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The Growing Fetus
Prenatal development during the nine months of pregnancy is divided into three
stages. From one stage to the next, dramatic biological and behavioral changes take
place—and environmental influences are plentiful. These developments are
illustrated in Figure 9.6. The three stage of prenatal development are as follows:
Germinal stage (conception to approximately two weeks)
Embryonic stage (approximately two weeks to eight weeks)
Fetal stage (approximately eight weeks to birth).
Description
Figure 9.6 Prenatal Development
TheVisualMD / Science Source
Germinal Stage
First, there is the germinal stage. Thanks to conception, life begins with one
remarkable new cell called a zygote—about the size of the period at the end of this
sentence, yet fully equipped with a rich genetic heritage. To set the stage for
conception to occur, toward the end of a female’s menstrual cycle, a follicle in the
female’s ovary matures and releases an egg cell, called an ovum, in a process called
ovulation. The mature egg takes an excursion traveling down the Fallopian tube
toward the uterus. Unless the ovum is fertilized by a sperm cell while in the Fallopian
tube, within about two weeks the egg will dry up and leave the body; this process is
called menstruation.
conception. The fertilization of an egg cell by a sperm cell.
zygote. A fertilized egg that undergoes a two-week period of rapid cell division
and develops into an embryo.
ovum. An unfertilized egg cell.
ovulation. The release of a mature egg cell from a woman’s ovary.
Immediately upon fertilization, the outside of a zygote thickens to block other
sperm cells from penetrating the egg. Very quickly following conception, a process of
cell division takes place: The first cell splits into two, four, eight, and so on. After two
and a half days, there are 12 to 16 cells. By the fourth day, there are more than 100
cells clustered together in a ball, traveling from the Fallopian tube into the uterus and
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increasing in both their number and their diversity. Some cells will form muscles and
bone; others will form the stomach, liver, and so on. At two weeks, the ball of cells
attaches to the uterine wall, braced and ready for eight and a half months in a new
home. By the time this zygote is born, it will consist of hundreds of trillions of cells.
Embryonic Stage
Once the zygote is firmly attached to the uterine wall, called implantation, the
germinal stage is over, and the zygote is called an embryo. Surprisingly, it is
estimated that about 30 to 50 percent of conceptions fail to implant and do not
survive (Moore, Persaud, & Tochia, 2013). If implantation fails, the zygote will pass
out of the female’s body, and the female will likely never know conception had ever
occurred.
embryo. The developing human organism, from two weeks to two months after
conception.
At this point, when implantation occurs, all parts of the body begin to form—an
oversize head with a primitive brain and central nervous system, eyes, ears, a nose,
and a mouth with lips and teeth, a heart and circulatory system, arms, legs, fingers,
toes, and a tail. During this stage, organs start to function, including a heart that
pumps blood and beats, quickly, for the first time. Also during this stage sexual
differentiation occurs: The male hormone testosterone is secreted in embryos that
are genetically male, but not in those destined to become female. All the pieces of an
individual are in place in an embryo, yet at eight weeks of age it is only an inch long
and weighs a tenth of an ounce. You could hold one in the palm of your hand.
Fetal Stage
From the ninth week on, the embryo becomes a conspicuously human fetus. At
first, the cartilage in the bones starts to harden and there is rapid growth of the brain,
heart, lungs, genitals, and other internal organs and body parts. Depending on its
age, the fetus can squirm, open its eyes, suck its thumb, kick its legs, and turn
somersaults. By the seventh month, the key life-support systems are sufficiently
developed so that the fetus can breathe, circulate blood, digest nutrients, and dispose
of wastes. At this point, the 2-pound fetus has a fighting chance to survive if born
prematurely. If born on schedule, it will weigh an average of 7 pounds.
fetus. The developing human organism, from nine weeks after conception to birth.
Although fetal development follows a biological clock, it is also influenced by
external factors. Depending on the stage of development, for example, exposure to
harmful substances called teratogens (from the Greek teras, meaning “monster”)
can have devastating effects. Babies of mothers who have alcoholism, for example,
often show a pattern of birth defects known as fetal alcohol syndrome (FAS).
