, such as receiving

information from the senses, instructing the body to move, and thinking. The order in

which cortical regions develop corresponds to the order in which various capacities emerge

in the infant and growing child. For example, a burst of activity occurs in the auditory and

visual cortexes and in areas responsible for body movement over the first year—a period of

dramatic gains in auditory and visual perception and mastery of motor skills (Johnson,

2011). Language areas are especially active from late infancy through the preschool years,

when language development flourishes (Pujol et al., 2006; Thompson, 2000).

The cortical regions with the most extended period of development are the frontal lobes.

The prefrontal cortex, lying in front of areas controlling body movement, is responsible for

thought—in particular, consciousness, inhibition of impulses, integration of information,

and use of memory, reasoning, planning, and problem-solving strategies. From age 2

months on, the prefrontal cortex functions more effectively. But it undergoes especially

rapid myelination and formation and pruning of synapses during the preschool and school

years, followed by another period of accelerated growth in adolescence, when it reaches an

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adult level of synaptic connections (Nelson, 2002; Nelson, Thomas, & de Haan, 2006; Sowell

et al., 2002).

FIGURE 4.5 The left side of the human brain, showing the cerebral cortex.

The cortex is divided into different lobes, each containing a variety of regions with specific functions. Some major regions are labeled here.

Lateralization and Plasticity of the Cortex.

The cerebral cortex has two hemispheres, or sides, that differ in their functions. Some tasks

are done mostly by the left hemisphere, others by the right. For example, each hemisphere

receives sensory information from the side of the body opposite to it and controls only that

side. For most of us, the left hemisphere is largely responsible for verbal abilities (such as

spoken and written language) and positive emotion (such as joy). The right hemisphere

handles spatial abilities (judging distances, reading maps, and recognizing geometric

shapes) and negative emotion (such as distress) (Banish & Heller, 1998; Nelson & Bosquet,

*

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2000). In left-handed people, this pattern may be reversed or, more commonly, the cerebral

cortex may be less clearly specialized than in right-handers.

Why does this specialization of the two hemispheres, called lateralization, occur? Studies

using fMRI reveal that the left hemisphere is better at processing information in a

sequential, analytic (piece-by-piece) way, a good approach for dealing with communicative

information—both verbal (language) and emotional (a joyful smile). In contrast, the right

hemisphere is specialized for processing information in a holistic, integrative manner, ideal

for making sense of spatial information and regulating negative emotion. A lateralized

brain may have evolved because it enabled humans to cope more successfully with

changing environmental demands (Falk, 2005). It permits a wider array of functions to be

carried out effectively than if both sides processed information exactly the same way.

*The eyes are an exception. Messages from the right half of each retina go to the right

hemisphere; messages from the left half of each retina go to the left hemisphere. Thus,

visual information from both eyes is received by both hemispheres.

Researchers study the timing of brain lateralization to learn more about brain plasticity. A

highly plastic cerebral cortex, in which many areas are not yet committed to specific

functions, has a high capacity for learning. And if a part of the cortex is

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damaged, other parts can take over tasks it would have handled. But once the hemispheres

lateralize, damage to a specific region means that the abilities it controls cannot be

recovered to the same extent or as easily as earlier.

At birth, the hemispheres have already begun to specialize. Most newborns show greater

activation (detected with either ERP or NIRS) in the left hemisphere while listening to

speech sounds or displaying a positive state of arousal. In contrast, the right hemisphere

reacts more strongly to nonspeech sounds and to stimuli (such as a sour-tasting fluid) that

evoke negative emotion (Davidson, 1994; Fox & Davidson, 1986; Hespos et al., 2010).

Nevertheless, research on brain-damaged children and adults offers dramatic evidence for

substantial plasticity in the young brain, summarized in the Biology and Environment box

on page 126. Furthermore, early experience greatly influences the organization of the

cerebral cortex. For example, deaf adults who, as infants and children, learned sign

language (a spatial skill) depend more than hearing individuals on the right hemisphere for

language processing (Neville & Bavelier, 2002). And toddlers who are advanced in language

development show greater left-hemispheric specialization for language than their more

slowly developing agemates (Luna et al., 2001; Mills et al., 2005). Apparently, the very

process of acquiring language and other skills promotes lateralization.

In sum, the brain is more plastic during the first few years than it will ever be again. An

overabundance of synaptic connections supports brain plasticity, ensuring that young

children will acquire certain capacities even if some areas are damaged. And although the

cortex is programmed from the start for hemispheric specialization, experience greatly

influences the rate and success of its advancing organization.

Sensitive Periods in Brain Development

Both animal and human studies reveal that early, extreme sensory deprivation results in

permanent brain damage and loss of functions—findings that verify the existence of

sensitive periods in brain development. For example, early, varied visual experiences must

occur for the brain’s visual centers to develop normally. If a 1-month-old kitten is deprived

of light for just three or four days, these areas of the brain degenerate. If the kitten is kept in

the dark during the fourth week of life and beyond, the damage is severe and permanent

(Crair, Gillespie, & Stryker, 1998). And the general quality of the early environment affects

overall brain growth. When animals reared from birth in physically and socially stimulating

surroundings are compared with those reared under depleted conditions, the brains of the

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stimulated animals are larger and heavier and show much denser synaptic connections

(Sale, Berardi, & Maffei, 2009).

Human Evidence: Victims of Deprived Early Environments.

For ethical reasons, we cannot deliberately deprive some infants of normal rearing

experiences and observe the impact on their brains and competencies. Instead, we must

turn to natural experiments, in which children were victims of deprived early

environments that were later rectified. Such studies have revealed some parallels with the

animal evidence just described.

For example, when babies are born with cataracts (clouded lenses, preventing clear visual

images) in both eyes, those who have corrective surgery within four to six months show

rapid improvement in vision, except for subtle aspects of face perception, which require

early visual input to the right hemisphere to develop (Le Grand et al., 2003; Maurer,

Mondloch, & Lewis, 2007). The longer cataract surgery is postponed beyond infancy, the less

complete the recovery in visual skills. And if surgery is delayed until adulthood, vision is

severely and permanently impaired (Lewis & Maurer, 2005).

