3.1 Growth in Infancy

In infancy, growth is so rapid and the consequences of neglect are so severe that gains are closely monitored. Medical checkups, including measurements of height, weight, and head circumference, occur often in developed nations because these measurements provide the first clues as to whether an infant is progressing as expected—or not.

Body Size

Weight gain is dramatic. Newborns lose a bit of weight in the first three days of life and then gain about 30 grams a day for several months. Birth weight typically doubles by 4 months and triples by a year. A typical 3175-gram newborn would be approximately 9525 grams at 12 months.

Physical growth in the second year is slower but still rapid. By 24 months, most children weigh almost 13 kilograms. They have added more than 30 centimetres in height—from about 51 centimetres at birth to about 86 centimetres at age 2 (see Figure 3.1). This means that 2-year-olds are half their adult height and about a fifth of their adult weight, four times heavier than they were at birth (see Figure 3.2).

FIGURE 3.1 Gender Differences Boys and girls grow at almost the same rate throughout childhood. Compare this graph to the one on weight (Figure 3.2), and note that already by age 2, genetic growth patterns are disturbed by overfeeding and underfeeding.

FIGURE 3.2 Eat and Sleep The rate of increasing weight in the first weeks of life makes it obvious why new babies need to be fed, day and night.

FIGURE 3.3 Growing Up Two-year-olds have already reached half their adult height and three-fourths of their adult brain size.

Each of these numbers is a norm, which is an average, or standard, for a particular population. The “particular population” for the norms just cited is North American infants. Remember, however, that genetic diversity means that some perfectly healthy newborns are smaller or larger than these norms, as we discussed in Chapter 2.

At each well-baby checkup (monthly at first), a doctor or nurse measures the baby’s growth and compares it to that baby’s previous numbers. A baby who always has been average (50th percentile) becomes worrisome if the percentile changes a lot, either up or down. Abnormal growth may signify a problem; that’s why early checkups are vital.

Prenatal and postnatal brain growth (measured by head circumference) is crucial for later cognition (Gilles & Nelson, 2012). If teething or a stuffed nose slow weight gain, nature protects the brain, a phenomenon called head-sparing. From two weeks after conception to 2 years, the brain grows more rapidly than any other part of the body, from about 25 percent of adult weight at birth to almost 75 percent. Over the same two years, brain circumference increases from about 36 to 48 centimetres (see Figure 3.3).

Brain Development

The brain is essential throughout life; it is discussed in every chapter of this book. We begin now with the basics—neurons, axons, dendrites, neurotransmitters, synapses, and the cortex, especially the prefrontal cortex.

Brain BasicsCommunication within the central nervous system (CNS)—the brain and spinal cord—begins with nerve cells, called neurons. At birth, the human brain has billions of neurons, most of them (about 70 percent) in the cortex, the brain’s six outer layers. The cortex is crucial: Most thinking, feeling, and sensing occur in the cortex (Johnson, 2010). The final part of the brain to mature is the prefrontal cortex, the area for anticipation, planning, and impulse control. It is virtually inactive in the first months of infancy, and gradually becomes more efficient in childhood and adolescence (Wahlstrom et al., 2010).

Different areas of the brain have specialized functions. Some regions deep within the skull maintain breathing and heartbeat, some in the midbrain underlie emotions and impulses, and some in the cortex allow perception and cognition. For instance, there is a visual cortex, an auditory cortex, and an area dedicated to the sense of touch for each body part—including for each finger of a person or each whisker of a rat (Barnett et al., 2006).

Within and between areas of the central nervous system, neurons are linked to other neurons by intricate networks of nerve fibres called axons and dendrites (see Figure 3.4). Each neuron has a single axon and numerous dendrites, which spread out like the branches of a tree. The axon of one neuron meets the dendrites of other neurons at intersections called synapses, tiny gaps that are critical communication links within the brain.