Obstetricians warn expectant mothers that malnutrition, X-rays, AIDS, German
measles and other viral infections, certain antibiotics, painkillers, large doses of
aspirin, heavy exposure to paint fumes, and a long list of drugs can all prove
dangerous (summarized in Table 9.1). Some teratogens cannot be avoided—and
some problems will arise without exposure to toxic substances. There are no
guarantees. Even a mother’s emotional state can affect the fetus. For example,
women who are depressed during the last trimester of pregnancy pass on stress
hormones to the fetus and give birth to infants that are initially passive and slow to
react to stimulation (Lundy et al., 1999). Expectant mothers cannot easily control
inner turmoil, but they can limit their intake of certain foods and drugs.
teratogens. Toxic substances that can harm the embryo or fetus during prenatal
development.
fetal alcohol syndrome (FAS). A specific pattern of birth defects (stunted growth,
facial deformity, and intellectual disability) often found in the offspring of mothers who
have alcoholism.
Table 9.1
Psychology Applied: How Does Alcohol Affect the
Fetus?
Legally and illegally, people have used drugs throughout history. The psychoactive
effects of various substances on the user are described in the chapter on
consciousness. But are there health risks to a fetus when the user is an expectant
mother? Is the fetus more vulnerable in some stages of pregnancy than others? In
the emerging area of “behavioral toxicology,” researchers examine the effects of
substances such as alcohol, tobacco, and cocaine. In animal studies, pregnant
mothers are randomly injected with a toxic substance and the effects on their
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y j
offspring are later measured. In human studies, researchers look for correlations
between substances that mothers consumed while pregnant and the later behavior of
their offspring. Because women who take drugs differ in other ways from those who
do not, and because the long-term effects of exposure can be subtle, it is important to
realize that the effects may be overestimated in some cases and underestimated in
others (Popova, Lange, Probst, Gmel, & Rehm, 2017).
Does alcohol, a universally popular drug, affect the fetus? In 1899, English doctor
William Sullivan studied the babies born to women in prison. He compared the
children of heavy-drinking mothers with those of nondrinkers and discovered that the
rate of stillbirths and infant mortality was two and a half times higher among the
children of drinking mothers. This discovery lay dormant for many years. Then in
1973, Kenneth Jones and others noticed that 11 babies of mothers with alcoholism
were born with a pattern of defects that included stunted growth, intellectual disability,
and facial deformities (widely spaced eyes, a flattened nose, and a thin upper lip).
They called this pattern fetal alcohol syndrome.
The devastating impact of alcohol is not that surprising in light of the immediate
effects it has on the fetus. Soon after a mother drinks—whether it’s beer, wine, or
hard liquor—the alcohol enters her bloodstream, passes through the placenta and
into the fetus’s blood, flows to the brain, and impairs breathing. Using ultrasound,
researchers observed that fetuses breathed less often after their mothers drank a
“screwdriver”—a mixture of orange juice and vodka—than when they had a glass of
orange juice (Lewis & Boylan, 1979). When an expectant mother drinks, to some
extent so does the fetus.
From animal experiments, and from studies with humans, the evidence is clear:
Mothers who have alcoholism are at risk of having a child who suffers from brain
damage, cognitive impairments, attention disorders, speech problems, hyperactivity,
and motor problems (Streissguth et al., 2004). What about social drinking during
pregnancy? Does an occasional glass of wine over dinner or bubbly champagne on
New Year’s Eve damage the fetus? For social drinking, too, the evidence suggests
that expectant mothers should exercise restraint (Lubbe, van Walbeek, & Vellios,
2017).
Sandra Jacobson and others (1993) tested infants born to females who drank
varying degrees of alcohol while pregnant. Infants whose mothers had two drinks a
day were slower to process information and less likely to engage in playful imitation.
In other research, Ann Streissguth and others (1993, 1999) interviewed hundreds of
pregnant women about their consumption of alcohol and other substances before and
after they knew they were pregnant. For 14 years, these women’s children were
tested periodically. The more alcohol the mother consumed, the more serious the
child’s problems were. Three findings gave cause for alarm. First, even small doses
were harmful. At 4 years old, the children of mothers who had only one drink a day
while pregnant lacked balance, manual dexterity, and a steady hand. At three drinks a
day, IQ scores were five points lower, and the damage was greatest when mothers
drank during the early weeks of pregnancy—before realizing they were pregnant.
