Brains grow rapidly before birth and throughout infancy, as you saw in Chapter 5. By age 2, most neurons have connected to other neurons and substantial pruning has occurred. The 2-year-old’s brain already weighs 75 percent of what it will weigh in adulthood; the 6-year-old’s brain is 90 percent of adult weight. (The major structures of the brain are diagrammed in Figure 8.1)

Since most of the brain is already present and functioning by age 2, what remains to develop? The most important parts!

Although the brains and bodies of other primates seem better than humans in some ways (they climb trees earlier and faster, for instance) and although many animals have abilities humans lack (smell in dogs, for instance), humans have intellectual capacities far beyond any other animal. Considered from an evolutionary perspective, our brains allowed the human species to develop “a mode of living built on social cohesion, cooperation and efficient planning … survival of the smartest” seems more accurate than survival of the fittest” (Corballis, 2011, p. 194).

As the prefrontal cortex matures, social understanding develops, distinguishing humans from other primates. For example, a careful series of tests given to 106 chimpanzees, 32 orangutans, and 105 human 2½-year-olds found that young children were “equivalent … to chimpanzees on tasks of physical cognition but far outstripped both chimpanzees and orangutans on tasks of social cognition” such as pointing or following someone’s gaze (Herrmann et al., 2007, p. 1365).

Children become better at controlling their emotions when they are with other people, no longer wailing, hitting, or laughing out loud on impulse. This is directly connected to brain development as time passes and family experiences continue, although how much of such control is brain-based and how much is a matter of earlier parenting is disputed (DeLisi, 2014; Kochanska et al., 2009). Nonetheless, gradual self-control is apparent.

For example, when a stranger greets them, many 2-year-olds are speechless, hiding behind their mothers if possible. By age 5, a kindergartner who hides is unusual. Adults may still feel shy, but they bravely respond. Brain scans of the prefrontal cortex and amygdala (soon described) taken at age 18 may show inhibition, but most inhibited people no longer act in extremely anxious ways (Schwartz et al., 2010). (Developmental Link: Emotional regulation is further discussed in Chapter 10.)

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After infancy, some brain growth is the result of proliferation of the communication pathways (dendrites and axons). However, most increased brain weight occurs because of myelination. Myelin (sometimes called the white matter of the brain) is a fatty coating on the axons that speeds signals between neurons (see Figure 8.2).

Although myelination continues for decades, the effects are especially apparent in early childhood (Silk & Wood, 2011). The areas of the brain that show greatest early myelination are the motor and sensory areas (Kolb & Whishaw, 2013), so preschoolers react more quickly to sounds and sights with every passing year.

Speed of thought from axon to neuron becomes pivotal when several thoughts must occur in rapid succession. By age 6, most children can see an object and immediately name it, catch a ball and throw it, write their ABCs in proper sequence, and so on. In fact, rapid naming of letters and objects—possible only when myelination is extensive—is a crucial indicator of later reading ability (Shanahan & Lonigan, 2010).

Of course, adults must be patient when listening to young children talk, helping them get dressed, or watching them write each letter of their names. Everything is done more slowly by 6-year-olds than by 16-year-olds because the younger children’s brains have less myelination, which slows information processing. However, thanks to myelination, older preschoolers are faster than toddlers, who sometimes forget what they were doing before they finish.

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One part of the brain that grows and myelinates rapidly during early childhood is the corpus callosum, a long, thick band of nerve fibers that connects the left and right sides of the brain. Growth of the corpus callosum makes communication between the hemispheres more efficient, allowing children to coordinate the two sides of their brains and hence, both sides of their bodies.

Serious disorders result when the corpus callosum fails to develop: This failure is one of several possible causes of autism (Frazier et al., 2012; Floris et al., 2013). Such failures lead to a variety of symptoms, always including intellectual disability (Cavalari & Donovick, 2014).

