Chat with us, powered by LiveChat ❚ C H A P T E R 2 Biological Foundations Heredity, Prenatal Development, and Birth ❚ C H A | Office Paper
+1(978)310-4246 credencewriters@gmail.com
  

❚ C H A P T E R 2

Biological Foundations

Heredity, Prenatal Development, and Birth

❚ C H A P T E R 3

Tools for Exploring the World

Physical Development in Infancy and Early
Childhood

❚ C H A P T E R 4

The Emergence of Thought and

Language

Cognitive Development in Infancy and Early
Childhood

❚ C H A P T E R 5

Entering the Social World

Socioemotional Development in Infancy and
Early Childhood

P A R T I

Prenatal Development, Infancy,
and Early Childhood

©
S

to
ck

by
te

/
G

et
ty

Im
ag

es

39

S
N

39

02-W4803-MRB1.indd 3902-W4803-MRB1.indd 39 9/5/08 2:39:56 PM9/5/08 2:39:56 PM

SECOND REVISED PAGES

S
N

40

2.1 I N T H E B E G I N N I N G : 2 3 PA I R S

O F C H R O M O S O M E S

Mechanisms of Heredity

Genetic Disorders

Heredity, Environment, and Development

❚ REAL PEOPLE: APPLYING HUMAN

DEVELOPMENT:

Ben and Matt Pick Their Niches

2.2 F R O M C O N C E P T I O N TO B I RT H

Period of the Zygote (Weeks 1–2)

❚ CURRENT CONTROVERSIES:

Conception in the 21st Century

Period of the Embryo (Weeks 3–8)

Period of the Fetus (Weeks 9–38)

2.3 I N F L U E N C E S O N P R E N ATA L

D E V E L O P M E N T

General Risk Factors

Teratogens: Drugs, Diseases, and
Environmental Hazards

How Teratogens Influence Prenatal
Development

Prenatal Diagnosis and Treatment

2.4 L A B O R A N D D E L I V E RY

Stages of Labor

Approaches to Childbirth

Adjusting to Parenthood

Birth Complications

❚ SPOTLIGHT ON RESEARCH:

Impaired Cognitive Functions in
Low Birth Weight Babies

Infant Mortality

S U M M A RY

K E Y T E R M S

L E A R N M O R E A B O U T I T

02-W4803-MRB1.indd 4002-W4803-MRB1.indd 40 9/5/08 2:40:01 PM9/5/08 2:40:01 PM

SECOND REVISED PAGES

41

C H A P T E R 2

S
N

41

Biological Foundations
Heredity, Prenatal Development, and Birth

I f you ask parents to name the most memorable experiences of their lives, many immediately
mention the events associated with the birth of their children. From the initial exciting news that

a woman is pregnant through birth 9 months later, the entire experience of pregnancy and birth

evokes awe and wonder.

The period before birth is the foundation for all human development and the focus of this chap-

ter. Pregnancy begins when egg and sperm cells unite and exchange hereditary material. In the first

section, you’ll see how this exchange takes place and, in the process, learn about inherited factors

that affect development. The second section of the chapter traces the events that transform sperm

and egg into a living, breathing human being. You’ll learn about the timetable that governs develop-

ment before birth and, along the way, get answers to common questions about pregnancy. We talk

about some of the problems that can occur during development before birth in the third section of

the chapter. The last section focuses on birth and the newborn baby. You’ll find out how an expectant

mother can prepare for birth and what labor and delivery are like.

©
M

as
te

rf
ile

02-W4803-MRB1.indd 4102-W4803-MRB1.indd 41 9/5/08 2:40:10 PM9/5/08 2:40:10 PM

SECOND REVISED PAGES

42 | CHAPTER 2

S
N

42

L
eslie and Glenn are excited at the thought of starting their own family. At the same time,

they’re nervous because Leslie’s grandfather had sickle-cell disease and died when he was

just 20 years old. Leslie is terrifi ed that her baby may inherit the disease that killed her grandfa-

ther. She and Glenn wish that someone could reassure them that their baby will be okay.

HOW CAN WE REASSURE LESLIE AND GLENN? For starters, we need to know more about
sickle-cell disease. Red blood cells carry oxygen and carbon dioxide to and from the
body. When a person has sickle-cell disease, the red blood cells are long and curved
like a sickle. These stiff , misshapen cells cannot pass through small capillaries, so oxy-
gen cannot reach all parts of the body. The trapped sickle cells also block the way of
white blood cells that are the body’s natural defense against bacteria. As a result, many
people with sickle-cell disease—including Leslie’s grandfather and many other Afri-
can Americans, who are more prone to this painful disease than other groups—die
from infections before the age of 20.

Sickle-cell disease is inherited and, because Leslie’s grandfather had the disorder, it
runs in her family. Will Leslie’s baby inherit the disease? To answer this question, we
need to examine the mechanisms of heredity.

| Mechanisms of Heredity

At conception, egg and sperm unite to create a new organism that incorporates some
characteristics of each parent. Each egg and sperm cell has 23 chromosomes, thread-
like structures in the nucleus that contain genetic material. When a sperm penetrates

L E A R N I N G O B J E C T I V E S

What are chromosomes and genes? How do they carry ❚
hereditary information from one generation to the next?

What are common problems involving chromosomes and ❚
what are their consequences?

How is children’s heredity influenced by the environment ❚
in which they grow up?

2.1 IN THE BEGINNING: 23 PAIRS OF CHROMOSOMES

Red blood cells carry oxygen throughout the

body.

Sickle-shaped blood cells associated with sickle-

cell disease cannot pass through the body’s

smallest blood vessels.

chromosomes

threadlike structures in the nuclei of cells

that contain genetic material

K
en

ne
th

E
w

ar
d/

B
io

G
ra

fx
/P

ho
to

R
es

ea
rc

he
rs

, I
nc

.

S
ci

en
ce

P
ho

to
/C

us
to

m
M

ed
ic

al
S

to
ck

P
ho

to

02-W4803-MRB1.indd 4202-W4803-MRB1.indd 42 9/5/08 2:40:16 PM9/5/08 2:40:16 PM

SECOND REVISED PAGES

BIOLOGIC AL FOUNDATIONS | 43

S
N

43

an egg, their chromosomes combine to produce 23 pairs of chromosomes. The fi rst 22
pairs of chromosomes are called autosomes. The 23rd pair determines the sex of the
child, so these are known as the sex chromosomes. When the 23rd pair consists of an
X and a Y chromosome, the result is a boy; two X chromosomes produce a girl.