FIGURE 3.4 How Two Neurons Communicate The link between one neuron and another is shown in the simplified diagram at right. The infant brain actually contains billions of neurons, each with one axon and many dendrites. Every electrochemical message to or from the brain causes thousands of neurons to fire, each transmitting the message across the synapse to neighbouring neurons. The electron micrograph directly below shows neurons greatly magnified, with their tangled but highly organized and well-coordinated sets of dendrites and axons.

CNRI/SCIENCE SOURCE

To be more specific, axons and dendrites do not touch at synapses. Instead, neurons communicate by sending electrochemical impulses (called neurotransmitters) through their axons to synapses, to be picked up by the dendrites of other neurons. The dendrites bring messages to the cell bodies of their neurons, which, in turn, convey the messages via their axons to the dendrites of other neurons.

Experiences and PruningAt birth, the brain contains at least 100 billion neurons, more than a person needs. However, the newborn’s brain has far fewer dendrites and synapses than the person will eventually possess. During the first months and years, rapid growth and refinement in axons, dendrites, and synapses occur, especially in the cortex. Dendrite growth is the major reason that brain weight triples from birth to age 2 years (Johnson, 2010).

An estimated fivefold increase in dendrites in the cortex occurs in the 24 months after birth, with about 100 trillion synapses being present at age 2 years. According to one expert, 40 000 new synapses are formed every second in the infant’s brain (Schore & McIntosh, 2011).

Same Boy, Much Changed All three photos show the same child, first at 3 months, then at 12 months, and finally at 24 months. Note the rapid growth in the first two years, especially apparent in head proportions and use of the legs.

© 2010 DIAN LOFTON. ALL RIGHT RESERVED.

This extensive postnatal brain growth is highly unusual for mammals. Why does it occur in humans? Although prenatal brain development is remarkable, it is limited by the simple fact that the human pelvis is relatively small and the head must be relatively small as well to make birth possible. Thus there is an acceleration of growth after birth. In fact, unlike other species, humans must nurture and protect their offspring for more than a decade as children’s brains continue to develop (Konner, 2010).

Early dendrite growth is called transient exuberance: exuberant because it is so rapid and transient because some of it is temporary. The expansive growth of dendrites is followed by pruning. Just as a gardener might prune a rose bush by cutting away some growth to enable more, or lovelier, roses to bloom, unused brain connections atrophy and die (Stiles & Jernigan, 2010).

The specifics of brain structure and growth depend on genes and maturation but even more on experience (Stiles & Jernigan, 2010). Some dendrites wither away because they are never used—that is, no experiences have caused them to send a message to other neurons. Expansion and pruning of dendrites occur for every aspect of early experience, from noticing musical rhythms to understanding emotions (Scott et al., 2007). Strangely enough, this loss of dendrites increases brainpower. The space between neurons in the human brain, for instance—especially in regions for advanced, abstract thought—is far greater than the space in chimpanzee brains (Miller, 2010). The densely packed neurons of chimps make them less intelligent than people, probably because humans have more space for dendrite formation. This allows more synapses and thus more complex thinking.

Some children with intellectual disabilities have “a persistent failure of normal synapse pruning” (Irwin et al., 2002). That makes thinking difficult. For example, one sign of autism is more rapid brain growth, suggesting too little pruning (Hazlett et al., 2011). Yet just as too little pruning creates problems, so does too much pruning. Brain sculpting is attuned to experience: The appropriate links in the brain need to be established, protected, and strengthened while inappropriate ones are eliminated. One group of scientists speculates that “lack of normative experiences may lead to overpruning of neurons and synapses, both of which may lead to reduction of brain activity” (Moulson et al., 2009). Another group suggests that infants who are often hungry, or hurt, or neglected develop brains that compensate—and cannot be reprogrammed even if circumstances change. The hungry baby becomes the obese adult, the neglected child rejects attention, and so on, always with the interaction of nature and nurture (van IJzendoorn et al., 2012).