To understand the significance of the corpus callosum, note that each side of the body and brain specializes and is therefore dominant for certain functions. This is lateralization, literally, “sidedness.” The entire human body is lateralized, apparent not only in right- or left-handedness but also in the feet, the eyes, the ears, and the brain itself. People prefer to kick a ball, wink an eye, or listen on the phone with their preferred foot, eye, or ear. Genes, prenatal hormones, and early experiences all affect which side does what. Lateralization advances with development of the corpus callosum (Kolb & Whishaw, 2013). The strength of lateralization varies—some people are more ambidextrous than others.

Left-handed people tend to have thicker corpus callosa than right-handed people do, perhaps because they need to readjust the interaction between the two sides of their bodies, depending on the task. For example, most left-handed people brush their teeth with their left hand because using their dominant hand is more natural, but they shake hands with their right hand because that is what the social convention requires.

Infants and toddlers usually prefer one hand to the other for grabbing spoons and rattles. Indeed, there is evidence that lateralization begins in the womb, so some newborns are already left- or right-handed (Ratnarajah et al., 2013). By age 2 most children have a dominant hand for scribbling and throwing. Preschool teachers notice that about 1 child in 10 prefers the left hand.

Many cultures try to make every child right-handed, with some success. When left-handed children are forced to use their right hands, most learn to write right-handedly. But neurologically, success is incomplete: Their brains are only partly reprogrammed, as evidenced when they choose their left hand to comb their hair, throw a ball, or wield a hammer.

Many cultures imply that being right-handed is best, an example of the difference-equals-deficit error, explained in Chapter 1. Consider language: In English, a “left-handed compliment” is insincere, and no one wants to have “two left feet” or to be “out in left field.” In Latin, dexter (as in dexterity) means “right” and sinister means “left” (and also “evil”). Gauche, the French word for left, means “socially awkward” in English. Many languages are written from left to right, which is easier for right-handed people.

The design of doorknobs, scissors, baseball mitts, instrument panels, and other objects favor the right hand. (Some manufacturers have special versions for lefties, but few young children know to ask for them.) In many Asian and African cultures, only the left hand is used for wiping after defecation; it is an insult to give someone anything with that “dirty” hand.

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Developmentalists advise against switching a child’s handedness, not only because this causes adult–child conflicts and may create neurological confusion but also because left lateralization is an advantage in some professions, especially those involving creativity and split-second actions.

A disproportionate number of artists, musicians, and sports stars were/are left-handed, including Pele, Babe Ruth, Monica Seles, Bill Gates, Oprah Winfrey, Jimi Hendrix, Lady Gaga, and Justin Bieber. Using both sides of the body is an advantage: LeBron James writes left-handed but plays basketball right-handed. Five of the past six presidents of the United States were/are lefties: Gerald Ford, Ronald Reagan, George H.W. Bush, Bill Clinton, and Barack Obama.

Acceptance of left-handedness is more widespread now than a century ago. More adults in Great Britain and the United States claim to be left-handed today (about 10 percent) than in 1900 (about 3 percent) (McManus et al., 2010). There seem to be more left-handed men than women, as well as more left-handers in North America than elsewhere. Whether this is cultural or genetic is not known.

Astonishing studies of humans whose corpus callosa were severed to relieve severe epilepsy, as well as research on humans and other vertebrates with intact corpus callosa, have revealed how the brain’s hemispheres specialize. Typically, the brain’s left half controls the body’s right side as well as areas dedicated to logical reasoning, detailed analysis, and the basics of language. The brain’s right half controls the body’s left side and areas dedicated to emotional and creative impulses, including appreciation of music, art, and poetry. Thus, the left side notices details and the right side grasps the big picture.

This left–right distinction has been exaggerated, especially when broadly applied to people (Hugdahl & Westerhausen, 2010). No one is exclusively left-brained or right-brained (except severely brain-damaged individuals). Moreover, the brain is plastic, especially in childhood, so a lost function in one hemisphere is sometimes replaced in the other hemisphere. (Developmental Link: Brain plasticity is discussed in Chapter 1.)