Studies of infants placed in orphanages who were later exposed to ordinary family rearing

confirm the importance of a generally stimulating physical and social environment for

psychological development. In one investigation, researchers followed the progress of a

large sample of children transferred between birth and 3½ years from extremely deprived

Romanian orphanages to adoptive families in Great Britain (Beckett et al., 2006; O’Connor et

al., 2000; Rutter et al., 1998, 2004, 2010). On arrival, most were impaired in all domains of

development. Cognitive catch-up was impressive for children adopted before 6 months, who

attained average mental test scores in childhood and adolescence, performing as well as a

comparison group of early-adopted British-born children.

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These children in an orphanage in Romania receive little adult contact or stimulation. The longer they remain in this barren environment, the more likely they are to display profound impairments in all domains of development.

But Romanian children who had been institutionalized for more than the first six months

showed serious intellectual deficits (see Figure 4.6). Although they improved in test scores

during middle childhood and adolescence, they remained substantially below average. And

most displayed at least three serious mental health problems, such as inattention,

overactivity, unruly behavior, and autistic-like symptoms (social disinterest, stereotyped

behavior) (Kreppner et al., 2007, 2010).

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Biology and Environment Brain Plasticity: Insights from Research on Brain-Damaged Children and Adults

This preschooler, who experienced brain damage in infancy, has been spared massive impairments because of early, high brain plasticity. A teacher guides his hand in drawing shapes to strengthen spatial skills, which are more impaired than language.

In the first few years of life, the brain is highly plastic. It can reorganize areas committed to

specific functions in ways that the mature brain cannot. Consistently, adults who suffered

brain injuries in infancy and early childhood show fewer cognitive impairments than

adults with later-occurring injuries (Holland, 2004; Huttenlocher, 2002). Nevertheless, the

young brain is not totally plastic. When it is injured, its functioning is compromised. The

extent of plasticity depends on several factors, including age at time of injury, site of

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damage, and skill area. Furthermore, plasticity is not restricted to childhood. Some

reorganization after injury also occurs in the mature brain.

Brain Plasticity in Infancy and Early Childhood

In a large study of children with injuries to the cerebral cortex that occurred before birth

or in the first six months of life, language and spatial skills were assessed repeatedly into

adolescence (Akshoomoff et al., 2002; Stiles, 2001a; Stiles et al., 2005, 2008). All the children

had experienced early brain seizures or hemorrhages. Brain-imaging techniques (fMRI and

PET) revealed the precise site of damage.

Regardless of whether injury occurred in the left or right cerebral hemisphere, the children

showed delays in language development that persisted until about 3½ years of age. That

damage to either hemisphere affected early language competence indicates that at first,

language functioning is broadly distributed in the brain. But by age 5, the children caught

up in vocabulary and grammatical skills. Undamaged areas—in either the left or the right

hemisphere—had taken over these language functions.

Compared with language, spatial skills were more impaired after early brain injury. When

preschool through adolescent-age youngsters were asked to copy designs, those with early

right-hemispheric damage had trouble with holistic processing—accurately representing

the overall shape. In contrast, children with left-hemispheric damage captured the basic

shape but omitted fine-grained details. Nevertheless, the children improved in drawing

skills with age—gains that do not occur in brain-injured adults (Akshoomoff et al., 2002;

Stiles et al., 2003, 2008).

Clearly, recovery after early brain injury is greater for language than for spatial skills. Why

is this so? Researchers speculate that spatial processing is the older of the two capacities in

our evolutionary history and, therefore, more lateralized at birth (Stiles, 2001b; Stiles et al.,

2002, 2008). But early brain injury has far less impact than later injury on both language

and spatial skills. In sum, the young brain is remarkably plastic.

The Price of High Plasticity in the Young Brain

Despite impressive recovery of language and (to a lesser extent) spatial skills, children with

early brain injuries show deficits in a wide range of complex mental abilities during the

school years. For example, their reading and math progress is slow. And in telling stories,

they produce simpler narratives than agemates without early brain injuries (although

many catch up in narrative skills by early adolescence) (Reilly, Bates, & Marchman, 1998;

Reilly et al., 2004). Furthermore, the more brain tissue destroyed in infancy or early

childhood, the poorer children score on intelligence tests (Anderson et al., 2006).

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High brain plasticity, researchers explain, comes at a price. When healthy brain regions

take over the functions of damaged areas, a “crowding effect” occurs: Multiple tasks must

be done by a smaller-than-usual volume of brain tissue (Stiles, 2012). Consequently, the

brain processes information less quickly and accurately than it would if it were intact.

Complex mental abilities of all kinds suffer into middle childhood, and often longer,

because performing them well requires considerable space in the cerebral cortex.

Brain Plasticity in Adulthood

Brain plasticity is not restricted to early childhood. Though far more limited,

reorganization in the brain can occur later, even in adulthood. For example, adult stroke

victims often display considerable recovery, especially in response to stimulation of

language and motor skills. Brain-imaging techniques reveal that structures adjacent to the

permanently damaged area or in the opposite cerebral hemisphere reorganize to support

the impaired ability (Kalra & Ratan, 2007; Murphy & Corbett, 2009).

In infancy and childhood, the goal of brain growth is to form neural connections that

ensure mastery of essential skills. Animal research reveals that plasticity is greatest while

the brain is forming many new synapses; it declines during synaptic pruning (Murphy &

Corbett, 2009). At older ages, specialized brain structures are in place, but after injury they

can still reorganize to some degree. The adult brain can produce a small number of new

neurons. And when an individual practices relevant tasks, the brain strengthens existing

synapses and generates new ones (Nelson, Thomas, & de Haan, 2006).

Plasticity seems to be a basic property of the nervous system. Researchers hope to discover

how experience and brain plasticity work together throughout life, so they can help people

of all ages—with and without brain injuries—develop at their best.

