15 Physical Development

Although the bulk of Unit 4 is primarily about cognitive and social development, people certainly develop in another obvious way, that is, physically. It is worth focusing exclusively on physical development at first, as it is one of the most obvious ways that people differ from each other. Although physical development is separated from cognitive and social development in this unit, you should realize that it does interact with them. First, many cognitive and social developments depend on prerequisite physical developments. Second, the different types of developments can influence each other. For example, in Module 17, you will learn about attachment, an infant’s emotional bond with a specific person, such as a parent. This is most clearly a social development. In order for an infant to be attached to a specific person, however, they must be able to recognize that person; this is a cognitive development. As the infant develops physically, they become able to move from location to location and can explore their environment. They can use the parent to whom they are attached as a secure base from which they feel confident to stray, so they can make discoveries that will enhance their further cognitive and social development.

This Module has three sections; it is organized principally by age. Section 15.1 describes the extraordinary changes that take place before birth and during childhood. At the moment of conception, the baby-to-be consists of exactly two cells; they divide and subdivide and differentiate rapidly so that nine months later an infant prepared to survive and learn in the world is born. Although the rate of change slows down dramatically after birth, physical developments in childhood are also remarkable, interesting, and important. Section 15.2 covers adolescent and adult development. In a striking reversal of the trend of decreasing rates of growth and change, the adolescent develops rapidly on the path to reaching sexual maturity. Adulthood is traditionally conceived as a period of decline. As you will see, the news is not nearly so pessimistic. Section 15.3 is the exception to the chronological organization of the first two sections in the module. The last section describes the changes that the brain undergoes from the prenatal period, all the way through to late adulthood.

15.1 Prenatal and child physical development

15.2 Adolescent and adult physical development

15.3 Brain development throughout the lifespan

READING WITH A PURPOSE

Remember and Understand

By reading and studying Module 15, you should be able to remember and describe:

• Physical development in the embryo and fetus: zygote, neural tube, testes, ovaries, androgens, amniotic sac, placenta, teratogens, fetal alcohol syndrome, fetal alcohol effect (15.1)
• Physical development in infancy and childhood (15.1)
• Physical developments in adolescence and adulthood: adolescent growth spurt, puberty, primary sex characteristics, secondary sex characteristics (15.2)
• Hormones and the endocrine system: hypothalamus and pituitary gland, gonads, androgens, testosterone, estrogens, progesterone, growth hormone
• Increasing rates of obesity in adulthood: basal metabolic rate, muscle mass (15.2)
• Brain development before birth: neural plate and neural tube, neural stem cells, migration (15.3)
• Brain development in infancy and childhood: myelinization, synaptogenesis (15.3)
• Brain development in adolescence and adulthood (15.3)

Apply

By reading and thinking about how the concepts in Module 15 apply to real life, you should be able to:

• Recognize the characteristic physical features of different aged infants, children and adolescents (15.1 and 15.2)

Analyze, Evaluate, and Create

By reading and thinking about Module 15, participating in classroom activities, and completing out-of-class assignments, you should be able to:

• Combine your knowledge of neurons and the brain from Unit 3 with the developments in Module 15 to predict some behavioral developments (Module 11 and 15; best done before reading the remainder of Unit 4)
• Speculate whether the physical characteristics of people you know, especially older adults, are more likely to be a result of physical development or lifestyle changes (15.1 and 15.2)

15.1 Prenatal and Child Physical Development

From two single cells—one among the largest in the human body, the other, the smallest—to a fully formed newborn infant in 266 days: It is a development in amount and form that will never be approached again in an individual human being.

Physical Development in the Embryo and Fetus

As you may recall from a biology class, when an egg is fertilized by a sperm cell, the resulting cell is called a zygote. The zygote, which contains the combined genetic information from the mother and father, quickly develops through the process of cell division. By about one week after fertilization, the cluster of about 100 cells attaches to the mother’s uterus, from which it begins to receive blood and nutrients; now it is called an embryo. At this stage, the embryo looks like a tiny, mostly-hollow ball of cells called a blastocyst.