Each chromosome actually consists of one molecule of deoxyribonucleic acid—DNA
for short. To understand the structure of DNA, imagine four diff erent colors of beads
placed on two strings. The strings complement each other precisely: Wherever a red
bead appears on one string, a blue bead appears on the other; wherever a green bead
appears on one string, a yellow one appears on the other. DNA is organized this way,
except that the four colors of beads are actually four diff erent chemical compounds:
adenine, thymine, guanine, and cytosine. The strings, which are made up of phos-
phates and sugars, wrap around each other and so create the double helix shown in
❚ Figure 2.1.

The order in which the chemical compound “beads” appear is really a code that
causes the cell to create specifi c amino acids, proteins, and enzymes—important bio-
logical building blocks. For example, three consecutive thymine “beads” make up the
instruction to create the amino acid phenylalanine. Each group of compounds that pro-
vides a specifi c set of biochemical instructions is a gene. Thus, genes are the functional

autosomes

fi rst 22 pairs of chromosomes

sex chromosomes

23rd pair of chromosomes; these deter-

mine the sex of the child

deoxyribonucleic acid (DNA)

molecule composed of four nucleotide

bases that is the biochemical basis of

heredity

gene

group of nucleotide bases that provides a

specifi c set of biochemical instructions

G

C

C

T

T

T

T

G

G

G

G

G

G

A

A

A

A

TA

A

C

C

C

C

C

G

T

G

G

G

A

C

C

C

T

G

C

G

A

T

C

T

G

C

Nucleotide bases
(A = Adenine,
T = Thymine,
G = Guanine,
C = Cytosine)

Strands of
phosphates
and sugars

Figure 2.1 ❚
DNA is organized in a double helix, with

strands of phosphates and sugars linked by

nucleotide bases.

Humans have 23 pairs of chromosomes, includ-

ing 22 pairs of autosomes and 1 pair of sex

chromosomes.

B
io

ph
ot

o
A

ss
oc

ia
te

s/
S

ci
en

ce
S

ou
rc

e/
Ph

ot
o

R
es

ea
rc

he
rs

, I
nc

.

02-W4803-MRB1.indd 4302-W4803-MRB1.indd 43 9/5/08 2:40:20 PM9/5/08 2:40:20 PM

SECOND REVISED PAGES

44 | CHAPTER 2

S
N

44

units of heredity because they determine the production of chemical substances that
are, ultimately, the basis for all human characteristics and abilities.

Altogether, a person’s 46 chromosomes include roughly 30,000 genes (Pennisi,
2005). Chromosome 1 has the most genes (nearly 3,000) and the Y chromosome has
the fewest (just over 200). Most of these genes are the same for all people—fewer
than 1% of genes cause diff erences between people (Human Genome Project, 2003).
Through biochemical instructions that are coded in DNA, genes regulate the develop-
ment of all human characteristics and abilities. The complete set of genes makes up a
person’s heredity and is known as the person’s genotype. Genetic instructions, in con-
junction with environmental infl uences, produce a phenotype, an individual’s physical,
behavioral, and psychological features.

How do genetic instructions produce the misshapen red blood cells of sickle-cell
disease? Genes come in diff erent forms that are known as alleles. In the case of red
blood cells, for example, two alleles can be present on chromosome 11. One allele
has instructions for normal red blood cells; another allele has instructions for sickle-
shaped red blood cells. The alleles in the pair of chromosomes are sometimes the same,
which is known as being homozygous. The alleles sometimes diff er, which is known
as being heterozygous. Leslie’s baby would be homozygous if it had two alleles for
normal cells or two alleles for sickle-shaped cells. The baby would be heterozygous if
it had one allele of each type.

How does a genotype produce a phenotype? With sickle-cell disease, for example,
how do genotypes lead to specifi c kinds of blood cells? The answer is simple if a per-
son is homozygous. When both alleles are the same—and therefore have chemical
instructions for the same phenotype—that phenotype results. If Leslie’s baby had an
allele for normal red blood cells on both of its 11th chromosomes, then the baby would
be almost guaranteed to have normal cells. If, instead, the baby had two alleles for
sickle-shaped cells, then it would almost certainly suff er from the disease.

When a person is heterozygous, the process is more complex. Often one allele is
dominant, which means that its chemical instructions are followed while those of the
other, recessive allele are ignored. In sickle-cell disease, the allele for normal cells
is dominant and the allele for sickle-shaped cells is recessive. This is good news for
Leslie: As long as either she or Glenn contributes the allele for normal red blood cells,
their baby will not develop sickle-cell disease.

❚ Figure 2.2 summarizes what we’ve learned about sickle-cell disease: A denotes
the allele for normal blood cells, and a denotes the allele for sickle-shaped cells. De-
pending on the alleles in Leslie’s egg and in the sperm that fertilizes that egg, three
outcomes are possible. Only if the baby inherits two recessive alleles for sickle-shaped
cells is it likely to develop sickle-cell disease. But this is unlikely in Glenn’s case: He is
positive that no one in his family has had sickle-cell disease, so he almost certainly has
the allele for normal blood cells on both of the chromosomes in his 11th pair.

Even though Glenn’s sperm will carry the gene for normal red blood cells, this
doesn’t guarantee that their baby will be healthy. Why? Sometimes one allele does
not dominate another completely, a situation known as incomplete dominance. In
incomplete dominance, the phenotype that results often falls between the phenotype
associated with either allele. This is the case for the genes that control red blood cells.
Individuals with one dominant and one recessive allele have sickle-cell trait: In most
situations they have no problems, but when seriously short of oxygen they suff er a tem-
porary, relatively mild form of the disease. Sickle-cell trait is likely to appear when the
person exercises vigorously or is at high altitudes (Sullivan, 1987). Leslie and Glenn’s
baby would have sickle-cell trait if it inherits a recessive gene from Leslie and a domi-
nant gene from Glenn.

The simple genetic mechanism responsible for sickle-cell disease—which involves
a single gene pair with one dominant allele and one recessive allele—is also respon-
sible for numerous other common traits, as shown in ● Table 2.1. In each of these in-
stances, individuals with the recessive phenotype have two recessive alleles, one from
each parent. Individuals with the dominant phenotype have at least one dominant
allele.

incomplete dominance

situation in which one allele does not

dominate another completely

sickle-cell trait

disorder in which individuals show

signs of mild anemia only when they are

seriously deprived of oxygen; occurs in

individuals who have one dominant allele

for normal blood cells and one recessive

sickle-cell allele

genotype

person’s hereditary makeup

phenotype

physical, behavioral, and psychological

features that result from the interaction

between one’s genes and the environment

alleles

variations of genes

homozygous

when the alleles in a pair of chromosomes

are the same

heterozygous

when the alleles in a pair of chromosomes

diff er from each other

dominant

form of an allele whose chemical instruc-

tions are followed

recessive

allele whose instructions are ignored in

the presence of a dominant allele

02-W4803-MRB1.indd 4402-W4803-MRB1.indd 44 9/5/08 2:40:22 PM9/5/08 2:40:22 PM

SECOND REVISED PAGES

BIOLOGIC AL FOUNDATIONS | 45

S
N

45

Sickle-cell
disease

Normal
child

Sickle-cell
trait

Sickle-cell
trait

MotherFather

A A

A

A

A

A a a

a

a

a

a

Figure 2.2 ❚
In single-gene inheritance, a heterozygous

father and a heterozygous mother can have a

healthy child, a child with sickle-cell trait, or a

child with sickle-cell disease.