Both sides of the brain are usually involved in every skill. That is why the corpus callosum is crucial. As myelination progresses, signals between the two hemispheres become quicker and clearer, enabling children to become better thinkers and less clumsy. For example, no 2-year-old can hop on one foot, but most 6-year-olds can—an example of brain balancing. Many songs, dances, and games that young children love involve moving their bodies in some coordinated way—difficult, but fun because of that. Logic (left brain) without emotion (right brain) is a severe impairment, as is the opposite (Damasio, 2012).

The entire frontal lobe continues to develop for many years after early childhood; dendrite density and myelination are still increasing in emerging adulthood. Nonetheless, neurological control advances significantly every year between ages 2 and 6, as is evident in several ways:

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One example of the maturing brain is in the game Simon Says. Players are supposed to follow the leader only when orders are preceded by the words “Simon says.” Thus, if leaders touch their noses and say, “Simon says touch your nose,” players are supposed to touch their noses, but when leaders touch their noses and say, “Touch your nose,” no one is supposed to follow the example. Young children lose at this game because they impulsively do what they see and hear. Older children can think before acting. The prefrontal cortex works!

Such advances can be observed in every child, but might personal experience rather than brain maturation be the reason? A convincing demonstration that something neurological, not experiential, is the primary reason for these changes comes from a series of experiments with shapes and colors.

These experiments begin with young children given a set of cards with clear outlines of trucks or flowers, some red and some blue. They are asked to “play the shape game,” putting trucks in one pile and flowers in another. Three-year-olds (and even some 2-year-olds) can do this correctly.

Then children are asked to “play the color game,” sorting the cards by color. Most children under age 4 fail. Instead, they sort by shape, as they had done before. This basic test has been replicated in many nations; 3-year-olds usually get stuck in their initial sorting pattern. By age 5 (and sometimes age 4), most children make the switch.

When this result was first obtained, experimenters thought perhaps the children didn’t have enough experience to know their colors; so the scientists switched the order, first playing “the color game.” Most 3-year-olds did that correctly, because most 3-year-olds know the difference between red and blue. Then, when asked to play “the shape game,” they sorted by color! Even with a new set of cards, such as yellow and green or rabbits and boats, 3-year-olds still tend to sort however they did originally, either by color or shape.

Researchers are looking into many possible explanations for this result (Müller et al., 2006; Marcovitch et al., 2010; Ramscar et al., 2013). All agree, however, that something in the brain must mature before children are able to switch from one way of sorting objects to another. (Developmental Link: Maturation of the prefrontal cortex is also discussed in Chapter 5, Chapter 11, and Chapter 14.)

Neurons have only two kinds of impulses: on–off or activate–inhibit. Each is signaled by biochemical messages from dendrites to axons to neurons. Both activation and inhibition are necessary for thoughtful adults, who neither leap too quickly nor hesitate too long. A balanced brain is best throughout life: One sign of cognitive loss in late adulthood is that some of the elderly become too cautious or too impulsive.

Many young children are notably unbalanced. They are impulsive, flitting from one activity to another. That explains why many 3-year-olds cannot stay quietly on one task, even in “circle time” in preschool, where each child is supposed to sit in place, not talking or touching anyone. Poor impulse control signfies a personality disorder in aduthood but not in early childhood. Few 3-year-olds are capable of sustained attention, which is required in primary school.

For some preschoolers, the see-saw tips in the opposite direction, when children play with a single toy for hours. Perseveration refers to the tendency to persevere in, or stick to, one thought or action, as evident in the card-sorting study just described (Hanania, 2010).