During the following weeks, the cells of the embryo change their shapes and begin to relocate, as the embryo organizes itself. The different areas of cells develop into different body parts and organs. For example, one set of cells develops into the neural tube, which will eventually become the central nervous system (spinal cord and brain). By around five weeks, all of the organs have started developing, and although the embryo is only one-half inch long, the eyes, heart, and the beginnings of the arms and legs are visible. How does the embryo “know” how to organize itself? The embryo’s genes direct the specialization, along with hormones that are produced by the embryo itself. Because the embryo is especially sensitive during this time to hormones, which are chemicals, the period during which the major organs are first forming is also a time of great sensitivity to other chemicals, such as toxins.

The sex organs are among the last parts to become differentiated in the developing fetus. Prior to the seventh week, male and female embryos have indistinguishable primitive sex organs; they resemble female organs, by the way. If the 7-week old embryo has a Y chromosome (i.e., if it is male), the male gonads, called testes, begin to develop. If there is no Y chromosome, the female gonads, ovaries, develop. In a sexually mature person, the testes produce sperm, and the ovaries produce eggs. At this point, sex hormones begin to play a role. The newly-formed testes (in males) begin to produce androgens, a group of hormones that play a role in male traits and reproductive activity. These hormones cause the primitive sex organs to develop into male organs. In the absence of androgens, the organs develop into female organs.

The developing fetus is housed in a very controlled environment, a fluid-filled sac called the amniotic sac; it protects the fetus by acting as a shock absorber and temperature regulator. Outside substances can only get in through the placenta, the structure found at the attachment point between the fetus and the mother’s uterus. The placenta allows the exchange of nutrients and waste products. To prevent harmful substances from reaching the fetus, the placenta also acts as a kind of filter. It is a remarkable system, but alas, it is not perfect.

Occasionally, harmful substances from the environment outside of the fetus can reach it; they are called teratogens. Have you ever noticed the cautions posted in x-ray areas? Women are warned to tell the x-ray technician if they might be pregnant. This is because x-rays are a teratogen; they can cause the fetus’s developing organs to become deformed. Other teratogens include cigarette smoke, some prescription drugs, other drugs, such as caffeine and marijuana, lead, and paint fumes.

Alcohol is a very well-known teratogen. If the mother drinks heavily (five to six drinks or more per day) during pregnancy, the child is at a greater risk of developing fetal alcohol syndrome. Children who suffer from fetal alcohol syndrome grow slowly and have distinctive facial features, such as wide-set eyes, thin upper lip, and flattened bridge of the nose. Many fetal alcohol syndrome children catch up and lose the distinctive facial features as they develop (Steinhausen et al., 1994). They are not so fortunate with the other symptoms, however. Fetal alcohol syndrome is also characterized by brain damage and many cognitive and behavioral deficits. The damage can be severe enough to be observed using standard brain imaging techniques but is often simply inferred from behavioral and cognitive testing. The deficits include lower intelligence and academic achievement, increases in learning disabilities, poorer language skills, and increases in distractibility and hyperactivity. These effects of alcohol on a developing fetus are not all-or-none (Astley & Clarren, 2000). Rather, they are graded, and even moderate drinking during pregnancy is associated with less severe versions of many of the same effects (these less severe versions are sometimes called fetal alcohol effect). Clearly, the best advice a pregnant woman can follow—and the advice given by the US Surgeon General—is to completely abstain from drinking alcohol. Women who drank alcohol before discovering that they were pregnant should stop immediately because further consumption would increase the risk of alcohol-related effects.

It is scary; sometimes it seems like the only way to keep a developing fetus safe is to live in a sterilized room and never go out, eat organic rice cakes only, and drink nothing except distilled water. If you have ever heard or wondered about a pregnant woman’s avoidance of wet paint, cigarette smoke (including second-hand smoke), caffeine, and even cat litter boxes, it is because of the possibility that substances contained in these common environmental elements can reach the fetus and disrupt its development. With a little bit of attention, guidance (from healthcare professionals and pregnancy books), and planning, however, the risk of damaging a fetus is actually very low. Still, many mothers-to-be choose to err on the side of caution and avoid substances that may pose little overall risk. This is probably a good idea because the consequences of a teratogen will last a lifetime.

During the remainder of the fetal stage, the fetus grows rapidly, and the organs develop so that the baby will be able to survive on its own when it is born. Obviously, the longer the fetus is able to develop in the uterus, the greater the chances of survival are. For example, a study in Sweden found that babies who are born at 22-26 weeks have about a 70% chance of surviving; those born at 22 weeks have only a 10% chance, while those born at 26 weeks have an 85% chance (The Express Group, 2009). In the US, infants overall have over a 99.3% chance of surviving to age 1.