● TA B L E 2 . 1

Some Common Phenotypes Associated With Single Pairs of Genes

Dominant Phenotype Recessive Phenotype

Curly hair Straight hair

Normal hair Pattern baldness (men)

Dark hair Blond hair

Thick lips Thin lips

Cheek dimples No dimples

Normal hearing Some types of deafness

Normal vision Nearsightedness

Farsightedness Normal vision

Normal color vision Red–green color blindness

Type A blood Type O blood

Type B blood Type O blood

Rh-positive blood Rh-negative blood

SOURCE: McKusick, 1995.

Most of the traits listed in Table 2.1 are biological and medical phenotypes. These
same patterns of inheritance can cause serious disorders, as we’ll see in the next
section.

| Genetic Disorders

Some people are aff ected by heredity in a special way: They have genetic disorders
that disrupt the usual pattern of development. Genetics can derail development in
two ways. First, some disorders are inherited. Sickle-cell disease is one example of

02-W4803-MRB1.indd 4502-W4803-MRB1.indd 45 9/5/08 2:40:22 PM9/5/08 2:40:22 PM

SECOND REVISED PAGES

46 | CHAPTER 2

S
N

46

an inherited disorder. Second, sometimes eggs or sperm do not include the usual
23 chromosomes but have more or fewer chromosomes instead. In the next few pages,
we’ll see how inherited disorders and abnormal numbers of chromosomes can alter a
person’s development.

Inherited Disorders
You know that sickle-cell disease is a disorder that aff ects people who inherit two
recessive alleles. Another disorder that involves recessive alleles is phenylketonuria
(PKU), a disorder in which babies are born lacking an important liver enzyme. This
enzyme converts phenylalanine—a protein found in dairy products, bread, diet soda,
and fi sh—into amino acids that are required for normal body functioning. Without
this enzyme, phenylalanine accumulates and produces poisons that harm the nervous
system, resulting in mental retardation (Diamond et al., 1997; Mange & Mange, 1990).

Most inherited disorders are like sickle-cell disease and PKU in that they are carried
by recessive alleles. Relatively few serious disorders are caused by dominant alleles.
Why? If the allele for the disorder is dominant, every person with at least one of these
alleles would have the disorder. Individuals aff ected with these disorders typically do
not live long enough to reproduce, so dominant alleles that produce fatal disorders
soon vanish from the species. An exception is Huntington’s disease, a fatal disease
characterized by progressive degeneration of the nervous system. Huntington’s disease
is caused by a dominant allele found on chromosome 4. Individuals who inherit this
disorder develop normally through childhood, adolescence, and young adulthood. Dur-
ing middle age, however, nerve cells begin to deteriorate, which produces symptoms
such as muscle spasms, depression, and signifi cant changes in personality (Shiwach,
1994). By this age, many adults with Huntington’s disease have already reproduced,
creating children who may well later display the disease themselves.

Abnormal Chromosomes
Sometimes individuals do not receive the normal complement of 46 chromosomes.
If they are born with extra, missing, or damaged chromosomes, development is al-
ways disturbed. The best example is Down syndrome. People with Down syndrome
have almond-shaped eyes and a fold over the eyelid. Their head, neck, and nose are
usually smaller than normal. During the fi rst several months of life, development of
babies with Down syndrome seems to be normal. Thereafter, their mental and behav-
ioral development begins to lag behind the average child’s. For example, a child with
Down syndrome might fi rst sit up without help at about 1 year, walk at 2, and talk
at 3, reaching each of these developmental milestones months or even years behind
children without Down syndrome. By childhood, most aspects of cognitive and social
development are seriously retarded. Rearing a child with Down syndrome presents
special challenges. During the preschool years, children with Down syndrome need
special programs to prepare them for school. Educational achievements of children
with Down syndrome are likely to be limited, and their life expectancy ranges from
25 to 60 years (Yang, Rasmussen, & Friedman, 2002). Nevertheless, as you’ll see in
Chapter 6, many individuals with Down syndrome lead full, satisfying lives.

What causes Down syndrome? Individuals with Down syndrome typically have
an extra 21st chromosome that is usually provided by the egg (Machatkova et al.,
2005). Why the mother provides two 21st chromosomes is unknown. However, the
odds that a woman will bear a child with Down syndrome increase markedly as she
gets older. For a woman in her late 20s, the risk of giving birth to a baby with Down
syndrome is about 1 in 1,000; for a woman in her early 40s, the risk is about 1 in 50.
Why? A woman’s eggs have been in her ovaries since her own prenatal development.
Eggs may deteriorate over time as part of aging or because an older woman has a
longer history of exposure to hazards in the environment, such as X-rays, that may
damage her eggs.

An extra autosome (as in Down syndrome), a missing autosome, or a damaged
autosome always has far-reaching consequences for development because the auto-

phenylketonuria (PKU)

inherited disorder in which the infant

lacks a liver enzyme

Huntington’s disease

progressive and fatal type of dementia

caused by dominant alleles

Children with Down syndrome typically have up-

ward slanting eyes with a fold over the eyelid, a

flattened facial profile, and a smaller than average

nose and mouth.

©
L

au
ra

D
w

ig
ht

P
ho

to
gr

ap
hy

02-W4803-MRB1.indd 4602-W4803-MRB1.indd 46 9/5/08 2:40:23 PM9/5/08 2:40:23 PM

SECOND REVISED PAGES

BIOLOGIC AL FOUNDATIONS | 47

S
N

47

somes contain huge amounts of genetic material. In fact, nearly half of all fertilized
eggs abort spontaneously within 2 weeks—primarily because of abnormal autosomes.
Thus, most eggs that cannot develop normally are removed naturally (Moore & Per-
saud, 1993).

Abnormal sex chromosomes can also disrupt development. ● Table 2.2 lists four
of the more frequent disorders associated with atypical numbers of X and Y chromo-
somes. Keep in mind that “frequent” is a relative term; although these disorders are
more frequent than PKU or Huntington’s disease, most are uncommon. Notice that
there are no disorders consisting solely of Y chromosomes. The presence of an X chro-
mosome appears to be necessary for life.