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FIGURE 4.6 Relationship of age at adoption to mental test scores at ages 6 and 11 among British and Romanian adoptees.

Children transferred from Romanian orphanages to British adoptive homes in the first six months of life attained average scores and fared as well as British early-adopted children, suggesting that they had fully recovered from extreme early deprivation. Romanian children adopted after 6 months of age performed well below average. And although those adopted after age 2 improved between ages 6 and 11, they continued to show serious intellectual deficits.

(Adapted from Beckett et al., 2006.)

Neurobiological findings indicate that early, prolonged institutionalization leads to a

generalized decrease in activity in the cerebral cortex, especially the prefrontal cortex,

which governs complex cognition and impulse control. Neural fibers connecting the

prefrontal cortex with other brain structures involved in control of emotion are also

reduced (Eluvathingal et al., 2006; Nelson, 2007b). And activation of the left cerebral

hemisphere, governing positive emotion, is diminished relative to right cerebral activation,

governing negative emotion (McLaughlin et al., 2011).

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Additional evidence confirms that the chronic stress of early, deprived orphanage rearing

disrupts the brain’s capacity to manage stress, with long-term physical and psychological

consequences. In another investigation, researchers followed the development of children

who had spent their first eight months or more in Romanian institutions and were then

adopted into Canadian homes (Gunnar et al., 2001; Gunnar & Cheatham, 2003). Compared

with agemates adopted shortly after birth, these children showed extreme stress reactivity,

as indicated by high concentrations of the stress hormone cortisol in their saliva—a

physiological response linked to persistent illness, retarded physical growth, and learning

and behavior problems, including deficits in attention and control of anger and other

impulses. The longer the children spent in orphanage care, the higher their cortisol levels—

even 6½ years after adoption. In other investigations, orphanage children displayed

abnormally low cortisol—a blunted physiological stress response that may be the central

nervous system’s adaptation to earlier, frequent cortisol elevations (Loman & Gunnar,

2010).

Appropriate Stimulation.

Unlike the orphanage children just described, Grace, whom Monica and Kevin had adopted

in Cambodia at 16 months of age, showed favorable progress. Two years earlier, they had

adopted Grace’s older brother, Eli. When Eli was 2 years old, Monica and Kevin sent a letter

and a photo of Eli to his biological mother, describing a bright, happy child. The next day,

the Cambodian mother tearfully asked an adoption agency to send her baby daughter to

join Eli and his American family. Although Grace’s early environment was very depleted,

her biological mother’s loving care—holding gently, speaking softly, playfully stimulating,

and breastfeeding—may have prevented irreversible damage to her brain.

In the Bucharest Early Intervention Project, about 200 institutionalized Romanian babies

were randomized into conditions of either care as usual or transfer to high-quality foster

families between ages 5 and 30 months. Specially trained social workers provided foster

parents with counseling and support. Follow-ups between 2½ and 4 years revealed that the

foster-care group exceeded the institutional-care group in intelligence test scores, language

skills, emotional responsiveness, and EEG and ERP assessments of brain activity (Nelson et

al., 2007; Smyke et al., 2009). On all measures, the earlier the foster placement, the better the

outcome. But consistent with an early sensitive period, the foster-care group remained

behind never-institutionalized agemates living with Bucharest families.

In addition to impoverished environments, ones that overwhelm children with expectations

beyond their current capacities interfere with the brain’s potential. In recent years,

expensive early learning centers have sprung up, in which infants are trained with letter

and number flash cards and slightly older toddlers are given a full curriculum of reading,

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math, science, art, gym, and more. There is no evidence that these programs yield smarter

“superbabies” (Hirsh-Pasek & Golinkoff, 2003). To the contrary, trying to prime infants with

stimulation for which they are not ready can cause them to withdraw, thereby threatening

their interest in learning and creating conditions much like stimulus deprivation!

How, then, can we characterize appropriate stimulation during the early years? To answer

this question, researchers distinguish between two types of brain development. The first,

experience-expectant brain growth, refers to the young brain’s rapidly developing

organization, which depends on ordinary experiences—opportunities to explore the

environment, interact with people, and hear language and other sounds. As a result of

millions of years of evolution, the brains of all infants, toddlers, and young children expect

to encounter these experiences

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and, if they do, grow normally. The second type of brain development, experience-

dependent brain growth, occurs throughout our lives. It consists of additional growth and

refinement of established brain structures as a result of specific learning experiences that

vary widely across individuals and cultures (Greenough & Black, 1992). Reading and

writing, playing computer games, weaving an intricate rug, and practicing the violin are

examples. The brain of a violinist differs in certain ways from the brain of a poet because

each has exercised different brain regions for a long time.

Experience-expectant brain growth occurs early and naturally, as caregivers offer babies

and preschoolers age-appropriate play materials and engage them in enjoyable daily

routines—a shared meal, a game of peekaboo, a bath before bed, a picture book to talk

about, or a song to sing. The resulting growth provides the foundation for later-occurring,

experience-dependent development (Huttenlocher, 2002; Shonkoff & Phillips, 2001). No

evidence exists for a sensitive period in the first five or six years for mastering skills that

depend on extensive training, such as reading, musical performance, or gymnastics. To the

contrary, rushing early learning harms the brain by overwhelming its neural circuits,

thereby reducing the brain’s sensitivity to the everyday experiences it needs for a healthy

start in life.

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Experience-expectant brain growth occurs naturally, through ordinary, stimulating experiences. This toddler exploring a mossy log enjoys the type of activity that best promotes brain development in the early years.

Changing States of Arousal

Rapid brain growth means that the organization of sleep and wakefulness changes

substantially between birth and 2 years, and fussiness and crying also decline. The newborn

baby takes round-the-clock naps that total about 16 to 18 hours (Davis, Parker &

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Montgomery, 2004). Total sleep time declines slowly; the average 2-year-old still needs 12 to

13 hours. But periods of sleep and wakefulness become fewer and longer, and the sleep–

wake pattern increasingly conforms to a night–day schedule. Most 6- to 9-month-olds take

two daytime naps; by about 18 months, children generally need only one nap. Finally,

between ages 3 and 5, napping subsides (Iglowstein et al., 2003).

