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.

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

·   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 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 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.

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 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

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.

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.

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 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 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.
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).

(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.)

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 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,  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 botheyes 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 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 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.

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 ).

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 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 ).

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.

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 ).

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, 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 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.

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 & 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

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?

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 ). 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 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.

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 .)

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.

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 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.
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?

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 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 (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.

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.

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).

· 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.

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 , 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 ) (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 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 ).

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 exploratory response to interesting stimuli (Jones,  2009 ). Furthermore, imitation is harder to induce in babies 2 to 3 months old than just after birth. Therefore, skeptics believe that the newborn imitative capacity is little more than an automatic response that declines with age, much like a reflex (Heyes,  2005 ).

Others claim that newborns—both primates and humans—imitate a variety of facial expressions and head movements with effort and determination, even after short delays—when the adult is no longer demonstrating the behavior (Meltzoff & Moore,  1999 ; Paukner, Ferrari, & Suomi,  2011 ). Furthermore, these investigators argue that imitation—unlike reflexes—does not decline. Human babies several months old often do not imitate an adult’s behavior right away because they first try to play familiar social games—mutual gazing, cooing, smiling, and waving their arms. But when an adult models a gesture repeatedly, older human infants soon get down to business and imitate (Meltzoff & Moore,  1994 ). Similarly, imitation declines in baby chimps around 9 weeks of age, when mother–baby mutual gazing and other face-to-face exchanges increase.

According to Andrew Meltzoff, newborns imitate much as older children and adults do—by actively trying to match body movements they see with ones they feel themselves make (Meltzoff,  2007 ). Later we will encounter evidence that young infants are remarkably adept at coordinating information across sensory systems.

Indeed, scientists have identified specialized cells in motor areas of the cerebral cortex in primates—called  mirror neurons —that underlie these capacities (Ferrari & Coudé,  2011 ). Mirror neurons fire identically when a primate hears or sees an action and when it carries out that action on its own(Rizzolatti & Craighero,  2004 ). Human adults have especially elaborate systems of mirror neurons, which enable us to observe another’s behavior (such as smiling or throwing a ball) while simulating the behavior in our own brain. Mirror neurons are believed to be the biological basis of a variety of interrelated, complex social abilities, including imitation, empathic sharing of emotions, and understanding others’ intentions (Iacoboni,  2009 ; Schulte-Ruther et al.,  2007 ).

Brain-imaging findings support a functioning mirror-neuron system as early as 6 months of age. Using NIRS, researchers found that the same motor areas of the cerebral cortex were activated in 6-month-olds and in adults when they observed a model engage in a behavior that could be imitated (tapping a box to make a toy pop out) as when they themselves engaged in the motor action (Shimada & Hiraki,  2006 ). In contrast, when infants and adults observed an object that appeared to move on its own, without human intervention (a ball hanging from the ceiling on a string, swinging like a pendulum), motor areas were not activated.

Still, Meltzoff’s view of newborn imitation as a flexible, voluntary capacity remains controversial. Mirror neurons, though possibly functional at birth, undergo an extended period of development (Bertenthal & Longo,  2007 ; Lepage & Théoret,  2007 ). Similarly, as we will see in  Chapter 5 , the capacity to imitate expands greatly over the first two years. But however limited it is at birth, imitation is a powerful means of learning. Using imitation, infants explore their social world, not only learning from other people but getting to know them by matching their behavioral states. As babies notice similarities between their own actions and those of others, they experience other people as “like me” and, thus, learn about themselves (Meltzoff,  2007 ). In this way, infant imitation may serve as the foundation for understanding others’ thoughts and feelings, which we take up in  Chapter 6 . Finally, caregivers take great pleasure in a baby who imitates their facial gestures and actions, which helps get the infant’s relationship with parents off to a good start.

ASK YOURSELF

REVIEW Provide an example of classical conditioning, of operant conditioning, and of habituation/recovery in young infants. Why is each type of learning useful?

CONNECT Which learning capacities contribute to an infant’s first social relationships? Explain, providing examples.

APPLY Nine-month-old Byron has a toy with large, colored push buttons on it. Each time he pushes a button, he hears a nursery tune. Which learning capacity is the manufacturer of this toy taking advantage of? What can Byron’s play with the toy reveal about his perception of sound patterns?

 Motor Development

Carolyn, Monica, and Vanessa each kept a baby book, filled with proud notations about when their children first held up their heads, reached for objects, sat by themselves, and walked alone. Parents are understandably excited about these new motor skills, which allow babies to master their bodies and the environment in new ways. For example, sitting upright gives infants a new perspective on the world. Reaching permits babies to find out about objects by acting on them. And when infants can move on their own, their opportunities for exploration multiply.