Fortunately, most of us receive the correct number of chromosomes and do not in-
herit life-threatening illnesses. For most people, heredity reveals its power in creating
a unique individual—a person unlike any other.

Now that you understand the basic mechanisms of heredity, we can learn how
heredity and environment work together to produce behavioral and psychological
development.

| Heredity, Environment, and Development

Many people mistakenly view heredity as a set of phenotypes unfolding automati-
cally from the genotypes that are set at conception. Nothing could be further from the
truth. Although genotypes are fi xed when the sperm fertilizes the egg, phenotypes are
not. Instead, phenotypes depend both on genotypes and on the environment in which
individuals develop.

To begin our study of heredity and environment, we need to look fi rst at the meth-
ods that developmental scientists use.

Behavioral Genetics: Mechanisms and Methods
Behavioral genetics is the branch of genetics that deals with inheritance of behavioral
and psychological traits. Behavioral genetics is complex, in part, because behavioral
and psychological phenotypes are complex. Traits controlled by single genes are usu-
ally “either–or” phenotypes. A person either has dimpled cheeks or not; a person ei-
ther has normal color vision or red-green color blindness; a person’s blood either clots
normally or it does not. In contrast, most important behavioral and psychological
characteristics are not of an “either–or” nature; rather, a range of diff erent outcomes
is possible. Take extraversion as an example. Imagine trying to classify 10 people that
you know well as either extroverts or introverts. This would be easy for a few ex-
tremely outgoing individuals (extroverts) and a few intensely shy persons (introverts).

● TA B L E 2 . 2

Common Disorders Associated With the Sex Chromosomes

Sex
Disorder Chromosomes Frequency Characteristics

Klinefelter’s XXY 1 in 500 male births Tall, small testicles, sterile, below-
syndrome normal intelligence, passive

XYY complement XYY 1 in 1,000 male births Tall, some cases apparently have
below-normal intelligence

Turner’s syndrome X 1 in 2,500–5,000 female Short, limited development of
births secondary sex characteristics,
problems perceiving spatial relations

XXX syndrome XXX 1 in 500–1,200 female Normal stature but delayed motor
births and language development

behavioral genetics

the branch of genetics that studies the

inheritance of behavioral and psychologi-

cal traits

02-W4803-MRB1.indd 4702-W4803-MRB1.indd 47 9/5/08 2:40:25 PM9/5/08 2:40:25 PM

SECOND REVISED PAGES

48 | CHAPTER 2

S
N

48

Most people are neither extroverts nor introverts but “in between.” The result is a
distribution of individuals ranging from extreme introversion at one end to extreme
extroversion at the other.

Many behavioral and psychological characteristics are distributed in this fashion,
including intelligence and many aspects of personality. When phenotypes refl ect the
combined activity of many separate genes, the pattern is known as polygenic inheri-
tance. Because so many genes are involved in polygenic inheritance, we usually can-
not trace the eff ects of each gene. But we can use a hypothetical example to show how
many genes work together to produce a behavioral phenotype that spans a continuum.
Let’s suppose that four pairs of genes contribute to extroversion, that the allele for
extroversion is dominant, and that the total amount of extroversion is simply the sum
of the dominant alleles. If we continue to use uppercase letters to represent dominant
alleles and lowercase letters to represent the recessive allele, then the four gene pairs
would be Aa, Bb, Cc, and Dd.

These four pairs of genes produce 81 diff erent genotypes and 9 distinct pheno-
types. For example, a person with the genotype AABBCCDD has 8 alleles for extro-
version (the proverbial party animal). A person with the genotype aabbccdd has no
alleles for extroversion (the proverbial wallfl ower). All other genotypes involve some
combination of dominant and recessive alleles, so these are associated with pheno-
types representing intermediate levels of extroversion. In fact, ❚ Figure 2.3 shows that
the most common outcome is for people to inherit exactly 4 dominant and 4 recessive
alleles, and 19 of the 81 genotypes (e.g., AABbccDd, aaBbcCDd) produce this pattern. A
few extreme cases (very outgoing or very shy), when coupled with many intermediate
cases, produce the familiar bell-shaped distribution that characterizes many behav-
ioral and psychological traits.

Remember, this example is completely hypothetical. Extroversion is not based on
the combined infl uence of eight pairs of genes. However, the sample shows how sev-
eral genes working together could produce a continuum of phenotypes. Something
like our example is probably involved in the inheritance of many human behavioral
traits, except that many more pairs of genes are involved. What’s more, the environ-
ment also infl uences the phenotype.

T H I N K A B O U T I T

Introversion–extroversion is an example

of a psychological characteristic that

defines a continuum. Think of other

psychological characteristics like this,

in which outcomes are not “either–or”

but are distributed across a range.

polygenic inheritance

when phenotypes are the result of the

combined activity of many separate genes

2

0

20

4

12

14

16

18

8

6

10

0 1 2

AABBCCDD

AaBBCCDD

AABbCCDD

AABBCCDd

AABBCcDD

aaBBCCDD

AaBbCCDD

AaBBCcDD

AaBBCCDd

AAbbCCDD

AABbCcDD

AABbCCDd

AABBccDD

AABBCcDd

AABBCCdd

aaBbCCDDaabbCCDDaabbCcDDaabbccDDaabbccDd

aaBBCcDDaaBbCcDDaabbCcDd aabbCCDdaabbCcdd

aaBBCCDdaaBbCCDdaabbCCdd aaBbccDDaaBbccdd

AabbCCDDaaBBccDDaaBbccDd aaBbCcDd

AaBbCcDDaaBBCcDdaaBbCCddaaBbCcdd

AaBbCCDdaaBBCCddaaBBccDdaaBBccdd

AaBBccDDAabbCcDDaaBBCcddAabbccDd

AaBBCcDdAabbCCDdAabbccDDAabbCcdd

AaBBCCddAaBbccDDAabbCcDdAaBbccdd

AAbbCcDDAaBbCcDdAabbCCdd

AAbbCCDdAaBbCCddAaBbccDd

AABbccDDAaBBccDdAaBbCcdd

AABbCcDdAaBBCcddAaBBccdd

AABbCCddAAbbccDDAAbbccDd

AABBccDdAAbbCcDdAAbbCcdd

AABBCcddAAbbCCdd

AABbccDd

AABbCcdd

AABBccdd

AABbccdd

AAbbccdd

Aabbccdd

aabbccdd

3 4 5 6 7 8
Number of dominant alleles for extroversion (phenotype)

N
u

m
b

er
o

f
d

if
fe

re
n

t
ge

n
ot

yp
es

Figure 2.3 ❚
Many behavioral phenotypes represent a con-

tinuum (with many people falling at the mid-

dle of the continuum), an outcome that can

be caused by many genes working together.