Thus there seemed to be no safe level of drinking—and the timing couldn’t be worse.
Third, the effects were lasting. When the children were retested at 14 years old, those
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prenatally exposed to alcohol still performed poorly on various cognitive tests and
had more trouble in school.
Fertility
While most pregnancies result in a single birth, multiple births are more common
today due to a variety of factors, such as women conceiving later in age and the
increased use of fertility drugs in developed countries (Diergaarde & Kurta, 2014).
Twins account for more than 90 percent of multiple births and can be identical
(monozygotic) or fraternal (dizygotic). Identical twins form when one fertilized egg
(ovum) splits, thus creating two ova with identical genetic information. By contrast,
fraternal twins form when two distinct eggs (ova) are each fertilized by a different
sperm; each ovum therefore contains distinctive genetic information that is no more
similar than would be found in individual siblings, although fraternal twins will uniquely
share the same amnion (the innermost membrane that surrounds the embryo) until
birth. About 3.4 percent of pregnancies result in the birth of twins, and pregnancies
that result in higher-order births, such as triplets or quadruplets, are much rarer; only
about 113 in every 100,000 births are higher-order births (Hamilton, Martin,
Osterman, Curtin, & Mathews, 2015).
For couples who engage in unprotected intercourse, or sex, with the intent to
conceive, conception should occur within one year (Greil, Slauson-Blevins, &
McQuillan, 2010). If conception does not occur, then one or both individuals may
have a medical condition called infertility. About 7 percent of males and about 11
percent of females of reproductive age in the United States have experienced fertility
problems (Chandra, Copen, & Stephen, 2014), which not only can be frustrating but
also can lead to psychological distress due to the “psychological rollercoaster” (Read
et al., 2014, p. 390) of trying to conceive. In both males and females, fertility declines
with age, although the decline is much greater in females. Females are about half as
fertile in their 30s as they are in their early 20s, and their chances of conception
decline substantially after age 35 (Practice Committee of the American Society for
Reproductive Medicine, 2013). Male fertility also declines with age but more
gradually, with substantial declines in the quality of a male’s sperm generally starting
after age 40. Infertility treatment has advanced due to the development of fertility
drugs starting in the 1950s and with the more recent development of assisted
reproductive technologies (ARTs) such as in vitro fertilization (IVF) and
intracytoplasmic sperm injection. While generally effective, these options are often
expensive, costing thousands of dollars, typically out of pocket. It is estimated that
approximately 1.6 percent of all infants born in the United States every year are
conceived using ART (Centers for Disease Control and Prevention, 2017).
LEARNING CHECK
A Developing Situation
Imagine you are developing from zygote to embryo to fetus to infant. For each of
the events below, identify the order in which they occur. Answer Z for Zygote, E for
Embryo, or F for Fetus.
(Answers: Zygote: b, d; Embryo: c; Fetus: a, e.)
THE REMARKABLE NEWBORN
LEARNING OBJECTIVES
Describe the abilities that a newborn infant can demonstrate and the adaptive
significance of their preferences.
Outline how newborn infants can “communicate” what they know to
researchers.
Define reflexes and explain how they are adaptive for the newborn.
Identify the kinds of sights and sounds that most attract the newborn.
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In 1890, William James described the newborn’s experience of the world as “one
great booming, buzzing confusion.” Some 80 later, Lewis Lipsitt (1971) signaled the
dawn of a new era in an article entitled “Babies: They’re a Lot Smarter Than They
Look.” Why is there such a discrepancy between James and Lipsitt? Babies are not
more capable today than in the past—they still sleep 16 to 20 hours a day, wet their
diapers, and cry a lot. The difference is that developmental psychologists now have
high-tech equipment and more sophisticated methods to measure what the newborn
cannot tell us in words.