Babies’ motor achievements have a powerful effect on their social relationships. When Caitlin crawled at 7½ months, Carolyn and David began to restrict her movements by saying no and expressing mild impatience. When she walked three days after her first birthday, the first “testing of wills” occurred (Biringen et al.,  1995 ). Despite her mother’s warnings, she sometimes pulled items from shelves that were off limits. “I said, ‘Don’t do that!’” Carolyn would say firmly, taking Caitlin’s hand and redirecting her attention.

At the same time, newly walking babies more actively attend to and initiate social interaction (Clearfield, Osborn, & Mullen,  2008 ; Karasik et al.,  2011 ). Caitlin frequently toddled over to her parents to express a greeting, give a hug, or show them objects of interest. Carolyn and David, in turn, increased their expressions of affection and playful activities. And when Caitlin encountered risky situations, such as a sloping walkway or a dangerous object, Carolyn and David intervened, combining emotional warnings with rich verbal and gestural information that helped Caitlin notice critical features of her surroundings, regulate her motor actions, and acquire language (Campos et al.,  2000 ; Karasik et al.,  2008 ). Caitlin’s delight as she worked on new motor skills triggered pleasurable reactions in others, which encouraged her efforts further. Motor, social, cognitive, and language competencies developed together and supported one another.

The Sequence of Motor Development

Gross-motor development refers to control over actions that help infants get around in the environment, such as crawling, standing, and walking. Fine-motor development has to do with smaller movements, such as reaching and grasping.  Table 4.2  shows the average age at which U.S. infants and toddlers achieve a variety of gross- and fine-motor skills. It also presents the age ranges during which most babies accomplish each skill, indicating large individual differences in rate of motor progress. Also, a baby who is a late reacher will not necessarily be a late crawler or walker. We would be concerned about a child’s development only if many motor skills were seriously delayed.

Historically, researchers assumed that motor skills were separate, innate abilities that emerged in a fixed sequence governed by a built-in maturational timetable. This view has long been discredited. Rather, motor skills are interrelated. Each is a product of earlier motor attainments and a contributor to new ones. And children acquire motor skills in highly individual ways. For example, before her adoption, Grace spent most of her days lying in a hammock. Because she was rarely placed on her tummy and on firm surfaces that enabled her to move on her own, she did not try to crawl. As a result, she pulled to a stand and walked before she crawled! Babies display such skills as rolling, sitting, crawling, and walking in diverse orders rather than in the sequence implied by motor norms (Adolph, Karasik, & Tamis-LeMonda,  2010 ).

TABLE 4.2 Gross- and Fine-Motor Development in the First Two Years

MOTOR SKILL	AVERAGE AGE ACHIEVED	AGE RANGE IN WHICH 90 PERCENT OF INFANTS ACHIEVE THE SKILL
When held upright, holds head erect and steady	6 weeks	3 weeks–4 months
When prone, lifts self by arms	2 months	3 weeks–4 months
Rolls from side to back	2 months	3 weeks–5 months
Grasps cube	3 months, 3 weeks	2–7 months
Rolls from back to side	4½ months	2–7 months
Sits alone	7 months	5–9 months
Crawls	7 months	5–11 months
Pulls to stand	8 months	5–12 months
Plays pat-a-cake	9 months, 3 weeks	7–15 months
Stands alone	11 months	9–16 months
Walks alone	11 months, 3 weeks	9–17 months
Builds tower of two cubes	11 months, 3 weeks	10–19 months
Scribbles vigorously	14 months	10–21 months
Walks up stairs with help	16 months	12–23 months
Jumps in place	23 months, 2 weeks	17–30 months
Walks on tiptoe	25 months	16–30 months

Note: These milestones represent overall age trends. Individual differences exist in the precise age at which each milestone is attained.

Sources: Bayley, 1969, 1993, 2005.

Photos: (top) © Laura Dwight Photography; (middle) © Laura Dwight Photography; (bottom) © Elizabeth Crews/The Image Works

Motor Skills as Dynamic Systems

According to  dynamic systems theory of motor development , mastery of motor skills involves acquiring increasingly complex systems of action. When motor skills work as a system, separate abilities blend together, each cooperating with others to produce more effective ways of exploring and controlling the environment. For example, control of the head and upper chest combine into sitting with support. Kicking, rocking on all fours, and reaching combine to become crawling. Then crawling, standing, and stepping are united into walking (Adolph & Berger,  2006 ; Thelen & Smith,  1998 ).