These changing arousal patterns are due to brain development, but they are also affected by

cultural beliefs and practices and individual parents’ needs (Super & Harkness, 2002). Dutch

parents, for example, view sleep regularity as far more important than the U.S. parents do.

And whereas U.S. parents regard a predictable sleep schedule as emerging naturally from

within the child, Dutch parents believe that a schedule must be imposed, or the baby’s

development might suffer (Super et al., 1996; Super & Harkness, 2010). At age 6 months,

Dutch babies are put to bed earlier and sleep, on average, 2 hours more per day than their

U.S. agemates.

Motivated by demanding work schedules and other needs, many Western parents try to get

their babies to sleep through the night as early as 3 to 4 months by offering an evening

feeding—a practice that may be at odds with young infants’ neurological capacities. Not

until the middle of the first year is the secretion of melatonin, a hormone within the brain

that promotes drowsiness, much greater at night than during the day (Sadeh, 1997).

Furthermore, as the Cultural Influences box on the following page reveals, isolating infants

to promote sleep is rare elsewhere in the world. When babies sleep with their parents, their

average sleep period remains constant at three hours from 1 to 8 months of age. Only at the

end of the first year, as REM sleep (the state that usually prompts waking) declines, do

infants move in the direction of an adultlike sleep–waking schedule (Ficca et al., 1999).

Even after infants sleep through the night, they continue to wake occasionally. In studies

carried out in Australia, Israel, and the United States, night wakings increased around 6

months and again between 1½ and 2 years and then declined (Armstrong, Quinn, & Dadds,

1994; Scher, Epstein, & Tirosh, 2004; Scher et al., 1995). As Chapter 6 will reveal, around the

middle of the first year, infants are forming a clear-cut attachment to their familiar

caregiver and begin protesting when he or she leaves. And the challenges of toddlerhood—

the ability to range farther from the caregiver and increased awareness of the self as

separate from others—often prompt anxiety, evident in disturbed sleep and clinginess.

When parents offer comfort, these behaviors subside.

LOOK AND LISTEN

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Interview a parent of a baby about sleep challenges. What strategies has the parent tried to

ease these difficulties? Are the techniques likely to be effective, in view of evidence on

infant sleep development?

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Cultural Influences Cultural Variation in Infant Sleeping Arrangements

This Vietnamese mother and child sleep together—a practice common in their culture and around the globe. Hard wooden sleeping surfaces protect cosleeping children from entrapment in soft bedding.

Western child-rearing advice from experts strongly encourages nighttime separation of

baby from parent. For example, the most recent edition of Benjamin Spock’s Baby and Child

Care recommends that babies sleep in their own room by 3 months of age, explaining, “By 6

months, a child who regularly sleeps in her parents’ room may feel uneasy sleeping

anywhere else” (Spock & Needlman, 2012, p. 62). And the American Academy of Pediatrics

(2012) has issued a controversial warning that parent–infant bedsharing may increase the

risk of sudden infant death syndrome (SIDS).

Yet parent–infant “cosleeping” is the norm for approximately 90 percent of the world’s

population, in cultures as diverse as the Japanese, the rural Guatemalan Maya, the Inuit of

northwestern Canada, and the !Kung of Botswana. Japanese and Korean children usually

lie next to their mothers in infancy and early childhood, and many continue to sleep with a

parent or other family member until adolescence (Takahashi, 1990; Yang & Hahn, 2002).

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Among the Maya, mother–infant bed-sharing is interrupted only by the birth of a new baby,

when the older child is moved next to the father or to another bed in the same room

(Morelli et al., 1992). Bedsharing is also common in U.S. ethnic minority families (McKenna

& Volpe, 2007). African-American children, for example, frequently fall asleep with their

parents and remain with them for part or all of the night (Buswell & Spatz, 2007).

Cultural values—specifically, collectivism versus individualism (see Chapter 2)—strongly

influence infant sleeping arrangements. In one study, researchers interviewed Guatemalan

Mayan mothers and American middle-SES mothers about their sleeping practices. Mayan

mothers stressed the importance of promoting an interdependent self, explaining that

cosleeping builds a close parent–child bond, which is necessary for children to learn the

ways of people around them. In contrast, American mothers emphasized an independent

self, mentioning their desire to instill early autonomy, prevent bad habits, and protect their

own privacy (Morelli et al., 1992).

Over the past two decades, cosleeping has increased in Western nations. An estimated 13

percent of U.S. infants routinely bedshare, and an additional 30 to 35 percent some-times

do (Buswell & Spatz, 2007; Willinger et al., 2003). Proponents of the practice say that it helps

infants sleep, makes breastfeeding more convenient, and provides valuable bonding time

(McKenna & Volpe, 2007).

During the night, cosleeping babies breastfeed three times longer than infants who sleep

alone. Because infants arouse to nurse more often when sleeping next to their mothers,

some researchers believe that cosleeping may actually help safeguard babies at risk for

SIDS (see page 110 in Chapter 3). Consistent with this view, SIDS is rare in Asian cultures

where cosleeping is widespread, including Cambodia, China, Japan, Korea, Thailand, and

Vietnam (McKenna, 2002; McKenna & McDade, 2005). And contrary to popular belief,

cosleeping does not reduce mothers’ total sleep time, although they experience more brief

awakenings, which permit them to check on their baby (Mao et al., 2004).

Infant sleeping practices affect other aspects of family life. For example, Mayan babies doze

off in the midst of ongoing family activities and are carried to bed by their mothers. In

contrast, for many American parents, bedtime often involves a lengthy, elaborate ritual.