Observing Infants
In The Scientist in the Crib, Alison Gopnik, Andrew Meltzoff, and Patricia Kuhl
(1999) first describe this exciting research and conclude that “babies and young
children know and learn more about the world than we could ever have imagined” (p.
viii). As they put it, the human infant is “the most powerful learning machine in the
universe” (p. 1). In an interview regarding her most recent book, The Gardener and
the Carpenter, Alison Gopnik (2016) captures the takeaway from her decades of
research: “Children learn much more from using their own brains to just observe and
play than they do by having someone sit down and teach them.”
Children learn at least as much from simply observing and playing as they do from
having someone sit down and teach them.
iStock.com/SanyaSM
One valuable research technique is based on the measurement of habituation,
the tendency for attention to a novel stimulus to wane over time. If a picture or a
sound is presented over and over again, an infant will eventually get bored, lose
interest, look away, and exhibit a change in heart rate—a sure sign that it has
“learned” the stimulus and “remembered” the previous exposures (Bornstein, 1989). If
the infant then perks up, regains interest, and spends more time looking when a new
stimulus is presented, this recovery response suggests that it has noticed a
difference between the old and new. Using this technique, researchers can determine
the age at which infants begin to distinguish among faces, voices, musical notes,
speech sounds, geometric shapes, and rudimentary concepts. In one experiment, for
example, Russell Adams and Mary Courage (1998) habituated 173 newborn infants
to white light and then flashed green, red, and yellow lights to determine how
sensitive they were to changes in color (compared to adults, they were not).
habituation. The tendency for attention to a stimulus to wane over time (often
used to determine whether an infant has “learned” a stimulus).
recovery. Following habituation to one stimulus, the tendency for a second
stimulus to arouse new interest (often used to test whether infants can discriminate
between stimuli).
Other techniques are also used. To determine what infants find interesting or
surprising, researchers put different objects in front of them—dots, lines, balls,
whatever—and use an eye-tracking device to record the amount of time spent looking
at each object. There are many subtle ways for newborns to communicate to
researchers. Head movements, facial expressions, and measured changes in brain
waves, heart rate, and respiration are just a few of the possibilities. Some
researchers worry that it’s too easy to overinterpret what infants know from indirect
measures. Marshall Haith (1998) asks, “How much of cognition is in the head of the
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infant and how much in the mind of the theoretician?” (p. 167). That’s why Carolyn
Rovee-Collier (2001) observes whether infants act upon objects and events they
have observed. Other researchers note that it’s easy to underinterpret what infants
know. As Mark Strauss put it, “You can tell the wheels are turning. They’re paying
attention to the world in incredibly subtle ways” (cited in Grunwald & Goldberg, 1993).
Reflexes
Owing to the grasping reflex, an infant grips a parent’s thumb.
iStock.com/damircudic
In light of recent research, developmental psychologists now have a profound
respect for the newborn’s capacities. To begin with, babies are prepared at birth with
many adaptive reflexes—automatic, unlearned reactions to certain types of
stimulation. Press into a newborn’s palm, and you will stimulate a grasping reflex
that causes the baby to clutch your hand so hard that it can support its own weight.
Touch the newborn’s right or left cheek with a nipple, or even a finger, and you will
stimulate a rooting reflex, causing the baby automatically to turn in that direction and
open its mouth. Touch the newborn’s lips, and it will try to squeeze your finger
between its tongue and palate, breathe through the nose, and begin to suck. Move
your finger to the back of the baby’s mouth, and it will try to swallow. None of these
reflexes is within an infant’s control, and most disappear within three or four months,
never to return. But while they last, grasping, turning, opening the mouth, sucking,
and swallowing are important parts of the newborn’s adaptive machinery.
grasping reflex. In infants, an automatic tendency to grasp an object that
stimulates the palm.
rooting reflex. In response to contact on the cheek, an infant’s tendency to turn
toward the stimulus and open its mouth.
Sensory Capacities
Contrary to what maternity doctors and nurses used to tell new mothers, the
newborn can see, hear, taste, smell, and feel pain. The question is not whether they
have these senses, but what their limitations are and what kinds of stimulation they
prefer. Researchers have used various techniques to study the development of
perception in infancy.