02-W4803-MRB1.indd 4802-W4803-MRB1.indd 48 9/5/08 2:40:25 PM9/5/08 2:40:25 PM

SECOND REVISED PAGES

BIOLOGIC AL FOUNDATIONS | 49

S
N

49

If many behavioral phenotypes involve countless genes, how can we hope to un-
ravel the infl uence of heredity? Twins and adopted children provide some important
clues to the role of heredity. In twin studies, researchers compare identical and fra-
ternal twins. Identical twins are called monozygotic twins because they come from
a single fertilized egg that splits in two. Because identical twins come from the same
fertilized egg, the same genes control their body structure, height, and facial features,
which explains why identical twins look alike. In contrast, fraternal or dizygotic twins
come from two separate eggs fertilized by two separate sperm. Genetically, fraternal
twins are just like any other siblings—on average, about half their genes are the same.
In twin studies, scientists compare identical and fraternal twins to measure the infl u-
ence of heredity. When identical twins are more alike than are fraternal twins, this
implicates heredity (Phelps, Davis, & Schartz, 1997).

A similar logic is used in adoption studies, in which adopted
children are compared with their biological parents and their
adoptive parents. The idea here is that biological parents pro-
vide their child’s genes whereas adoptive parents provide the
child’s environment. Consequently, if a behavior has important
genetic roots, then adopted children should behave more like
their biological parents than like their adoptive parents.

These and other methods are not foolproof. Perhaps you
recognized a potential fl aw in twin studies: Parents and other
people may treat monozygotic twins more similarly than they
treat dizygotic twins. This would make monozygotic twins more
similar than …

S
N

82

3.1 T H E N E W B O R N

The Newborn’s Reflexes

Assessing the Newborn

The Newborn’s States

Temperament

3.2 P H YS I C A L D E V E L O P M E N T

Growth of the Body

The Emerging Nervous System

3.3 M OV I N G A N D G R A S P I N G :

E A R LY M OTO R S K I L L S

Locomotion

Fine Motor Skills

3.4 C O M I N G TO K N OW T H E

WO R L D : P E R C E P T I O N

Smell, Taste, and Touch

Hearing

Seeing

❚ SPOTLIGHT ON RESEARCH:

How Infants Become Face Experts

Integrating Sensory Information

3.5 B E C O M I N G S E L F – AWA R E

Origins of Self-Concept

Theory of Mind

❚ REAL PEOPLE: APPLYING HUMAN
DEVELOPMENT:

“Seeing Is Believing . . .” for 3-Year-Olds

S U M M A RY

K E Y T E R M S

L E A R N M O R E A B O U T I T

03-W4803-RS1.indd 8203-W4803-RS1.indd 82 9/5/08 1:18:33 PM9/5/08 1:18:33 PM

SECOND REVISED PAGES

83

S
N

83

C H A P T E R

Tools for Exploring the World
Physical, Perceptual, and Motor Development

T hink about what you were like 2 years ago. Whatever you were doing, you probably look, act,
think, and feel in much the same way today as you did then. Two years in an adult’s life usually doesn’t

result in profound changes, but 2 years makes a big difference early in life. The changes that occur in

the first few years after birth are incredible. In less than 2 years, an infant is transformed from a seem-

ingly helpless newborn into a talking, walking, havoc-wreaking toddler. No changes at any other point

in the life span come close to the drama and excitement of these early years.

In this chapter, our tour of these 2 years begins with the newborn and then moves to physical

growth—changes in the body and the brain. The third section of the chapter examines motor skills.

You’ll discover how babies learn to walk and how they learn to use their hands to hold and then

manipulate objects. In the fourth section, we’ll examine changes in infants’ sensory abilities that allow

them to comprehend their world.

As children begin to explore their world and learn more about it, they also learn more about

themselves. They learn to recognize themselves and begin to understand more about their thoughts

and others’ thoughts. We’ll explore these changes in the last section of the chapter.

3
©

S
to

ck
by

te
/

G
et

ty
Im

ag
es

03-W4803-RS1.indd 8303-W4803-RS1.indd 83 9/5/08 1:18:38 PM9/5/08 1:18:38 PM

SECOND REVISED PAGES

84 | CHAPTER 3

S
N

84

Lisa and Steve, proud but exhausted parents, are astonished at how their lives revolve around 10-day-old Dan’s eating and sleeping. Lisa feels as if she is feeding Dan around the
clock. When Dan naps, Lisa thinks of many things she should do but usually naps herself because

she is so tired. Steve wonders when Dan will start sleeping through the night so that he and

Lisa can get a good night’s sleep themselves.

T H E N EW BO R N BABY T H AT T H R ILLS PAREN T S LIK E LISA AN D ST EVE IS AC T UALLY R AT H ER
H O MELY. Newborns arrive covered with blood and vernix, a white-colored “wax” that
protected the skin during the many months of prenatal development. In addition, the
baby’s head is temporarily distorted from its journey through the birth canal, and the
newborn has a beer belly and is bowlegged.

What can newborns like Dan do? We’ll answer that question in this section and,
as we do, you’ll learn when Lisa and Steve can expect to resume getting a full night’s
sleep.

| The Newborn’s Reflexes

Most newborns are well prepared to begin interacting with their
world. The newborn is endowed with a rich set of reflexes, un-
learned responses that are triggered by a specifi c form of stimulation.
● Table 3.1 shows the variety of refl exes commonly found in new-
born babies.

You can see that some refl exes are designed to pave the way for
newborns to get the nutrients they need to grow: The rooting and
sucking refl exes ensure that the newborn is well prepared to begin
a new diet of life-sustaining milk. Other refl exes seem designed to
protect the newborn from danger in the environment. The eye blink,
for example, helps newborns avoid unpleasant stimulation.

Still other refl exes serve as the foundation for larger, voluntary
patterns of motor activity. For example, the stepping refl ex motions
look like precursors to walking, so it probably won’t surprise you to
learn that babies who practice the stepping refl ex often learn to walk
earlier than those who don’t practice this (Zelazo, 1993).

Refl exes are also important because they can be a useful way to determine whether
the newborn’s nervous system is working properly. For example, infants with damage
to the sciatic nerve, which is found in the spinal cord, do not show the withdrawal
refl ex. Infants who have problems with the lower part of the spine do not show the
Babinski refl ex. If these or other refl exes are weak or missing altogether, a thorough
physical and behavioral assessment is called for. Similarly, many of these refl exes nor-
mally vanish during infancy; if they linger then this, too, indicates the need for a thor-
ough physical examination.