Perhaps bedtime struggles, so common in Western homes but rare elsewhere in the world,

are related to the stress young children feel when they must fall asleep without assistance

(Latz, Wolf, & Lozoff, 1999).

Critics warn that bedsharing will promote emotional problems, especially excessive

dependency. Yet a study following children from the end of pregnancy through age 18

showed that young people who had bedshared in the early years were no different from

others in any aspect of adjustment (Okami, Weisner, & Olmstead, 2002). Another concern is

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that infants might become trapped under the parent’s body or in soft bedding and

suffocate. Parents who are obese or who use alcohol, tobacco, or illegal drugs do pose a

serious risk to their sleeping babies, as does the use of quilts and comforters or an overly

soft mattress (American Academy of Pediatrics, 2012; Willinger et al., 2003).

But with appropriate precautions, parents and infants can cosleep safely (McKenna &

Volpe, 2007). In cultures where cosleeping is widespread, parents and infants usually sleep

with light covering on hard surfaces, such as firm mattresses, floor mats, and wooden

planks, or infants sleep in a cradle or hammock next to the parents’ bed (McKenna, 2001,

2002). And when sharing the same bed, infants typically lie on their back or side facing the

mother—positions that promote frequent, easy communication between parent and baby

and arousal if breathing is threatened.

Finally, breastfeeding mothers usually assume a distinctive sleeping posture: They face the

infant, with knees drawn up under the baby’s feet and arm above the baby’s head. Besides

facilitating feeding, the position prevents the infant from sliding down under covers or up

under pillows (Ball, 2006). Because this posture is also seen in female great apes while

sharing sleeping nests with their infants, researchers believe it may have evolved to

enhance infant safety.

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ASK YOURSELF

REVIEW How do overproduction of synapses and synaptic pruning support infants’ and

children’s ability to learn?

CONNECT Explain how inappropriate stimulation—either too little or too much—can

impair cognitive and emotional development in the early years.

APPLY Which infant enrichment program would you choose: one that emphasizes gentle

talking and touching and social games, or one that includes reading and number drills and

classical music lessons? Explain.

REFLECT What is your attitude toward parent–infant cosleeping? Is it influenced by your

cultural background? Explain.

Influences on Early Physical Growth

Physical growth, like other aspects of development, results from a complex interplay

between genetic and environmental factors. Heredity, nutrition, and emotional well-being

all affect early physical growth.

Heredity

Because identical twins are much more alike in body size than fraternal twins, we know

that heredity is important in physical growth (Estourgie-van Burk et al., 2006; Touwslager et

al., 2011). When diet and health are adequate, height and rate of physical growth are largely

influenced by heredity. In fact, as long as negative environmental influences such as poor

nutrition and illness are not severe, children and adolescents typically show catch-up

growth—a return to a genetically influenced growth path once conditions improve. Still, the

brain, the heart, the digestive system, and many other internal organs may be permanently

compromised (Hales & Ozanne, 2003). (Recall the consequences of inadequate prenatal

nutrition for long-term health, discussed on page 92 in Chapter 3.)

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Genetic makeup also affects body weight: The weights of adopted children correlate more

strongly with those of their biological than of their adoptive parents (Kinnunen, Pietilainen,

& Rissanen, 2006). At the same time, environment—in particular, nutrition—plays an

especially important role.

Nutrition

Nutrition is especially crucial for development in the first two years because the baby’s

brain and body are growing so rapidly. Pound for pound, an infant’s energy needs are twice

those of an adult. Twenty-five percent of babies’ total caloric intake is devoted to growth,

and infants need extra calories to keep rapidly developing organs functioning properly

(Meyer, 2009).

Midwives in India support a mother as she learns to breastfeed her infant. Breastfeeding is especially important in developing countries, where it helps protect babies against life-threatening infections and early death.

Breastfeeding versus Bottle-Feeding.

Babies need not only enough food but also the right kind of food. In early infancy,

breastfeeding is ideally suited to their needs, and bottled formulas try to imitate it. Applying

What We Know on the following page summarizes major nutritional and health advantages

of breastfeeding.

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Because of these benefits, breastfed babies in poverty-stricken regions are much less likely

to be malnourished and 6 to 14 times more likely to survive the first year of life. The World

Health Organization recommends breastfeeding until age 2 years, with solid foods added at

6 months. These practices, if widely followed, would save the lives of more than a million

infants annually (World Health Organization, 2012b). Even breastfeeding for just a few

weeks offers some protection against respiratory and intestinal infections, which are

devastating to young children in developing countries. Also, because a nursing mother is

less likely to get pregnant, breastfeeding helps increase spacing between siblings, a major

factor in reducing infant and childhood deaths in nations with widespread poverty. (Note,

however, that breastfeeding is not a reliable method of birth control.)

Yet many mothers in the developing world do not know about these benefits. In Africa, the

Middle East, and Latin America, most babies get some breastfeeding, but fewer than 40

percent are exclusively breastfed for the first six months, and one-third are fully weaned

from the breast before 1 year (UNICEF, 2009). In place of breast milk, mothers give their

babies commercial formula or low-grade nutrients, such as rice water or highly diluted cow

or goat milk. Contamination of these foods as a result of poor sanitation is common and

often leads to illness and infant death. The United Nations has encouraged all hospitals and

maternity units in developing countries to promote breastfeeding as long as mothers do not

have viral or bacterial infections (such as HIV or tuberculosis) that can be

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transmitted to the baby. Today, most developing countries have banned the practice of

giving free or subsidized formula to new mothers.

Partly as a result of the natural childbirth movement, breastfeeding has become more

common in industrialized nations, especially among well-educated women. Today, 74

percent of American mothers breastfeed, but more than half stop by 6 months (Centers for

Disease Control and Prevention, 2011a). Not surprisingly, mothers who return to work

sooner wean their babies from the breast earlier (Kimbro, 2006). But mothers who cannot

be with their infants all the time can still combine breast- and bottle-feeding. The U.S.

Department of Health and Human Services (2010a) advises exclusive breastfeeding for the

first 6 months and inclusion of breast milk in the baby’s diet until at least 1 year.