Vision and Visual Preferences
At birth, parts of the eye and the visual cortex are not fully developed, and the
newborn is nearsighted. In fact, to see an object as would an adult with 20/20 vision,
the newborn needs to be 20 to 30 times closer—with the best distance being about 8
inches (Banks & Salapatek, 1983). The problem is that the newborn’s lenses do not
focus on objects at a distance and cannot detect subtle differences in light, shading,
or color. For that reason, soft pastel colors in the crib do not arouse as much interest
as a newspaper with bold print or a checkerboard that has stark black-on-white
contrast (Adams & Maurer, 1984). What would it be like to see through the eyes of a
newborn baby? According to Daphne Maurer and Charles Maurer (1988), the world
would look like “a badly focused snapshot that has been fading in the sun for so
many years that you can barely identify the subject” (p. 127).
Newborns may be limited in their vision, but their sensory abilities develop quickly
and they have marked preferences for certain kinds of stimulation. Just hours after
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birth, for example, infants distinguish between light and dark, stare at objects that
show contrast, and can track slow movement with their eyes. In an especially
intriguing study, Robert Fantz (1961) recorded the amount of time that 2- to 5-day-old
infants spent gazing at each of the six disks illustrated in Figure 9.7—a human face, a
bull’s-eye, newsprint, and three solids colored red, white, and yellow. As shown, the
infants preferred to look at the patterns over solids. Lo and behold, their favorite
pattern was the human face. This finding raised an intriguing question: Do faces just
happen to provide the right kind of visual stimulation, or is the human nervous system
primed to pay special attention to social stimuli? Is the attraction a mere happy
coincidence or the clever design of evolution?
Description
Figure 9.7 Visual Preferences in Newborns
Research supports the evolutionary interpretation. To be sure, newborns look at
any object that has complexity, contrast, and a symmetrical pattern of eyelike dots in
an outline—whether that object resembles a face or not (Kleiner, 1987). But infants
tested within an hour or two of birth exhibit a unique level of interest in facelike
stimuli. Mark Johnson and others (1991) presented newborns with head-shaped
forms that depict a properly featured face, a scrambled face, or a blank, featureless
face. The experimenter moved each pattern slowly across each infant’s field of view
and recorded the extent to which the infants rotated their heads and eyes to follow
the visual stimulus. The infants tracked the facelike pattern more than they did the
scrambled and blank patterns. It appears that humans are born with a special
orientation toward the face (Mondloch et al., 1999; Morton & Johnson, 1991; Valenza,
Simion, Cassia, & Umilta, 1996). They can even distinguish between the faces of
their own mothers and those of female strangers (Pascalis, de Schonen, Morton,
DeRuelle, & Fabre-Grenet, 1995; Walton, Bower, & Bower, 1992).
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Facial recognition among infants helps them recognize familiar faces, such as the
faces of their parents or loved ones.
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Other lines of research also indicate that newborns are “tuned in” to the face as a
social object. Andrew Meltzoff and Keith Moore (1983) found that within 72 hours of
birth, babies not only look at faces but also often mimic gestures such as moving the
head, pursing the lips, or sticking out the tongue. This rudimentary form of imitation
occurs even when the model is a stranger (Meltzoff & Moore, 1992). In another study,
newborns were recorded as they watched an adult wear a happy, sad, or surprised
expression. Observers who later saw only the tape were able to guess the adult’s
expression from changes in the baby’s face (Field, Woodson, Greenberg, & Cohen,
1982). What do these findings mean? It’s not clear that newborns are capable of
deliberate and coordinated imitation. But they do react automatically to certain facial
cues—almost as if babies were born with a “social reflex,” to the delight of parents all
over the world.
Hearing and Auditory Preferences
Slam the nursery door while a newborn is asleep, and they will open their eyes
wide and fling out their arms. Clearly, the newborn can hear. But what does it hear,
and how well? As with vision, the human auditory system is not completely developed
at birth. The baby’s outer ear is small, its eardrum does not vibrate effectively, and the
auditory cortex is still immature. For the first week or so, the baby’s ears are also
clogged with amniotic fluid, which muffles sound. The result of all this is that the
newborn baby is hard of hearing, compared to adults.