This 99.3% survival rate corresponds to an infant mortality rate of 5.8. This means that for every 1,000 live births in the US, 5.8 infants will die before they reach one year old. The United States’ rate is higher than you might guess. Monaco (1.60),  Japan (2.0), and Iceland (2.10) have the lowest infant mortality rates in the world. The US rate is only the 55^th best in the world, worse than such countries as Canada, Czechia, Ireland, Belgium, Hong Kong, France, Germany, Slovenia, and the Netherlands. A staggering 18 countries have infant mortality rates above 60. Afghanistan’s rate is 110 per 1000 live births. Let us repeat that. In Afghanistan, for every 1,000 live births, 110 children will not survive to see their first birthday. (The infant mortality rates can be found in the CIA World Factbook, 2017). Countries that have extremely high infant mortality rates are, without exception, very poor. The children die from the disease (including AIDS), parasites, malnourishment, and poor sanitary conditions (Population Reference Bureau, 2004).

Within the US there are substantial differences in infant mortality for different ethnic groups. According to the Centers for Disease Control and Prevention (2016), in the year 2016, the Asian mortality rate was 3.6, White and Hispanic infant mortality rates were around 5.0, Native Hawaiian or other  Pacific Islanders had a rate of 7.4, American Indian/Alaska Native had a rate of 9.4, and African Americans had a rate of 11.4, an infant death rate similar to  Tonga’s (in 2017), the 99th ranked country in the world. The US Government’s Centers for Disease Control and Prevention Office of Minority Health had set up a goal to eliminate the racial and ethnic differences in infant mortality by the year 2010, but they were obviously unsuccessful. They have focused on the likely causes, such as medical problems and illnesses, lack of prenatal care, poor nutrition, smoking and substance abuse, but it is clear that more effort is necessary (CDC Office of Minority Health, 2004).

Physical Development in Infancy and Childhood

Newborns enter the world with a set of programmed behaviors. Several of these reflexes are clearly designed to help the infant to survive. For example, if you stroke the cheek of a newborn, they will turn their head toward the stroke; this is called the rooting reflex and it helps the newborn find their mother’s nipple. Newborns will also reflexively suck anything that touches their lips. Contrary to some people’s beliefs, newborns can see, just not very well (in the words of Module 12, their visual acuity is poor). Their clearest vision is for objects that are about nine inches away, almost exactly the distance between a nursing infant and their mother’s face. As you will see in Modules 16 and 17, newborns are actually quite a bit more capable than you might think, and they are prepared to make enormous strides in cognition and social relationships.

Children are usually referred to as infants until they are two years old, although some people refer to children between one and two as toddlers. Physical development, or at least growth in size, slows dramatically during the first year, a trend that continues until the adolescent growth spurt. Think about what would happen if growth did not slow down. The fetus grew from 2 to 20 inches during the last 26 weeks before birth. If the new baby grew 18 inches every 26 weeks, it would be about 4 feet, 8 inches tall at one year. Parents complain now about their children outgrowing clothes too quickly. In the US, one-year-old babies actually average about 29 inches in height and weigh about 22 pounds, and two-year-olds average 34 inches and 28 pounds (National Center for Health Statistics, 2000).

Young parents sometimes have mixed feelings about one of the most important physical developments, their first child’s developing locomotion skills. Many parents compare notes with peers, swelling with pride when their child can crawl or walk earlier than another child. On the other hand, her newfound ability to move themself to a different location shows them how unprepared they really are for the rigors of vigilant parenting. For the first six months, a parent can be pretty sure that their child will be where they left them if they needed to leave them alone for a minute or two. An infant that can move around, though, requires constant attention and an extremely “child-proofed” house (e.g., electrical cords and all small objects safely out of reach, outlets covered, stairs barricaded).

Most children learn to walk some time around 1 year of age. The stages of development on the path to walking differ little across children, but the length of time the children stay on a particular stage does differ a lot. Sometime around four to five months old, infants learn to roll over. By seven months, most can sit up, and they begin crawling by around eight to nine months. Many infants can stand while holding on to something at the time they learn to crawl. From, then, they typically learn to walk while holding on to objects, then to full, albeit extremely unsteady walking (usually sometime around 12 to 14 months). Infants “wobble” when they walk; the side-to-side movements of each step can be larger than the forward progress. Also, each step covers a different distance, making for a very unsteady and irregular gait (Clark et al., 1988). As the infants get older, these irregularities even out and gait becomes more steady.