Women who do not breastfeed sometimes worry that they are depriving their baby of an

experience essential for healthy psychological development. Yet breastfed and bottle-fed

infants in industrialized nations do not differ in quality of the mother–infant relationship or

in later emotional adjustment (Fergusson & Woodward, 1999; Jansen, de Weerth, & Riksen-

Walraven, 2008). Some studies report a slight advantage in intelligence test performance for

children and adolescents who were breastfed, after controlling for many factors. Most,

however, find no cognitive benefits (Der, Batty, & Deary, 2006).

Applying What We Know Reasons to Breastfeed

Nutritional and Health Advantages Explanation

Provides the correct balance of fat and protein

Compared with the milk of other mammals, human milk is higher in fat and lower in protein. This balance, as well as the unique proteins and fats contained in human milk, is ideal for a rapidly myelinating nervous system.

Ensures nutritional completeness

A mother who breastfeeds need not add other foods to her infant’s diet until the baby is 6 months old. The milks of all mammals are low in iron, but the iron contained in breast milk is much more easily absorbed by the baby’s system. Consequently, bottle-fed infants need iron-fortified formula.

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Nutritional and Health Advantages Explanation

Helps ensure healthy physical growth

One-year-old breastfed babies are leaner (have a higher percentage of muscle to fat), a growth pattern that persists through the preschool years and that may help prevent later overweight and obesity.

Protects against many diseases

Breastfeeding transfers antibodies and other infection-fighting agents from mother to child and enhances functioning of the immune system. Compared with bottle-fed infants, breastfed babies have far fewer allergic reactions and respiratory and intestinal illnesses. Breast milk also has anti-inflammatory effects, which reduce the severity of illness symptoms. Breastfeeding in the first four months is linked to lower blood cholesterol levels in adulthood and, thereby, may help prevent cardiovascular disease.

Protects against faulty jaw development and tooth decay

Sucking the mother’s nipple instead of an artificial nipple helps avoid malocclusion, a condition in which the upper and lower jaws do not meet properly. It also protects against tooth decay due to sweet liquid remaining in the mouths of infants who fall asleep while sucking on a bottle.

Ensures digestibility

Because breastfed babies have a different kind of bacteria growing in their intestines than do bottle-fed infants, they rarely suffer from constipation or other gastrointestinal problems.

Smooths the transition to solid foods

Breastfed infants accept new solid foods more easily than bottle- fed infants, perhaps because of their greater experience with a variety of flavors, which pass from the maternal diet into the mother’s milk.

Sources: American Academy of Pediatrics, 2005; Buescher, 2001; Michels et al., 2007; Owen et al., 2008; Rosetta & Baldi, 2008; Weyermann, Rothenbacher, & Brenner, 2006.

Are Chubby Babies at Risk for Later Overweight and Obesity?

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From early infancy, Timmy was an enthusiastic eater who nursed vigorously and gained

weight quickly. By 5 months, he began reaching for food on his mother’s plate. Vanessa

wondered: Was she overfeeding Timmy and increasing his chances of becoming

overweight?

Most chubby babies thin out during toddlerhood and early childhood, as weight gain slows

and they become more active. Infants and toddlers can eat nutritious foods freely without

risk of becoming overweight. But recent evidence does indicate a strengthening relationship

between rapid weight gain in infancy and later obesity (Botton et al., 2008; Chomtho et al.,

2008). The trend may be due to the rise in overweight and obesity among adults, who

promote unhealthy eating habits in their young children. Interviews with 1,500 U.S. parents

of 4- to 24-month-olds revealed that many routinely served older infants and toddlers

french fries, pizza, candy, sugary fruit drinks, and soda. On average, infants consumed 20

percent and toddlers 30 percent more calories than they needed. At the same time, as many

as one-fourth ate no fruits and one-third no vegetables (Siega-Riz et al., 2010).

How can concerned parents prevent their infants from becoming overweight children and

adults? One way is to breastfeed for the first six months, which is associated with slower

early weight gain (Gunnarsdottir et al., 2010). Another is to avoid giving them foods loaded

with sugar, salt, and saturated

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fats. Once toddlers learn to walk, climb, and run, parents can also provide plenty of

opportunities for energetic play. Finally, because research shows a correlation between

excessive television viewing and overweight in older children, parents should limit the time

very young children spend in front of the TV.

Malnutrition

Osita is an Ethiopian 2-year-old whose mother has never had to worry about his gaining too

much weight. When she weaned him at 1 year, there was little for him to eat besides starchy

rice-flour cakes. Soon his belly enlarged, his feet swelled, his hair fell out, and a rash

appeared on his skin. His bright-eyed curiosity vanished, and he became irritable and

listless.

In developing countries and war-torn areas where food resources are limited, malnutrition

is widespread. Recent evidence indicates that about 27 percent of the world’s children suffer

from malnutrition before age 5 (World Health Organization, 2010). The 10 percent who are

severely affected suffer from two dietary diseases.

Marasmus is a wasted condition of the body caused by a diet low in all essential nutrients. It

usually appears in the first year of life when a baby’s mother is too malnourished to

produce enough breast milk and bottle-feeding is also inadequate. Her starving baby

becomes painfully thin and is in danger of dying.

Osita has kwashiorkor, caused by an unbalanced diet very low in protein. The disease

usually strikes after weaning, between 1 and 3 years of age. It is common in regions where

children get just enough calories from starchy foods but little protein. The child’s body

responds by breaking down its own protein reserves, which causes the swelling and other

symptoms that Osita experienced.

Children who survive these extreme forms of malnutrition grow to be smaller in all body

dimensions and suffer from lasting damage to the brain, heart, liver, or other organs

(Müller & Krawinkel, 2005). When their diets do improve, they tend to gain excessive weight

(Uauy et al., 2008). A malnourished body protects itself by establishing a low basal

metabolism rate, which may endure after nutrition improves. Also, malnutrition may

disrupt appetite control centers in the brain, causing the child to overeat when food

becomes plentiful.