William Greenough and his colleagues explored the plasticity of the brain and discovered that experiences influence how the brain develops and matures. They believed that there are two types of categorization schemes present in the brain, depending on the type of information that is to be stored. The first type of scheme is the experience-expectant information storage, which includes the environmental information that is common to all people (e.g., seeing contrast, borders, or patterns). The second type of scheme is called experience-dependent, which stores important and specific information, unique to the individual (e.g., sources of food). Thus, new synaptic connections are made in response to each person’s experiences and what needs to be remembered (Greenough et al., 1987).

Plasticity of the BrainDuring the prenatal stage, the brain develops rapidly, as various parts take on their specialized functions. Some of this development is automatic, due to genetically predetermined pathways. However, the brain is also very vulnerable to environmental influences, and these influences can set the stage for major neurological patterns in the future (e.g., competence, health, and well-being) (Mustard, 2006). Thus, the brain is high in plasticity, meaning that it can be modified and changed by environmental circumstances. For example, infants raised in an environment with minimal sensory stimulation have been found to have a different brain structure and weight than infants who have been raised in enriched settings (Cicchetti, 2003; Couperus & Nelson, 2006).

One advantage of the brain’s plasticity is the ability to compensate or take over the functions of certain areas that may have been damaged by disease or accident. However, as just noted, such plasticity also means the brain is highly vulnerable to impoverished or restricted environments, and this can lead to damage that is significant enough to have serious implications for future development.

Some researchers have found that language and literacy assessment can provide an indication of overall brain development in the early years. The sounds of the language that infants are exposed to have been found to influence how the auditory neurons function. For example, during the first 7 to 8 months, if infants are exposed to two languages (e.g., English and French), they are more likely to speak each language idiomatically, that is, with no discernible accent. Those who learn two languages during their early years have a larger left brain (Mustard, 2006), which may assist with language acquisition and fluency.

Electric Excitement Milo’s delight at his mother’s facial expressions is visible, not just in his eyes and mouth, but also in the neurons of the outer layer of his cortex. Electrodes map his brain activation, region by region and moment by moment. Every month of life up to age 2 shows increased electrical excitement.

LAWRENCE MIGDALE/SCIENCE SOURCE

A VIEW FROM SCIENCE

Face Recognition

Unless you have prosopagnosia (face blindness, relatively rare), one part of your brain, the fusiform face area, is astonishingly adept at face recognition. This area is primed among newborns, although it has yet to reflect experience: Newborns stare at monkey faces as well as at human ones, and at pictures and toys with faces as well as at live faces.

Soon, experiences (such as seeing one’s parents again and again) refine perception (de Heering et al., 2010). By 3 months, most babies smile more readily at familiar people and are more accurate at differentiating faces from their own species and their own ethnic group (called the own-race effect).

The own-race effect is the result of limited multi-ethnic experiences, not innate prejudice. Children of one ethnicity, adopted and raised exclusively among people of another ethnicity, recognize differences among people of their adopted group more readily than differences among people of their biological group.

The importance of early experience is further evidenced in two studies. In one, over the course of three months, 6-month-old infants were repeatedly (more than 30 times) shown a book of pictures of six monkey faces, each with a name written on the page (see photo).

Distinguishing Between Faces If you heard that Dario was quite different from Louis or Boris, would you stare at unfamiliar monkey faces more closely in the future? For 6-month-olds, the answer is yes.

REPRINTED FROM NEUROPSYCHOLOGIA, 48, SCOTT, L. ET AL. EXPERIENCE-DEPENDENT NEURAL SPECIALIZATION DURING INFANCY, 1857-1861. COPYRIGHT 2010 WITH PERMISSION FROM ELSEVIER.

For one-third of the babies, the parents read the names while showing the pictures; another one-third of the parents said only “monkey” as they turned each page; and the final one-third simply turned pages. At 9 months, all the infants viewed pictures of six unfamiliar monkeys. The infants who had repeatedly seen named monkeys were better at distinguishing one new monkey from another than the infants who saw the same picture book but did not hear each monkey’s name (Scott & Monesson, 2010).