What about the pride that some parents feel from their children’s walking accomplishments? Do they really deserve any credit? Infants’ walking skills develop through increases in strength and balance; these increases come about via a combination of growth of the body, maturation of neurons, and experience. Experience can have some impact on the age at which an infant begins to walk, but the other two components must be in place. No matter how much practice you give a 4-month old infant, it will not help them walk at that time. Once the infant’s body is ready, however, early practice does seem to accelerate learning to walk. For example, one study found that daily “practice” over the first eight weeks after birth, in which parents guided the infants through a walking reflex, led the infants to walk two months earlier (9 months versus 11 months) than a “passively exercised” control group (Zelazo et al.,  1972). Newborns have a walking reflex; if you hold them upright and allow their feet to contact a moving surface, they will move their legs as if walking. Infants whose parents worked them through this walking exercise for two and a half minutes per day for three weeks walked earlier than a group whose parents pumped their legs “bicycle-style” for the same amount of time. So, parents might have some impact on the age at which their children begin to walk.

You should realize, however, that it is not necessarily a good thing to have an infant who walks early. First, infants who begin walking at later ages will quickly catch up to earlier walkers, and later practice is the most important factor for improving walking skills (Adolph et al.,  2003). Second, infants’ skulls are not yet fully formed; infants who walk very early may be more prone to injuries from falls because their skulls may not be ready for it (Gott, 1972).

There are really no physical milestones in later childhood as momentous as learning to walk. Rather, the remainder of the period is marked by continuing slow growth and development of more complex skills. Gross motor skills, such as running, jumping, skipping, and balancing begin developing first and continue to develop throughout childhood. For example, children’s ability to balance, the key skill underlying all standing skills, improves throughout the first decade (Roncesvalles et al., 2001). We once tested a four-year-old, an eight-year-old, and an eleven-year-old on their ability to balance on one foot with their eyes closed. The four-year-old lasted three seconds. The eight and eleven-year-old were able to balance for a full two minutes, but the eight-year-old needed to hop around a lot in order to make it. Fine motor skills, the ones that use small muscles of the hands and fingers and require a fair amount of precision, begin developing later than gross motor skills. Perhaps the easiest way to see this is through children’s drawing skills. Children progress from scribbling at age two, to drawing simple shapes at three, to drawing recognizable pictures by about four or five (Kellogg, 1967).

Growth slows to about two to three inches and five pounds per year. It is as if the little body is lying in wait for the dramatic increases of puberty, which marks the beginning of adolescence.

15.2 Adolescent and Adult Physical Development

From the prenatal period through infancy and childhood, we see a pattern in physical development, namely a slower and slower rate of change. With the start of adolescence, there is a stark reversal of that trend. Seemingly overnight, the physical growth rate increases dramatically, and the individual who was a little boy or girl yesterday rapidly comes to resemble a man or woman. When we reach the end of adolescence and enter adulthood, physical growth stops completely. Common wisdom holds that people quickly reach their peak in adulthood, make it “over the hill,” and begin the gradual, but accelerating and inevitable decline. As is often the case, however, common wisdom is not exactly right. Let us now turn to physical development in adolescence and adulthood and see what, in fact, does typically happen.

Physical Changes in Adolescence

We’ll begin with the two immense physical changes that occur during adolescent development: sexual maturity and rapid growth, commonly known as the adolescent growth spurt. Puberty is the term used to describe the period during which the body reaches sexual maturity; it roughly corresponds to adolescence, or around the teenage years. But let us ignore these obvious signs of physical development for a moment and focus on the brain and biochemistry. Hormones help to explain how both the growth spurt and puberty take place.

In Module 11, we briefly described the hypothalamus and pituitary gland; you may recall that the hypothalamus directs the pituitary gland to release hormones. It is time to give you some details about the hormones released by the endocrine system, to which the pituitary gland belongs. The endocrine system is composed of several glands throughout the body; the principal function of these glands is to release chemicals called hormones. These hormones travel through the bloodstream to reach target areas elsewhere in the body, typically other glands or nervous system parts.