Learning and behavior are also seriously affected. In one long-term study of marasmic

children, an improved diet led to some catch-up growth in height, but not in head size

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(Stoch et al., 1982). The malnutrition probably interfered with growth of neural fibers and

myelination, causing a permanent loss in brain weight. And animal evidence reveals that a

deficient diet alters the production of neurotransmitters in the brain—an effect that can

disrupt all aspects of development (Haller, 2005). These children score low on intelligence

tests, show poor fine-motor coordination, and have difficulty paying attention (Galler et al.,

1990; Liu et al., 2003). They also display a more intense stress response to fear-arousing

situations, perhaps caused by the constant, gnawing pain of hunger (Fernald & Grantham-

McGregor, 1998).

Inadequate nutrition is not confined to developing countries. Because government-

supported supplementary food programs do not reach all families in need, an estimated 21

percent of U.S. children suffer from food insecurity—uncertain access to enough food for a

healthy, active life. Food insecurity is especially high among single-parent families (35

percent) and low-income ethnic minority families—for example, Hispanics and African

Americans (25 and 27 percent, respectively) (U.S. Department of Agriculture, 2011a).

Although few of these children have marasmus or kwashiorkor, their physical growth and

ability to learn are still affected.

Left photo: This baby of Niger, Africa, has marasmus, a wasted condition caused by a diet low in all essential nutrients. Right photo: The swollen abdomen of this toddler, also of Niger, is a symptom of kwashiorkor, which results from a diet very low in protein. If these children survive, they are likely to be growth stunted and to suffer from lasting organ damage and serious cognitive and emotional impairments.

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ASK YOURSELF

REVIEW Explain why breastfeeding can have lifelong consequences for the development of

babies born in poverty-stricken regions of the world.

CONNECT How are bidirectional influences between parent and child involved in the

impact of malnutrition on psychological development?

APPLY Eight-month-old Shaun is well below average in height and painfully thin. What

serious growth disorder does he likely have, and what type of intervention, in addition to

dietary enrichment, will help restore his development? (Hint: See page 92 in Chapter 3.)

REFLECT Imagine that you are the parent of a newborn baby. Describe feeding practices

you would use, and ones you would avoid, to prevent overweight and obesity.

Learning Capacities

Learning refers to changes in behavior as the result of experience. Babies come into the

world with built-in learning capacities that permit them to profit from experience

immediately. Infants are capable of two basic forms of learning, which were introduced in

Chapter 1: classical and operant conditioning. They also learn through their natural

preference for novel stimulation. Finally, shortly after birth, babies learn by observing

others; they can imitate the facial expressions and gestures of adults.

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FIGURE 4.7 The steps of classical conditioning.

This example shows how a mother classically conditioned her baby to make sucking movements by stroking the baby’s forehead at the beginning of feedings.

Classical Conditioning

Newborn reflexes, discussed in Chapter 3, make classical conditioning possible in the young

infant. In this form of learning, a neutral stimulus is paired with a stimulus that leads to a

reflexive response. Once the baby’s nervous system makes the connection between the two

stimuli, the neutral stimulus produces the behavior by itself. Classical conditioning helps

infants recognize which events usually occur together in the everyday world, so they can

anticipate what is about to happen next. As a result, the environment becomes more orderly

and predictable. Let’s take a closer look at the steps of classical conditioning.

As Carolyn settled down in the rocking chair to nurse Caitlin, she often stroked Caitlin’s

forehead. Soon Carolyn noticed that each time she did this, Caitlin made sucking

movements. Caitlin had been classically conditioned. Figure 4.7 shows how it happened:

1. Before learning takes place, an unconditioned stimulus (UCS) must consistently produce

a reflexive, or unconditioned, response (UCR). In Caitlin’s case, sweet breast milk (UCS)

resulted in sucking (UCR).

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2. To produce learning, a neutral stimulus that does not lead to the reflex is presented just

before, or at about the same time as, the UCS. Carolyn stroked Caitlin’s forehead as each

nursing period began. The stroking (neutral stimulus) was paired with the taste of milk

(UCS).

3. If learning has occurred, the neutral stimulus by itself produces a response similar to

the reflexive response. The neutral stimulus is then called a conditioned stimulus (CS),

and the response it elicits is called a conditioned response (CR). We know that Caitlin

has been classically conditioned because stroking her forehead outside the feeding

situation (CS) results in sucking (CR).

If the CS is presented alone enough times, without being paired with the UCS, the CR will no

longer occur, an outcome called extinction. In other words, if Carolyn repeatedly strokes

Caitlin’s forehead without feeding her, Caitlin will gradually stop sucking in response to

stroking.

Young infants can be classically conditioned most easily when the association between two

stimuli has survival value. In the example just described, learning which stimuli regularly

accompany feeding improves the infant’s ability to get food and survive (Blass, Ganchrow, &

Steiner, 1984).

In contrast, some responses, such as fear, are very difficult to classically condition in young

babies. Until infants have the motor skills to escape unpleasant events, they have no

biological need to form these associations. After age 6 months, however, fear is easy to

condition. In Chapter 6, we will discuss the development of fear and other emotional

reactions.

Operant Conditioning

In classical conditioning, babies build expectations about stimulus events in the

environment, but their behavior does not influence the stimuli that occur. In operant

conditioning, infants act, or operate, on the environment, and stimuli that follow their

behavior change the probability that the behavior will occur again. A stimulus that

increases the occurrence of a response is called a reinforcer. For example, sweet liquid

reinforces the sucking response in newborns. Removing a desirable stimulus or presenting

an unpleasant one to decrease the occurrence of a response is called punishment. A sour-

tasting fluid punishes newborns’ sucking response, causing them to purse their lips and stop

sucking entirely.