L E A R N I N G O B J E C T I V E S

How do reflexes help newborns interact with the world? ❚

How do we determine whether a baby is healthy and adjust- ❚
ing to life outside the uterus?

What behavioral states are common among newborns? ❚

What are the different features of temperament? Do they ❚
change as children grow?

3.1 THE NEWBORN

R
ob

er
t

V.
K

ai
l

This newborn baby (Ben Kail at 20 seconds old)

is covered with vernix and is bow-legged; his

head is distorted from the journey down the

birth canal.

refl exes

unlearned responses triggered by specifi c

stimulation

03-W4803-RS1.indd 8403-W4803-RS1.indd 84 9/5/08 1:18:40 PM9/5/08 1:18:40 PM

SECOND REVISED PAGES

TOOLS FOR EXPLORING THE WORLD | 85

S
N

85

| Assessing the Newborn

Imagine that a mother has just asked you if her newborn baby is healthy.
How would you decide? You would probably check to see whether the baby
seems to be breathing and if her heart seems to be beating. In fact, breathing
and heartbeat are two vital signs included in the Apgar score, which provides
a quick, approximate assessment of the newborn’s status by focusing on the
body systems needed to sustain life. The other vital signs are muscle tone,
presence of refl exes such as coughing, and skin tone. Each of the fi ve vital
signs receives a score of 0, 1, or 2, where 2 is the optimal score. For example, a
newborn whose muscles are completely limp receives a 0; a baby who shows
strong movements of arms and legs receives a 2. The fi ve scores are added to-
gether, with a total score of 7 or more indicating a baby who is in good physi-
cal condition. A score of 4–6 means that the newborn needs special attention
and care. A score of 3 or less signals a life-threatening situation that requires
emergency medical care (Apgar, 1953).

For a comprehensive evaluation of the newborn’s well-being, pediatri-
cians and other child-development specialists sometimes administer the Neo-
natal Behavioral Assessment Scale or NBAS for short (Brazelton & Nugent,
1995). The NBAS is used with newborns to 2-month-olds to provide a detailed
portrait of the baby’s behavioral repertoire. The scale includes 28 behavioral
items along with 18 items that test refl exes. The baby’s performance is used to
evaluate the functioning of these four systems:

Autonomic ■ : the newborn’s ability to control body functions such as
breathing and temperature regulation

Motor ■ : the newborn’s ability to control body movements and activity
level

State ■ : the newborn’s ability to maintain a state (e.g., staying alert or staying
asleep)

Social ■ : the newborn’s ability to interact with people

Newborns step reflexively when they are held

upright and moved forward.

©
B

ub
bl

es
P

ho
to

lib
ra

ry
/

A
la

m
y

● TA B L E 3 . 1

Some Major Reflexes Found in Newborns

Name Response Age When Reflex Disappears Significance

Babinski A baby’s toes fan out when the sole 8–12 months Perhaps a remnant of evolution
of the foot is stroked from heel to toe

Blink A baby’s eyes close in response to Permanent Protects the eyes
bright light or loud noise

Moro A baby throws its arms out and 6 months May help a baby cling to its mother.
then inward (as if embracing) in
response to loud noise or when
its head falls

Palmar A baby grasps an object placed 3–4 months Precursor to voluntary walking
in the palm of its hand

Rooting When a baby’s cheek is stroked, 3–4 weeks (replaced Helps a baby find the nipple
it turns its head toward the by voluntary head turning)
stroking and opens its mouth

Stepping A baby who is held upright by 2–3 months Precursor to voluntary walking
an adult and is then moved
forward begins to step rhythmically

Sucking A baby sucks when an object 4 months (replaced by Permits feeding
is placed in its mouth voluntary sucking)

03-W4803-RS1.indd 8503-W4803-RS1.indd 85 9/5/08 1:18:42 PM9/5/08 1:18:42 PM

SECOND REVISED PAGES

86 | CHAPTER 3

S
N

86

The NBAS is based on the view that newborns are remarkably competent individuals
who are well prepared to interact with the environment. Refl ecting this view, exam-
iners go to great lengths to bring out a baby’s best performance. They do everything
possible to make a baby feel comfortable and secure during testing. And if the infant
does not at fi rst succeed on an item, the examiner provides some assistance (Alberts,
2005).

The NBAS, along with a thorough physical examination, can determine whether a
newborn is functioning normally. Scores from the NBAS can, for example, be used to
diagnose disorders of the central nervous system.

| The Newborn’s States

Newborns spend most of each day alternating among four diff erent states (St. James-
Roberts & Plewis, 1996; Wolff , 1987):

Alert inactivity ■ —The baby is calm with eyes open and attentive; the baby
seems to be deliberately inspecting the environment.

Waking activity ■ —The baby’s eyes are open but they seem unfocused; the arms
or legs move in bursts of uncoordinated motion.

Crying ■ —The baby cries vigorously, usually accompanied by agitated but unco-
ordinated motion.

Sleeping ■ —The baby alternates from being still and breathing regularly to mov-
ing gently and breathing irregularly; eyes are closed throughout.

Of these states, crying and sleeping have captured the attention of parents and re-
searchers alike.

Crying
Newborns spend 2–3 hours each day crying or on the verge of crying. If you’ve not
spent much time around newborns, you might think that all crying is pretty much

alike. In fact, scientists and parents can identify three distinctive types of cries
(Snow, 1998). A basic cry starts softly and then gradually becomes more intense; it
usually occurs when a baby is hungry or tired. A mad cry is a more intense version
of a basic cry; and a pain cry begins with a sudden, long burst of crying followed
by a long pause and gasping. Thus, crying represents the newborn’s fi rst venture
into interpersonal communication. By crying, babies tell their parents that they
are hungry or tired, angry or hurt. By responding to these cries, parents are en-
couraging their newborn’s eff orts to communicate.

Parents are naturally concerned when their baby cries, and if they can’t quiet
a crying baby, their concern mounts and can easily give way to frustration and
annoyance. It’s no surprise, then, that parents develop little tricks for soothing
their babies. Many Western parents lift a baby to the shoulder and walk or gently
rock the baby. Sometimes they will also sing lullabies, pat the baby’s back, or give
the baby a pacifi er. Yet another method is to put a newborn into a car seat and go
for a drive; this technique was used once, as a last resort, at 2 a.m. with Ben Kail
when he was 10 days old. After about the 12th time around the block, he fi nally
stopped crying and fell asleep!

Another useful technique is swaddling, in which an infant is wrapped tightly
in a blanket. Swaddling is used in many cultures around the world, including Tur-
key and Peru, as well as by Native Americans. Swaddling provides warmth and
tactile stimulation that usually works well to soothe a baby (Delaney, 2000).