The glands that are important for sexual development and the growth spurt are the pituitary gland and the gonads, or sex glands. The pituitary gland is often called the master gland because one of its key functions is to release hormones that direct the activity of other glands, such as the gonads. The gonads, testes in males and ovaries in females, serve the dual function of producing sex hormones, and producing the sperm cells and ova (eggs). The most important sex hormones are androgens (especially testosterone, one specific kind of androgen), estrogens, and progesterone. Both testes and ovaries produce all three of the sex hormone types, but the testes produce more androgens and the ovaries produce more estrogens and progesterone. Consequently, androgens are often referred to as male sex hormones, whereas estrogens and progesterone are referred to as female sex hormones.

During adolescence, sex hormones trigger the development of primary and secondary sex characteristics. Primary sex characteristics are the maturation of the reproductive organs. They become fully functioning and capable of reproduction during puberty. Secondary sex characteristics are other features that signal the maturation of the reproductive organs and tend to distinguish men physically from women. They include growth of facial, body, pubic, and underarm hair; voice changes; changes in body shape; and growth of girls’ breasts. Basically, the pituitary gland increases its release of hormones that direct the testes and ovaries to release their own hormones. In boys and me, the increase in androgens leads to masculine physical features; in girls and women, the increase in estrogens leads to feminine physical features.

Different body parts grow at different rates, so the body proportions change dramatically during the growing period. This sometimes leads to anxiety and embarrassment about the adolescents’ beliefs that their feet or hands are too big, and about clumsiness or awkwardness (Downs, 1990).

Females finish growing taller at about 17, males at about 21; again, however, there is large variability. Weight and height increases occur at around the same time for males. For females, however, weight sometimes begins to increase earlier than height, leading some females and their parents to worry about weight gain (Spear, 2002).

So, at the end of the adolescent period, we see a bit of a parallel with what happened after the dramatic growth over the first year of life. This time, however, instead of a slowing of growth, there is an outright stopping. As you will see, however, the cessation of physical growth does not mean that development and change stop, and it certainly does not mean that unavoidable decline is right around the corner.

Physical Changes in Adulthood

It is common for adults to gain weight as they age. Both physical and lifestyle changes associated with aging contribute to these common increases. Further, being overweight can lead to a reduction in physical activity that can accelerate age-related changes, making a bit of a vicious cycle. Specifically, basal metabolic rate and muscle mass both decline as we age, beginning at around age 30 (Poehlman et al., 1990; Poehlman et al., 1993). Basal metabolic rate (BMR) is the amount of energy that our body expends when it is at rest; it represents the energy requirements (or the calories burned) for the basic functions of life, such as breathing, maintaining our heart rate, and supporting the cells of our body. As we age, those basic requirements decrease, meaning we burn fewer calories at rest. The cause of the decline is at least partially related to another loss due to aging, namely muscle mass, or the amount of lean muscle tissue in our bodies. Because our bodies use more energy maintaining muscle cells than fat cells, the loss of muscle cells leads to a lower BMR.

For the rest of us who are not at our maximum physical capacities, the actual physical decline is so gradual that we barely notice it for years. When most people complain about physical decline beginning in their 30’s, they are very likely reporting on the results of the lifestyle side of the equation. As people settle into careers, many of them behind a desk, and take on family and other responsibilities, they find it difficult to exercise regularly and wind up leading far more sedentary lives (this, of course, also contributes to the increase in weight). Thus, the decline they experience is more of a detraining effect than anything else. The best news about the actual age-related physical decline is that it can be slowed with physical activity. What this means in practical terms is that unless you are currently at your maximum possible fitness, you can continue to increase strength and fitness for many years, as the benefits of activity will more than offset the small declines in your maximum capacity. For example, if you engage in muscle-building exercise (i.e., strength training), you can prevent the decline in muscle mass and the consequent decrease in BMR for many years. The declines do become more noticeable around age 55 to 60 (Lim, 1999), so even if you continue to exercise strenuously, you will probably begin to notice a drop off around then. One potential problem to keep in mind is that it becomes more difficult to exercise strenuously as we age. Range of motion, flexibility, as well as recovery time after exertion all deteriorate, making injuries more likely and slower to heal, and requiring more rest between workouts. Again, these declines begin gradually and accelerate as we age.

Hearing Loss

We would like to conclude this section on physical changes in adulthood with a description of hearing loss. We confess that we had a hard time deciding where to include this information. To be sure, it is a physical change that most commonly affects older adults. As you will see in a minute, though, it also affects cognitive and social aspects of our lives. And, importantly, it is something that has become a larger risk for much younger people, too. It might not even belong in the Development unit of the book at all, because many people do not experience it, and according to the World Health Organization (2021), about half of all hearing loss is preventable. 