Though not in perfect form, the newborn reacts to life’s sounds in consistent ways.
If you stand on the baby’s right or left side and shake a rattle, you’ll notice that they
slowly turns their head in your direction, as if trying to locate the source of the sound.
Newborns cannot easily detect low-pitch sounds, but they are particularly sensitive to
high-pitch sounds, melodies, and the human voice (Aslin, 1989). They can tell the
difference between tones that are one note apart on the musical scale, between the
mother’s voice and that of another female, and between speech sounds that are as
similar as pa and ba. As measured by changes in sucking rate, newborn infants can
distinguish among multisyllable words that vary in stress patterns or rhythm
(Sansavini, Bertoncini, & Giovanelli, 1997). By 20 weeks old, they’re more likely to
turn toward the sound of their own names than to similar other names (Mandel,
Jusczyk, & Pisoni, 1995). They also seem to enjoy music. By recording the amount of
time spent looking at stereo speakers as they deliver sound, researchers have found
that infants show a measurable preference for Mozart and European folk songs over
sounds that are “dissonant” or unpleasant to most adults (Trainor & Heinmiller, 1998;
Zentner & Kagan, 1998).
In light of these various findings, it’s interesting to consider the way adults talk to
babies. In cultures all over the world, men, women, and children use baby talk, or
“motherese”—a form of speech that is slow, clear, simple, high in pitch, rhythmic,
songlike, and practically giddy, just the kinds of sounds that seize a newborn’s
attention. Infants like listening to higher-pitched voices (Trainor & Zacharias, 1998).
They also prefer to hear baby talk than ordinary adult conversation. So when babies
listen to recordings of women talking, they turn their head more toward the sound
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when the female is talking to a baby than to another adult (Fernald, 1985; Fernald et
al., 1989).
Sensitivity to Number
Of all the discoveries about infants and what they know, perhaps the most
startling come from classic experiments suggesting that babies are mini
mathematicians. Karen Wynn (1992) showed 5-month-old infants one or two Mickey
Mouse dolls being put on a puppet stage and covered by a screen. Next, they saw
her add a doll to the one behind the screen (1 + 1) or take one away (2 – 1). The
screen was then lowered so the babies could see how many dolls were now on
stage. Sometimes the number was correct (2 in addition, 1 in subtraction); at other
times, it was incorrect (1 in addition, 2 in subtraction). Can infants add and subtract?
If they could, reasoned Wynn, they would “expect” correct outcomes and be surprised
by incorrect outcomes. That is what happened. By recording their looking time, Wynn
found that the babies looked longer at the incorrect—and apparently unexpected—
outcomes. Using similar methods, Wynn and others have discovered that 5-montholds have a rudimentary ability not only to add and subtract but also to tell the
difference between two objects or event sequences and three (Canfield & Smith,
1996; Wynn, 1996).
Figure 9.8 Can Infants Add and Subtract?
iStock.com/lostinbids
This research has sparked a controversy over whether infants are born with an
innate sensitivity to numbers. Studies have shown that when infants are habituated to
a certain number of objects, say two blocks or rubber ducks, they look longer at a
new display containing one or three objects than another two-object display. In other
words, they seem to notice the difference in number. Or do they? Melissa Clearfield
and Kelly Mix (1999) argue that these infants may have responded to a change in the
overall amount of material in the display, not in the discrete number of objects
present. To test this hypothesis, they habituated infants, 6 to 8 months old, to two
black squares on a white board. Then they showed either three small squares that
combined to produce the same amount of black space or two large squares that
produced a larger combined black space. The result: The babies looked longer at the
total change in space, or surface area (the two large squares) than at the change in
number (the three small squares). Other studies have found similar results
(Feigenson, Carey, & Spelke, 2002; Mix, Huttenlocher, & Levine, 2002; vanMarle &
Wynn, 2011).
The human newborn is not a miniature adult and is obviously too helpless to
survive on its own. Thanks to recent research, however, we are in a better position to
appreciate a newborn’s capacities. From the moment of birth, babies are equipped
with primitive but adaptive reflexes. They’re also prepared to experience certain
forms of stimulation, especially those provided by human contact—faces, voices, and
mother’s scent. And they are capable of rudimentary forms of learning and memory.