Parents are sometimes reluctant to respond to their crying infant for fear of
producing a baby who cries constantly. Yet they hear their baby’s cry as a call for
help that they shouldn’t ignore. What to do? Should parents respond? “Yes, usu-

ally” is probably the best answer (Hubbard & van Ijzendoorn, 1991). If parents respond
immediately, every time their infant cries, the result may well be a fussy, whiny baby.

alert inactivity

state in which a baby is calm with eyes

open and attentive; the baby seems to be

deliberately inspecting the environment

waking activity

state in which a baby’s eyes are open but

seem unfocused while the arms or legs

move in bursts of uncoordinated motion

crying

state in which a baby cries vigorously, usu-

ally accompanied by agitated but uncoor-

dinated movement

sleeping

state in which a baby alternates from be-

ing still and breathing regularly to moving

gently and breathing irregularly; the eyes

are closed throughout

basic cry

cry that starts softly and gradually be-

comes more intense; often heard when

babies are hungry or tired

In many countries worldwide, infants are

wrapped tightly in blankets as a way to keep

them soothed.

©
D

ea
n

C
on

ge
r

/
C

or
bi

s
T H I N K A B O U T I T

Newborns seem to be extremely well

prepared to begin to interact with their

environment. Which of the theories

described in Chapter 1 predict such

preparedness? Which do not?

03-W4803-RS1.indd 8603-W4803-RS1.indd 86 9/5/08 1:18:44 PM9/5/08 1:18:44 PM

SECOND REVISED PAGES

TOOLS FOR EXPLORING THE WORLD | 87

S
N

87

Instead, parents need to consider why their infant is crying and the intensity of the
crying. When a baby wakes during the night and cries quietly, a parent might wait be-
fore responding, giving the baby a chance to calm itself. However, when parents hear
a loud noise from an infant’s bedroom followed by a mad cry, they should respond
immediately. Parents need to remember that crying is actually the newborn’s fi rst at-
tempt to communicate with others. They need to decide what the infant is trying to
tell them and whether that warrants a quick response or whether they should let the
baby soothe itself.

Sleeping
Crying may get parents’ attention, but sleep is what newborns
do more than anything else. They sleep 16–18 hours daily. The
problem for tired parents is that newborns sleep in naps taken
round-the-clock. Newborns typically go through a cycle of wake-
fulness and sleep about every 4 hours. That is, they will be awake
for about an hour, sleep for 3 hours, and then start the cycle
anew. During the hour when newborns are awake, they regularly
move between the diff erent waking states several times. Cycles
of alert inactivity, waking activity, and crying are common.

As babies grow older, the sleep–wake cycle gradually begins
to correspond to the day–night cycle (St. James-Roberts & Plewis,
1996). By 3 or 4 months, many babies sleep for 5–6 hours straight,
and by 6 months many are sleeping for 10–12 hours at night, a
major milestone for bleary-eyed parents like Lisa and Steve.

By six months, most North American infants are sleeping in a crib in their own
room. Although this practice seems “natural” to North American parents, in much
of the rest of the world children sleep with their parents throughout infancy and
the preschool years. Such parent–child “co-sleeping” is commonly found in cultures
where people defi ne themselves less as independent individuals and more as part of
a group. For parents in cultures that value such interdependence—including Egypt,
Italy, Japan, and Korea, as well as the Maya in Guatemala and the Inuit in Canada—
co-sleeping is an important step in forging parent–child bonds, just as sleeping alone
is an important step toward independence in cultures that value self-reliance (Morelli
et al., 1992; Nelson, Schiefenhoevel, & Haimerl, 2000; Worthman & Brown, 2007).

How does co-sleeping work? Infants may sleep in a cradle placed next to their par-
ents’ bed or in a basket that’s in their parents’ bed. When they outgrow this arrange-
ment, they sleep in the bed with their mother; depending on the culture, the father
may sleep in the same bed, in another bed in the same room, in another room, or in
another house altogether!

You might think that co-sleeping would make children more dependent on their
parents, but research provides no evidence of this (Cortesi et al., 2004; Okami, Weisner,
& Olmstead, 2002). Plus, co-sleeping has the benefi t of avoiding the lengthy and elabo-
rate rituals that are often required to get youngsters to sleep in their own room alone.
With co-sleeping, children and parents simply go to bed together with few struggles.

Roughly half of newborns’ sleep is irregular or rapid-eye-movement (REM) sleep,
a time when the body is quite active. During REM sleep, newborns move their arms
and legs; they may grimace and their eyes may dart beneath their eyelids. Brain waves
register fast activity, the heart beats more rapidly, and breathing is more rapid. In
regular or nonREM sleep, breathing, heart rate, and brain activity are steady and
newborns lie quietly without the twitching associated with REM sleep. REM sleep be-
comes less frequent as infants grow. By 4 months, only 40% of sleep is REM sleep. By
the fi rst birthday, REM sleep will drop to 25%—not far from the adult average of 20%
(Halpern, MacLean, & Baumeister, 1995).

The function of REM sleep is still debated. Older children and adults dream dur-
ing REM sleep, and brain waves during REM sleep resemble those of an alert, awake
person. Consequently, many scientists believe that REM sleep provides stimulation for

T H I N K A B O U T I T

When Mary’s 4-month-old son cries,

she rushes to him immediately and does

everything possible to console him. Is

this a good idea?

Co-sleeping, in which infants and young children

sleep with their parents, is common in many

countries around the world.

©
A

ub
re

y
W

ad
e

/
Pa

no
s

Pi
ct

ur
es

irregular or rapid-eye-movement (REM)

sleep

irregular sleep in which an infant’s eyes

dart rapidly beneath the eyelids while the

body is quite active

regular (nonREM) sleep

sleep in which heart rate, breathing, and

brain activity are steady

mad cry

more intense version of a basic cry

pain cry

cry that begins with a sudden long burst,

followed by a long pause and gasping

03-W4803-RS1.indd 8703-W4803-RS1.indd 87 9/5/08 1:18:46 PM9/5/08 1:18:46 PM

SECOND REVISED PAGES

88 | CHAPTER 3

S
N

88

the brain that fosters growth in the nervous system (Halpern et al., 1995; Roff warg,
Muzio, & Dement, 1966).

By the toddler and preschool years, sleep routines are well established. Most 2-year-
olds spend about 13 hours sleeping, compared to just under 11 hours for 6-year-olds.
By age 4, most youngsters give up their afternoon nap and sleep longer at nighttime to
compensate. This can be a challenging time for parents and caregivers who use nap-
time as an opportunity to complete some work or to relax.