Most hearing loss is not reversible (reference), and it is much more than a nuisance. It is a significant health and life problem. Hearing loss can contribute to communication and employment difficulties, mental health problems, such as loneliness and depression, and even dementia (WHO, 2021; Griffiths et al. 2020). 

How common is it? 13% of people 18 and over in the US report some trouble hearing. The percentage increases with age, so 26.8% of people 65 and older have trouble (Madans, Weeks, & Elgaddal, 2019). The World Health Organization estimates that almost 2.5 billion people worldwide will have some hearing loss, with more than 700 million needing medical assistance. The large majority of at risk people live in low-income countries (WHO, 2021). 

Causes of hearing loss in young people include genetic and prenatal factors, trauma at birth (for example lack of oxygen), low birth weight, and ear infections. Among adults, causes include smoking, degeneration of sensory neurons, and excessive ear wax. In addition, there is a set of causes that commonly lead to hear loss at any ages. These include: head and ear trauma, infections and other illnesses, and nutritional deficiencies. 

And exposure to loud noises. A 2012 study by the CDC found that up to 10,000,000 adults (6%) in the US under 70 and 40,000,000 of adults overall (24%) have hearing test or interview results that indicate noise-induced hearing loss (NIDCD, 2022). It is important that you realize that this type of hearing loss is permanent. Long-term exposure to sounds as mild as a blender or heavy traffic when you are inside the car can lead to hearing loss. You can suffer hearing loss after 15 minutes of exposure to noise equal to being in a nightclub (RNID, 2024). With such seemingly low levels of noise presenting a risk, you can probably guess where we are going with this. 

Simply put, the problem of noise-induced hearing loss is very likely to get worse. A 2021 meta-analysis estimated that up to 1.3 billion young people are at risk of permanent hearing loss because their listening practices are not safe. You probably already realize attending live performances (not just music because crowd noise can reach unsafe levels) are common unsafe listening practices. Maybe you are aware that listening to music through earbuds is risky, as well. Indeed, earbuds can easily produce sounds well above levels that can cause damage in 15 minutes (Pienkowski, 2021). Researchers have begun to discover that even playing video games can often lead to unsafe volume levels (Dillard et al. 2024).

To be honest, a major reason that we have included this section about hearing loss in the book is to make sure you realize how dangerous the listening habits that you and many of your friends and family might have. Please do limit your exposure to loud noises and remember that what qualifies as “loud” is probably a lot quieter than you think. 

Remember, half of all hearing loss is preventable. 

15.3 Brain Development Throughout the Lifespan 

It is true that brain development is a physical development in many ways no different from the others in this module. Because the brain, as the source of all of our behavior and mental processes, bears a special relationship to psychology, however, it is worth pulling it out (so to speak) and describing its changes in a section separate from the other physical developments. As you learn about the types of developments that take place at different times during the lifespan, as well as the different brain areas involved, you will begin to understand and appreciate many of the differences in the psychology of infants, children, adolescents, and adults.

Prenatal Brain Development

Recall that when the embryo begins organizing itself, one of the new specialized areas is the neural tube. The neural tube actually develops from a section called the neural plate, which appears by three weeks after conception. The cells in the neural plate are called neural stem cells; they have the ability to develop into any cells of the nervous system, such as glial cells (the cells that support and communicate with neurons) and immature neurons (Varoqueaux & Brose, 2002). The cells at the top of the developing neural tube will become the brain; three distinct sections that will become the hindbrain, midbrain, and forebrain can be seen during the second month after conception. The rest of the neural tube develops into the spinal cord.

Once the neural tube is formed, the number of neurons increases rapidly. Then, they begin to move, or migrate to their eventual location. Migration is guided by chemicals contained in the particular areas through which the neurons move (Gleeson & Walsh, 2000; Golman & Lushkin, 1998). Although the cells are changing while they travel, they will not develop into a specific type of neuron or glial cell until they reach their destinations.

The main changes that the cells undergo while they are migrating is axon growth. Both axon and dendrite growth, which begins in earnest after the migration is completed, prepare the developing nervous system for the process of synaptogenesis, the formation of new synapses. Recall that a synapse is the small area where neural communication takes place, a “connection” between the axon of a sending neuron and the dendrite or cell body of a receiving neuron. Some synaptogenesis takes place before birth, but the bulk of it happens after the infant is born.