In a particularly intriguing program of research, Carolyn Rovee-Collier (1988) hung a
mobile over the crib of 6-week-olds, attaching the mobile by a ribbon to one of their
legs so they could move it. When these infants were brought back two weeks later,
they remembered which leg to kick. Newborn capacities like these have existed for
generations, but only now are we beginning to appreciate them.
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THE INFANT AND THE GROWING CHILD
LEARNING OBJECTIVES
Describe biological, cognitive, and social development during childhood.
Outline how the brain and body develop in the first few years of life.
Describe the developmental changes that take place in the way children
think, reason, and speak.
Distinguish among the four stages of development that Jean Piaget identified.
The newborn has come a long way—both in our understanding and in its own
short history of development. But there is much more to come. First, there is an
infant, from the Latin word meaning “without language.” The infant grows into a
walking and talking toddler, who then graduates on the first day of school to the
category of child. Puberty spurs the adolescent, though what it means to be an adult
is anybody’s guess. Throughout the rest of this chapter, we look at biological,
cognitive, and social aspects of development and consider the interplay among them.
Biological Development
During the first year or so, babies grow at a pace never to be equaled again. On
average, babies double their birth weight in five months and triple it by their first
birthday. They grow 10 inches in height during the first year, and another 4 to 6
inches the year after. To the distress of caregivers without hand-me-downs, a baby’s
clothing size changes almost every month. Though it is not always accurate, there’s a
general rule of thumb you might find remarkable: By their second birthday, most
toddlers have reached half of their adult height.
Physical Growth
Matching the observable changes in body size, other aspects of growth also
proceed at a fast pace. Cartilage turns to bone, muscle fibers thicken, and teeth
break through the gums. Most impressive are the changes that occur in the brain and
nervous system—and what these changes mean for cognitive and social
development. At birth, an infant’s brain weighs close to a pound and is fully equipped
with all of the 100 to 200 billion neurons it will have in its lifetime. But the brain and
nervous system are immature, and relatively few synaptic connections have formed.
Starting in the first year, the neural axons grow longer, the dendrites increase in
number, and a surplus of new synaptic connections are carved into the brain—
trillions, more than can possibly be used. Then in childhood, the brain undergoes a
pruning process in which often-used synaptic connections survive while unused
connections are eliminated. The neurons also become more tightly wrapped in myelin
sheath, the fatty substance that enhances the speed of neural transmission. This
process of myelination continues to early adolescence.
The brain’s maturation is closely linked to psychological development. For
example, natural increases in the number of synaptic connections, or “pathways,” in
the brain are often accompanied by advances in cognitive ability (Gage & Baars,
2018; Gazzaniga, 1984). The same is true of myelination. At birth, the brainstem and
spinal cord—which govern simple reflexes—are well myelinated and in working order.
The visual cortex is less developed, as is the newborn’s vision. Those parts of the
cortex that control attention and information processing are not fully myelinated until
the ages of 4 to 7 years, which is when children become capable of reading and
simple arithmetic (Parmelee & Sigman, 1983). In short, growth spurts in the brain
correspond nicely to developments of the mind.
Motor Skills
From infancy to childhood, physical growth—as measured by gains in height and
weight—is the most predictable change that takes place. The second most
predictable is the coordination of motor skills. On average, babies can lift their heads
at 2 months, sit without support at 5.5 months, crawl at 10 months, and walk at 12
months. Children differ somewhat in their rate of motor development, but the
sequence of events is usually the same: Lifting the head precedes sitting, which
precedes crawling, standing, and walking. Infants differ in the way they learn to crawl
and then walk. For example, some begin by inching along on their bellies before
crawling on hands and knees; others do not (Adolph, Vereijken, & Denny, 1998).
More advanced activities like running, jumping, climbing stairs, and throwing a ball
develop later and are less rigid in their sequence. So are fine motor skills like gripping
a pencil, tying a shoelace, and using a fork (Wade & Whiting, 1986).
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