Each new skill is a joint product of four factors: (1) central nervous system development, (2) the body’s movement capacities, (3) the goals the child has in mind, and (4) environmental supports for the skill. Change in any element makes the system less stable, and the child starts to explore and select new, more effective motor patterns.

The broader physical environment also profoundly influences motor skills. Infants with stairs in their home learn to crawl up stairs at an earlier age and also more readily master a back-descent strategy—the safest but also the most challenging position because the baby must turn around at the top, give up visual guidance of her goal, and crawl backward (Berger, Theuring, & Adolph,  2007 ). And if children were reared on the moon, with its reduced gravity, they would prefer jumping to walking or running!

LOOK AND LISTEN

Spend an hour observing a newly crawling or walking baby. Note the goals that motivate the baby to move, along with the baby’s effort and motor experimentation. Describe parenting behaviors and features of the environment that promote mastery of the skill.

When a skill is first acquired, infants must refine it. For example, in trying to crawl, Caitlin often collapsed on her tummy and moved backward. Soon she figured out how to propel herself forward by alternately pulling with her arms and pushing with her feet, “belly-crawling” in various ways for several weeks (Vereijken & Adolph,  1999 ). As babies attempt a new skill, related, previously mastered skills often become less secure. As the novice walker experiments with balancing the body vertically over two small moving feet, balance during sitting may become temporarily less stable (Chen et al.,  2007 ). In learning to walk, toddlers practice six or more hours a day, traveling the length of 29 football fields! Gradually their small, unsteady steps change to a longer stride, their feet move closer together, their toes point to the front, and their legs become symmetrically coordinated (Adolph, Vereijken, & Shrout,  2003 ). As movements are repeated thousands of times, they promote new synaptic connections in the brain that govern motor patterns.

Dynamic systems theory shows us why motor development cannot be genetically determined. Because it is motivated by exploration and the desire to master new tasks, heredity can map it out only at a general level. Rather than being hardwired into the nervous system, behaviors are softly assembled, allowing for different paths to the same motor skill (Adolph,  2008 ; Thelen & Smith,  2006 ).

FIGURE 4.10 Reaching “feet first.”

When sounding toys were held in front of babies’ hands and feet, they reached with their feet as early as 8 weeks of age, a month or more before they reached with their hands. This 2½-month-old skillfully explores an object with her foot.

Dynamic Motor Systems in Action.

To find out how babies acquire motor capacities, some studies have tracked their first attempts at a skill until it became smooth and effortless. In one investigation, researchers held sounding toys alternately in front of infants’ hands and feet, from the time they showed interest until they engaged in well-coordinated reaching and grasping (Galloway & Thelen,  2004 ). As  Figure 4.10  shows, the infants violated the normative sequence of arm and hand control preceding leg and foot control, shown in  Table 4.2 . They first reached for the toys with their feet—as early as 8 weeks of age, at least a month before reaching with their hands!

Why did babies reach “feet first”? Because the hip joint constrains the legs to move less freely than the shoulder constrains the arms, infants could more easily control their leg movements. When they first tried reaching with their hands, their arms actually moved away from the object! Consequently, foot reaching required far less practice than hand reaching. As these findings confirm, rather than following a strict, predetermined pattern, the order in which motor skills develop depends on the anatomy of the body part being used, the surrounding environment, and the baby’s efforts.

Cultural Variations in Motor Development.

Cross-cultural research further illustrates how early movement opportunities and a stimulating environment contribute to motor development. Over half a century ago, Wayne Dennis ( 1960 ) observed infants in Iranian orphanages who were deprived of the tantalizing surroundings that induce infants to acquire motor skills. These babies spent their days lying on their backs in cribs, without toys to play with. As a result, most did not move on their own until after 2 years of age. When they finally did move, the constant experience of lying on their backs led them to scoot in a sitting position rather than crawl on their hands and knees. Because babies who scoot come up against furniture with their feet, not their hands, they are far less likely to pull themselves to a standing position in preparation for walking. Indeed, by 3 to 4 years of age, only 15 percent of the Iranian orphans were walking alone.

Cultural variations in infant-rearing practices affect motor development.  TAKE A MOMENT…  Take a quick survey of several parents you know: Should sitting, crawling, and walking be deliberately encouraged? Answers vary widely from culture to culture. Japanese mothers, for example, believe such efforts are unnecessary (Seymour,  1999 ). Among the Zinacanteco Indians of southern Mexico and the Gusii of Kenya, rapid motor progress is actively discouraged. Babies who walk before they know enough to keep away from cooking fires and weaving looms are viewed as dangerous to themselves and disruptive to others (Greenfield,  1992 ).