After birth, the infant’s brain does continue growing rapidly, approximately tripling in weight over the first year. A newborn’s brain has about 86 billion neurons. An adult’s brain has about 86 billion neurons. So, rather than adding new neurons, infant brain growth occurs primarily within the existing neurons. Myelin sheaths develop to cover many axons (this process is called myelinization), and dendrites develop many new branches. The increase in dendrite branches allows the massive synaptogenesis to occur.

Synaptogenesis is very selective. Specific axons hook up with specific target areas, resulting in the creation of many different types of synapses. It is among the more amazing engineering feats in the universe, with the brain ending up with some 100 trillion synapses, an extraordinarily complex network of interconnected neurons. Synapse formation is particularly massive in the cortex, the most important brain area for higher intellectual functions. At the end of the major synaptogenesis during infancy, there are no “stray” neurons. Every neuron in the brain is connected, through synapses, with many others.

The major ways the developing brain accomplishes the final wiring is through the overproduction and later pruning of synapses and the death of unused neurons. Sometime during the first or second year, the number of synapses in particular brain areas reaches a maximum (the exact time depends on the brain area), each area containing many more synapses than will be present in the adult brain. Then, the pruning of the oversupply of synapses occurs largely through the “use it or lose it” principle. Synapses are activated by environmental stimulation; those that are not used die off, as do neurons that are left unconnected.

Research has shown that when infants are raised in impoverished environments, brain development suffers (Rutter, 1998; Shonkoff & Phillips, 2000). Many parents, and more than a few marketers, have responded to these kinds of research findings by surrounding (or recommending to parents that they surround) their children with educational toys, games, and videos. These toys aren’t bad, but, the trouble is, that the research findings are likely being misinterpreted. Children that are in impoverished environments aren’t struggling because of a lack of educational toys, they are struggling because of a lack of social stimulation. It appears to be social stimulation, such as talking and singing, playing, and providing consistent and loving care, that is most needed for healthy brain development. Although the more rigorous academic kind of stimulation may be beneficial (future research may be able to tell us this), it almost certainly will not be if it takes the place of a more nurturing kind of environment.

The pruning of synapses makes it sound as if there is only loss after early childhood. Recall that neurons do continue to be generated throughout life, particularly in the hippocampus. Also, after the conclusion of the synaptogenesis burst in infancy, the brain continues to create new synapses; it does so at least throughout adolescence, just never as much as it did during infancy. Throughout life, the brain will continue to strengthen some synapses, often through using them, and weaken or eliminate others (Varoqueaux & Brose, 2002).

Adolescent and Adult Brain Development

Synaptogenesis slows dramatically by adolescence. Another critical process for brain development, myelinization, continues, however. In particular, the addition of myelin sheaths to axons is most pronounced in the prefrontal cortex during adolescence and early adulthood, making it the latest area of the brain to mature. This late development may very well be related to the cognitive developments that take place in adolescence (Kwon & Lawson, 2000).

Obviously, learning continues throughout life. Learning in the adult brain appears to involve the strengthening and modification of existing synapses, rather than the creation of new ones (Bourgeois, Goldman-Rakic, & Rakic, 2000; Varoqueaux & Brose, 2002). As a consequence, although the adult brain does undergo a reduction in the number of synapses over time, the reduction does not dramatically affect our ability to learn. Although the brain does continue to produce new neurons from stem cells throughout life, researchers have not yet discovered how these neurons are put to use.

Without a doubt, the big story about the adult brain is the decline. Synapses decline, plasticity (the brain’s ability to reorganize itself) declines, and so on. The story is not nearly as pessimistic as you might think, however. Biologist Robert Sapolsky (2004) notes that it is a myth that we lose enormous numbers of neurons as we age, an error based on researchers’ conclusions that the brains of people suffering from dementia showed the same brain changes as normal aging people. We do lose neurons as we age, but not nearly as many as common wisdom holds. Neuron loss is not distributed evenly throughout the brain either, which explains many other characteristics of aging people. The hippocampus, an important structure for memory, is one of the biggest losers. Finally, because of the reduction in synaptogenesis, the brain plasticity that we observe so readily in younger people, particularly children, is less dramatic in the older brain. Older people’s brains can recover from various types of damage, such as injuries or strokes, but the recovery is slow and it is usually not complete.