The second study was premised on the fact that many children and adults do not notice the individuality of newborns. Some even claim that “all babies look alike.” However, the study found that 3-year-olds with younger siblings were much better at recognizing differences between photos of unfamiliar newborns than were 3-year-olds with no younger brothers or sisters (Cassia et al., 2009). This shows that experience matters, contributing to development of dendrites in the fusiform face area.

Harm and ProtectionMost infants develop well within their culture, and head-sparing usually ensures that baby brains are sufficiently nourished. For brain development, it does not matter whether a baby hears French or Farsi, or sees emotions dramatically or subtly (e.g., throwing oneself to the floor or merely pursing the lips). However, infant brains do not develop well without certain experiences that all humans need.

Because of brain plasticity and its consistent development, parents and others in the infants’ social world play an important role in the baby’s brain development. There are many simple and efficient ways of promoting healthy development. To begin with, parents need to provide a stimulating environment, which can include talking and singing to the baby, playing, massaging, and engaging in other sensory activities, all of which provide fodder for brain connections. Severe lack of stimulation (e.g., no talking at all) stunts the brain. As the saying goes, “use it or lose it.” As one review of early brain development explains, “enrichment and deprivation studies provide powerful evidence of…widespread effects of experience on the complexity and function of the developing system” (Stiles & Jernigan, 2010).

This does not mean that babies require spinning, buzzing, multi-textured, and multicoloured toys. In fact, such toys may be a waste of time and money since infants can be overstimulated by them; babies usually cry or go to sleep when that happens, to avoid bombardment. While there is no evidence that such overstimulation harms the infant brain, it is clear that babies are just as fascinated by simple objects and facial expressions as they are by complicated ones.

A simple application of what has been learned about the prefrontal cortex is that hundreds of objects, from the very simple to the quite elaborate, can capture an infant’s attention. There is also a tragic implication: The brain is not yet under thoughtful control, since the prefrontal cortex is not well developed. Unless adults understand this, they might get angry if an infant keeps crying. Infants cry as a reflex to pain (usually digestive pain); they are too immature to decide to stop crying, as adults do.

If a frustrated caregiver reacts to crying by shaking the baby, it can cause a life-threatening condition called shaken baby syndrome (SBS), which occurs when infants are shaken back and forth sharply and quickly. Because the brain is still developing, shaking stops the crying because blood vessels in the brain rupture and fragile neural connections break. Pediatricians consider shaken baby syndrome an example of abusive head trauma (Christian et al., 2009). Death is the worst possible result; lifelong intellectual impairment is the more likely one.

According to one study of 364 injured children under the age of 5 years who were admitted to pediatric hospitals across Canada, 69 (19 percent) of the children died outright of their injuries. Of those who survived, 55 percent suffered long-term neurological damage and 65 percent had visual impairments. Only 22 percent of survivors showed no signs of continuing developmental impacts when they were released from hospital (King et al., 2003). The study also found that in cases where the abuser was identified, it was usually a parent, either the biological father (50 percent), the stepfather or partner (20 percent), or the biological mother (12 percent). Such statistics have encouraged researchers and government agencies to develop remedial programs that will raise parental awareness of the serious nature of SBS, and offer advice on ways to avoid the frustration that can lead parents to shake a baby who won’t stop crying.

In 2004, the state of New York passed a law requiring mothers of newborns to watch a 15-minute video on SBS before leaving hospital. One of the project coordinators has credited this program with reducing the incidence of SBS in western New York by 50 percent (CBC, 2005). A temporary program in Vancouver that distributed information booklets and DVDs to mothers of newborns also showed promising results (Barr et al., 2009). All such efforts indicate the importance of education in lowering the rates of what Health Canada has called “a preventable tragedy” (Public Health Agency of Canada, 2012b).