Many explanations for perseveration are plausible, and the tendency is unmistakable. Often young children repeat one phrase or question again and again, and often once they start giggling they find it hard to stop. Another example of perseveration occurs when a child has a tantrum when told to end an activity. (Wise teachers give a warning—”Cleanup in 5 minutes”—which may help.) The tantrum itself may perseverate. Crying may become uncontrollable because the child is stuck in the emotion that triggered the tantrum.

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Impulsiveness and perseveration are opposite manifestations of the same underlying cause: immaturity of the prefrontal cortex. No young child is perfect at regulating attention; impulsiveness and perseveration are evident in every young child.

A longitudinal study of children from ages 3 to 6 found their ability to pay attention increased steadily, and that led to academic learning and behavioral control (fewer outbursts or tears) (Metcalfe et al., 2013). That development continues in middle childhood as brain maturation (innate) and emotional regulation (learned) allow most children to pay attention and switch activities as needed. By adolescence, teenagers change tasks at the sound of the bell.

Exceptions include children with attention-deficit/hyperactivity disorder (ADHD), who are too impulsive for their age. An imbalance between the left and right sides of the prefrontal cortex and abnormal growth of the corpus callosum seem to underlie (and perhaps cause) ADHD (Gilliam et al., 2011).

As with all biological maturation, development of impulse control and behavioral flexibility is related to upbringing. A study of Korean preschoolers found that they developed impulse control and reduced perseveration sooner than a comparable group of English children. This study included the shape–color task: At age 3, 40 percent of Korean children but only 14 percent of British ones successfully shifted from sorting by shape to color. The researchers considered many possible reasons and concluded that “a cultural explanation is more likely” (Oh & Lewis, 2008, p. 96).

Now that we have considered the prefrontal cortex, we turn to another region of the brain, sometimes called the limbic system, the major system for emotions. Emotional expression and emotional regulation advance during early childhood. (Developmental Link: Emotional regulation is discussed further in Chapter 10 and Chapter 15.) Crucial to that advance are three parts of the brain—the amygdala, the hippocampus, and the hypothalamus.

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The amygdala is a tiny structure deep in the brain. It registers emotions, both positive and negative, especially fear (Kolb & Whishaw, 2013). Increased amygdala activity is one reason some young children have terrifying nightmares or sudden terrors, overwhelming the prefrontal cortex. Similarly, a child may refuse to enter an elevator or may be hysterical at a nightmare.

The amygdala responds to comfort but not to logic. If a child is terrified of, say, a dream of a lion in the closet, an adult should not laugh but might open the closet door and command the lion to go home.

Another structure in the emotional network is the hippocampus, located next to the amygdala. A central processor of memory, especially memory for locations, the hippocampus responds to the anxieties of the amygdala by summoning memory. A child can remember, for instance, whether previous elevator riding was scary or fun.

Early memories of location are fragile because the hippocampus is still developing. Nonetheless, emotional memories from early childhood can interfere with expressed, rational thinking: An adult might have a panic attack but not know why.

The interaction of the amygdala and the hippocampus is sometimes helpful, sometimes not; fear can be constructive or destructive (LaBar, 2007). Studies of animals find that when the amygdala is surgically removed, the animals are fearless in situations that should scare them. For instance, a cat without an amygdala will stroll nonchalantly past monkeys—something no normal cat would do (Kolb & Whishaw, 2013).

A third part of the limbic system, the hypothalamus, responds to signals from the amygdala (arousing) and from the hippocampus (usually dampening) by producing cortisol, oxytocin, and other hormones that activate parts of the brain and body (see Figure 8.3). Ideally, this hormone production occurs in moderation. Both temperamental inhibition and parental responses affect whether or not the hypothalamus will overreact, making the preschooler too anxious—as about 20 percent of 4- to 7-year-olds are (Paulus et al., 2014).