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Many stimuli besides food can serve as reinforcers of infant behavior. For example,

newborns will suck faster on a nipple when their rate of sucking produces interesting sights

and sounds, including visual designs, music, or human voices (Floccia, Christophe, &

Bertoncini, 1997). As these findings suggest, operant conditioning is a powerful tool for

finding out what stimuli babies can perceive and which ones they prefer.

As infants get older, operant conditioning includes a wider range of responses and stimuli.

For example, researchers have hung mobiles over the cribs of 2- to 6-month-olds. When the

baby’s foot is attached to the mobile with a long cord, the infant can, by kicking, make the

mobile turn. Under these conditions, it takes only a few minutes for infants to start kicking

vigorously (Rovee-Collier, 1999; Rovee-Collier & Barr, 2001). As you will see in Chapter 5,

operant conditioning with mobiles is frequently used to study infants’ memory and their

ability to group similar stimuli into categories. Once babies learn the kicking response,

researchers see how long and under what conditions they retain it when exposed again to

the original mobile or to mobiles with varying features.

Operant conditioning also plays a vital role in the formation of social relationships. As the

baby gazes into the adult’s eyes, the adult looks and smiles back, and then the infant looks

and smiles again. As the behavior of each partner reinforces the other, both continue their

pleasurable interaction. In Chapter 6, we will see that this contingent responsiveness

contributes to the development of infant–caregiver attachment.

Habituation

At birth, the human brain is set up to be attracted to novelty. Infants tend to respond more

strongly to a new element that has entered their environment, an inclination that ensures

that they will continually add to their knowledge base. Habituation refers to a gradual

reduction in the strength of a response due to repetitive stimulation. Looking, heart rate,

and respiration rate may all decline, indicating a loss of interest. Once this has occurred, a

new stimulus—a change in the environment—causes responsiveness to return to a high

level, an increase called recovery. For example, when you walk through a familiar space,

you notice things that are new and different—a recently hung picture on the wall or a piece

of furniture that has been moved. Habituation and recovery make learning more efficient

by focusing our attention on those aspects of the environment we know least about.

Researchers investigating infants’ understanding of the world rely on habituation and

recovery more than any other learning capacity. For example, a baby who first habituates to

a visual pattern (a photo of a baby) and then recovers to a new one (a photo of a bald man)

appears to remember the first stimulus and perceive the second one as new and different

from it. This method of studying infant perception and cognition, illustrated in Figure 4.8,

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can be used with newborns, including preterm infants (Kavšek & Bornstein, 2010). It has

even been used to study the fetus’s sensitivity to external stimuli—for example, by

measuring changes in fetal heart rate when various repeated sounds are presented (see

page 85 in Chapter 3).

Recovery to a new stimulus, or novelty preference, assesses infants’ recent memory. TAKE A

MOMENT… Think about what happens when you return to a place you have not seen for a

long time. Instead of attending to novelty, you are likely to focus on aspects that are

familiar: “I recognize that—I’ve been here before!” Like adults, infants shift from a novelty

preference to a familiarity preference as more time intervenes between habituation and

test phases in research. That is, babies recover to the familiar stimulus rather than to a

novel stimulus (see Figure 4.8)

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(Bahrick, Hernandez-Reif, & Pickens, 1997; Courage & Howe, 1998; Flom & Bahrick, 2010;

Richmond, Colombo, & Hayne, 2007). By focusing on that shift, researchers can also use

habituation to assess remote memory, or memory for stimuli to which infants were exposed

weeks or months earlier.

As Chapter 5 will reveal, habituation research has greatly enriched our understanding of

how long babies remember a wide range of stimuli. And by varying stimulus features,

researchers can use habituation and recovery to study babies’ ability to categorize stimuli

as well.

FIGURE 4.8 Using habituation to study infant perception and cognition.

In the habituation phase, infants view a photo of a baby until their looking declines. In the test phase, infants are again shown the baby photo, but this time it appears alongside a photo of a bald-headed

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man. (a) When the test phase occurs soon after the habituation phase (within minutes, hours, or days, depending on the age of the infants), participants who remember the baby face and distinguish it from the man’s face show a novelty preference; they recover to (spend more time looking at) the new stimulus. (b) When the test phase is delayed for weeks or months, infants who continue to remember the baby face shift to a familiarity preference; they recover to the familiar baby face rather than to the novel man’s face.

Imitation

Babies come into the world with a primitive ability to learn through imitation—by copying

the behavior of another person. For example, Figure 4.9 shows a human newborn imitating

two adult facial expressions (Meltzoff & Moore, 1977). The newborn’s capacity to imitate

extends to certain gestures, such as head and index-finger movements, and has been

demonstrated in many ethnic groups and cultures (Meltzoff & Kuhl, 1994; Nagy et al., 2005).

As the figure illustrates, even newborn primates, including chimpanzees (our closest

evolutionary relatives), imitate some behaviors (Ferrari et al., 2006; Myowa-Yamakoshi et

al., 2004).

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FIGURE 4.9 Imitation by human and chimpanzee newborns.

The human infants in the middle row imitating (left) tongue protrusion and (right) mouth opening are 2 to 3 weeks old. The chimpanzee imitating both facial expressions is 2 weeks old.

(From A. N. Meltzoff & M. K. Moore, 1977, “Imitation of Facial and Manual Gestures by Human Neonates,” Science, 198, p. 75. Copyright © 1977 by AAAS. Reprinted with permission of the AAAS and A. N. Meltzoff. And from M. Myowa-Yamakoshi et al., 2004, “Imitation in Neonatal Chimpanzees [Pan Troglodytes].” Developmental Science, 7, p. 440. Copyright 2004 by Blackwell Publishing. Reproduced with permission of John Wiley & Sons Ltd.)

Although newborns’ capacity to imitate is widely accepted, a few studies have failed to

reproduce the human findings (see, for example, Anisfeld et al., 2001). And because

newborn mouth and tongue movements occur with increased frequency to almost any

arousing change in stimulation (such as lively music or flashing lights), some researchers

argue that certain newborn “imitative” responses are actually mouthing—a common early

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