The fact that infant brains respond to their circumstances suggests that waiting until evidence shows that a young child has been mistreated is waiting too long. In the first months of life, babies adjust to their world, becoming withdrawn and quiet if their caregivers are depressed or becoming loud and demanding if that is the only way they get fed. Such adjustments set patterns that are destructive later on. Thus, understanding development as dynamic and interactive means helping caregivers from the start, not waiting until destructive systems are established (Tronick & Beegly, 2011). The word “systems” is crucial here. Almost every baby experiences something stressful—a caregiver yelling, or a fall off the bed, or a painful stomach. Fortunately, self-righting—an inborn drive to remedy deficits—is built into the human system. Infants with few toys develop their brains by using whatever objects are available, and infants whose mothers are neglectful may develop close bonds with someone else who provides daily stimulation.

Human brains are designed to grow and adapt; plasticity is apparent from the beginning of life (Tomalski & Johnson, 2010). It is the patterns, not the moments, of neglect or maltreatment that harm the brain.

Sleep

One consequence of brain maturation is the ability to sleep through the night. Newborns cannot do this. Normally they sleep 15–17 hours a day, in one- to three-hour segments. Hours of sleep decrease rapidly with maturity: The norm per day for the first 2 months is 14¼ hours; for the next 3 months, 13¼ hours; and for 6 to 17 months, 12¾ hours. Variation is particularly apparent in the early weeks. As reported by parents (who might exaggerate), one new baby in 20 sleeps nine hours or fewer per day, and one in 20 sleeps 19 hours or more (Sadeh et al., 2009).

Sleep specifics vary not only because of biology (age and genes), but also because of the social environment. With responsive parents, full-term newborns sleep more than low-birth-weight babies, who are hungry every two hours. Babies who are fed cow’s milk and cereal sleep more soundly—easier for parents but not ideal for the baby. Social environment has a direct effect: If parents respond to pre-dawn cries with food and play, babies wake up early every morning (Sadeh et al., 2009).

Over the first months, the relative amount of time in various stages of sleep changes. Babies born preterm may always seem to be dozing. Full-term newborns dream a lot; about half their sleep is REM (rapid eye movement) sleep, with flickering eyes and rapid brain waves. That indicates dreaming. REM sleep declines over the early weeks, as does “transitional sleep,” the dozing, half-awake stage. At 3 or 4 months, quiet sleep (also called slow-wave sleep) increases, as does time alert and wide awake.

Overall, 25 percent of parents of children under age 3 years report that their babies have sleeping problems, according to an Internet study of more than 5000 North Americans (Sadeh et al., 2009). Sleep problems are more troubling for parents than for infants. This does not render them insignificant; overtired parents may be less patient and responsive (Bayer et al., 2007). Patience is also needed to ensure the baby’s sleep position is properly “back to sleep,” to protect against sudden infant death syndrome (see Chapter 1).

One problem for parents is that advice about where infants should sleep varies, from contending that infants should sleep beside their parents—who must immediately respond to every cry (Nicholson & Parker, 2009)—to advising that infants need their own room, should be allowed to “cry it out” so they will not be spoiled, and can learn to soothe themselves. Both sets of advice make sense, as the following explains.

Protective Sleeping It matters little what infants sleep in—bassinet, cradle, crib, or Billum bag made from local plants in Papua, New Guinea, as shown here. In fact, this kind of bag is very useful since babies can easily be carried in it. It can also be used for carrying food, tools, and much else. What does matter is the infant’s sleeping position—always on the back, like this healthy infant.

PETER SOLNESS/GETTY IMAGES

OPPOSING PERSPECTIVES

Where Should Babies Sleep?

Traditionally, most middle-class North American infants slept in cribs in their own rooms; it was feared that they would be traumatized by the parents’ sexual interactions. By contrast, infants in Asia, Africa, and Latin America slept with their parents, a practice called co-sleeping. People in those cultures believed that parent—child separation at night was cruel.