As the limbic system develops, young children watch their parents’ emotions closely. If a parent looks worried when entering an elevator, the child may fearfully cling to the parent when the elevator moves. If this sequence recurs often enough, the child may become hypersensitive to elevators, as fear from the amygdala joins memories from the hippocampus, increasing cortisol production via the hypothalamus. If, instead, the parent makes elevator riding fun (letting the child push the buttons, for instance), initial fears subside, and the child’s brain will be aroused to enjoy elevators—even when there is no need to go from floor to floor.

Knowing the varieties of fears and joys is helpful when a teacher takes a group of young children on a trip. To stick with the elevator example, one child might be terrified while another child might rush forward, pushing the close button before the teacher enters. Every experience (elevators, fire engines, train rides, animals at the zoo, a police officer) is likely to trigger a range of emotions, without much reflection, in a group of 3-year-olds: A class trip needs several adults, ready to respond to whatever reactions the children have.

During infancy and early childhood, extreme stress may cause cortisol to flood the brain and destroy part of the hippocampus. There is “extensive evidence of the disruptive impacts of toxic stress” (Shonkoff et al., 2012). Too much of that hormone early in life may lead to permanent deficits in learning and health, causing major depression, post-traumatic stress disorder, and attention-deficit/hyperactivity disorder in childhood and adolescence.

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Yet some stress is needed for normal brain growth. Emotionally arousing experiences—meeting new friends, entering school, visiting a strange place—seem beneficial if a young child has someone or something to moderate the stress. Parental support and child temperament at age 3 are crucial moderators. When past support and experience are in place, cortisol will not be overwhelming. A study of 5- and 6-year-olds exposed to a stressful experience found that cortisol rose dramatically in some children but not at all in others, probably because of individual variations in genes and early childhood experiences (de Weerth et al., 2013).

In an experiment, brain scans and hormone measurements were taken of 4- to 6-year-olds immediately after a fire alarm. (Teoh & Lamb, 2013). As measured by their cortisol levels, some children were upset and some were not. Two weeks later, they were questioned about the event. Those with higher cortisol reactions to the alarm remembered more details than did those with less stress, which suggests that some stress aided memory. There are good evolutionary reasons for that: People need to remember experiences that arouse their emotions, so they can avoid, or adjust to, similar experiences in the future.

Generally, a balance between arousal and reassurance is needed. For instance, if children are witnesses to a crime (a stressful experience), a child’s memory is more accurate when an interviewer is warm and attentive, listening carefully but not suggesting some answers instead of others (Teoh & Lamb, 2013).

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Studies of maltreated children suggest that excessive stress hormone levels in early childhood permanently damage the brain. The impaired hypothalamus produces hormones that affect emotions lifelong (Evans & Kim, 2013; Wilson et al., 2011).

Sadly, this topic leads again to those adopted Romanian children mentioned in Chapter 7. When some of them (presumably those who experienced more stress) saw pictures of happy, sad, frightened, or angry faces, their limbic systems were less reactive than were those of Romanian children who were living with their biological parents. The adopted children’s brains were also less lateralized, suggesting less specialized, less efficient thinking (Nelson et al., 2014). Thus, early stress had probably impaired their brains.

Romania no longer permits wholesale international adoptions. Nonetheless, as mentioned earlier, some children are raised in institutions. They are confined to cribs most of the time, and are severely impaired in motor skills, as well as language and social bonding. As you remember, adoption (local or international) should occur before age 1 for the best outcomes.

That is not always possible, however. In one study, at about age 2 some institutionalized Romanian children were randomly assigned to well-paid and trained foster parents. By age 4, they were smarter (by about 10 IQ points) than those who remained institutionalized (Nelson et al., 2007). This research suggests that ages 2 to 4 constitute a sensitive time for learning because families are able to balance challenge and comfort for each individual, allowing normal brain maturation.

Unfortunately, although these children did better than those who remained in institutions, they lagged behind another group who were with their parents the whole time. That group did not receive excellent physical and medical care, as the foster children did, but they were loved and stimulated by growing up in their families. In neurological development, that made all the difference (Marshall, 2014).

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