Even today, at baby’s bedtime, Asian and African mothers worry more about separation, whereas European and North American mothers worry more about lack of privacy. A study in 19 nations confirms that parents act on their fears: The extremes were 82 percent of babies in Vietnam sleeping with their parents compared to 6 percent of babies in New Zealand (Mindell et al., 2010) (see Figure 3.5).

FIGURE 3.5 Awake at Night Why the disparity between Asian and non-Asian rates of co-sleeping? It may be that Western parents use a variety of gadgets and objects—monitors, night lights, pacifiers, cuddle cloths, sound machines—to accomplish the same thing Asian parents do by having their infant next to them.

At first, this may seem to be a matter of income: Families of low socioeconomic status (SES) are less likely to have an extra room. But even wealthy Japanese families often co-sleep, and many poor Western families find a separate space for their children to sleep. Co-sleeping is a matter of custom, not merely income (Kohyama et al., 2011).

The argument for co-sleeping is that it is easier to respond to infants in the middle of the night, especially if a baby is hungry or scared. When parents opt for co-sleeping, they are less exhausted since they can reach over to feed or comfort their baby. Breastfeeding, often done every hour or two at first, is easier with co-sleeping—one reason many high-SES North Americans now practise it.

Yet the argument against co-sleeping rests on a chilling statistic: Sudden infant death is more common when babies sleep beside their parents (Gettler & McKenna, 2010; Ruys et al., 2007). In fact, the Public Health Agency of Canada (2011a) strongly recommends that infants under 4 months be placed on their backs in their own crib, cradle, or bassinet. Young infants should not share an adult’s bed, couch, or armchair at bedtime.

Furthermore, adult beds, unlike cribs, are often soft, with comforters, mattresses, and pillows that increase the risk of suffocation (Alm, 2007). A commercial solution is a “co-sleeper,” which is a baby bed designed to be next to the parents’ bed but not in it. That way, the dangers of bed-sharing are avoided. Privacy issues remain, however.

One reason for opposing views is that every adult is affected by his or her long-past babyhood and seeks to avoid the mistakes of his or her parents. This phenomenon is called ghosts in the nursery because the parents bring decades-old memories into the bedrooms of their children. Those ghosts can push for either co-sleeping or separate rooms.

Infant at Risk? Sleeping in the parents’ bed is a risk factor for SIDS in North America, However, in Japan, 97 percent of infants sleep next to their parents, yet infant mortality is only 3 per 1000. What explains this difference in infant mortality?

YAGI STUDIO/GETTY IMAGES

One study found that, compared with Israeli adults who had slept near their parents as infants, those who had slept communally with other infants (as sometimes occurred on a kibbutz) often interpreted their own infants’ nighttime cries as distress requiring comfort (Tikotzky et al., 2010). In other words, a ghost from the past is affecting current behaviour; when parents think their crying babies are frightened, lonely, and distressed, they want to respond quickly. Quick responses are more possible with co-sleeping. On the other hand, if parents’ only private time and place is in their bedroom at night, the marriage may suffer if the baby sleeps there.

A developmental perspective begins with what we know: Infants learn from their earliest experiences. If babies become accustomed to bed-sharing, they will crawl into their parents’ bed when they are long past infancy. Parents might lose sleep for years because they wanted more sleep when their babies were small.

Of course, that concern reflects a cultural norm as well. According to an ethnographic study, by the time Mexican Mayan children are 5 years old, they choose when, how long, and with whom to sleep (Gaskins, 1999), a practice that bewilders many other North Americans.

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Developmentalists hesitate to declare any particular pattern best (Tamis-LeMonda et al., 2008) because the issue is “tricky and complex” (Gettler & McKenna, 2010). Sleeping alone may encourage independence and individuality—traits appreciated in some cultures, and not so much in others. Interestingly, one British study found that mothers and fathers who adjusted to sleeping with their infants felt this bonding experience was more rewarding than disruptive. They also found that it encouraged more involvement on the part of the father in nighttime caregiving (Ball et al., 2000).