Following an active day, most preschool children drift off to sleep easily. However,
most children will have an occasional night when bedtime is a struggle. Furthermore,
for approximately 20 to 30% of preschool children, bedtime struggles occur nightly
(Lozoff , Wolf, & Davis, 1985). More often than not, these bedtime problems refl ect the
absence of a regular bedtime routine that’s followed consistently. The key to a pleasant
bedtime is to establish a nighttime routine that helps children to “wind down” from
busy daytime activities. This routine should start at about the same time every night
(“It’s time to get ready for bed . . .”) and end at about the same time (when the parent
leaves the child and the child tries to fall asleep). This nighttime routine may be any-
where from 15 to 45 minutes long, depending on the child. Also, as children get older,
parents can expect them to perform more of these tasks independently. A 2-year-old

will need help all along the way, but a 5-year-old can do many of these tasks alone.
But remember to follow the routine consistently; this way, children know that
each step is getting them closer to bedtime and falling asleep.

Sudden Infant Death Syndrome
For many parents of young babies, however, sleep is a cause of concern. In sud-
den infant death syndrome (SIDS), a healthy baby dies suddenly for no apparent
reason. Approximately 1–3 of every 1,000 American babies dies from SIDS. Most
of them are between 2 and 4 months of age (Wegman, 1994).

Scientists don’t know the exact causes of SIDS, but one idea is that 2- to
4-month-old infants are particularly vulnerable to SIDS because many newborn
refl exes are waning during these months and thus infants may not respond ef-
fectively when breathing becomes diffi cult. They may not refl exively move their
head away from a blanket or pillow that is smothering them (Lipsitt, 2003).

Researchers have also identifi ed several risk factors associated with SIDS. Ba-
bies are more vulnerable if they were born prematurely or with low birth weight.
They are also more vulnerable when their parents smoke. SIDS is more likely
when a baby sleeps on its stomach (face down) than when it sleeps on its back
(face up). Finally, SIDS is more likely during winter, when babies sometimes be-
come overheated from too many blankets and sleepwear that is too heavy (Carroll
& Loughlin, 1994). Evidently, SIDS infants, many of whom were born prematurely
or with low birth weight, are less able to withstand physiological stresses and im-
balances that are brought on by cigarette smoke, breathing that is temporarily
interrupted, or overheating (Simpson, 2001).

In 1992, based on mounting evidence that SIDS occurred more often when
infants slept on their stomachs, the American Academy of Pediatrics (AAP) began
advising parents to put babies to sleep on their backs or sides. In 1994 the AAP
joined forces with the U.S. Public Health Service to launch a national program
to educate parents about the dangers of SIDS and the importance of putting
babies to sleep on their backs. The “Back to Sleep” campaign was widely publi-
cized through brochures, posters like the one shown in ❚ Figure 3.1, and videos.
Since the Back to Sleep campaign began, research shows that far more infants are
now sleeping on their backs and that the incidence of SIDS has dropped (NIH,
2000b).

However, it became clear that African American infants were still twice as
likely to die from SIDS, apparently because they were much more likely to be
placed on their stomachs to sleep. Consequently, in the 21st century the National
Institutes of Health has partnered with groups such as the Women in the NAACP

sudden infant death syndrome (SIDS)

when a healthy baby dies suddenly for no

apparent reason

Figure 3.1 ❚
This poster is one part of an effective cam-

paign to reduce SIDS by encouraging parents

to have their babies sleep on their backs.

National Institute of Child Health and Development.

03-W4803-RS1.indd 8803-W4803-RS1.indd 88 9/5/08 1:18:47 PM9/5/08 1:18:47 PM

SECOND REVISED PAGES

TOOLS FOR EXPLORING THE WORLD | 89

S
N

89

and the National Council of 100 Black Women to train thousands of people to convey
the Back to Sleep message in a culturally appropriate manner to African American
communities (NICHD, 2004). The goal is for African American infants to benefi t from
the life saving benefi ts of the Back to Sleep program. The message for all parents—
particularly if their babies were premature or small-for-date—is to keep their babies
away from smoke, to put them on their backs to sleep, and to not overdress them or
wrap them too tightly in blankets (Willinger, 1995).

| Temperament

So far, we’ve talked as if all babies are alike. But if you’ve seen a number of babies to-
gether, you know this isn’t true. Perhaps you’ve seen some babies who are quiet most
of the time alongside others who cried often and impatiently? Maybe you’ve known
infants who responded warmly to strangers next to others who seemed shy? These
characteristics of infants indicate a consistent style or pattern to an infant’s behavior,
and collectively they defi ne an infant’s temperament.

Alexander Thomas and Stella Chess (Thomas, Chess, & Birch, 1968) pioneered the
study of temperament with the New York Longitudinal Study, in which they traced the
lives of 141 individuals from infancy through adulthood. Thomas and Chess gathered
their initial data by interviewing the babies’ parents and asking individuals unfamiliar
with the children to observe them at home. Based on these interviews and observa-
tions, Thomas and Chess suggested that infants’ behavior varied along nine tempera-
mental dimensions. One dimension was activity, which referred to an infant’s typical
level of motor activity. A second was persistence, which referred to the amount of time
an infant devoted to an activity, particularly when obstacles were present.

The New York Longitudinal Study launched research on infant temperament, but
today we know that Thomas and Chess overestimated the number of temperamental
dimensions. Instead of nine dimensions, scientists now propose from two to six di-
mensions. For example, Mary K. Rothbart (2004; Rothbart & Hwang, 2005) has devised
an infl uential theory of temperament that includes three diff erent dimensions:

Surgency/extroversion ■ refers to the extent to which a child is generally happy,
active, vocal, and regularly seeks interesting stimulation.

Negative aff ect ■ refers to the extent to which a child is angry, fearful, frus-
trated, shy, and not easily soothed.

Effortful control ■ refers to the extent to which a child
can focus attention, is not readily distracted, and can
inhibit responses.

These dimensions of temperament emerge in infancy, con-
tinue into childhood, and are related to dimensions of person-
ality that are found in adolescence and adulthood (Gartstein,
Knyazev, & Slobodskaya, 2005). However, the dimensions are
not independent:, infants who are high on eff ortful control
tend to be high on surgency/extroversion and low on negative
aff ect. In other words, babies who can control their attention
and inhibit responses tend to be happy and active but not an-
gry or fearful.

Hereditary and Environmental Contributions to
Temperament
Most theories agree that temperament refl ects both heredity
and experience. The infl uence of heredity is shown in twin stud-
ies: Identical twins are more alike in most aspects of tempera-
ment than fraternal twins, including activity level, extroversion,

temperament

consistent style or pattern of behavior

Twin studies show the impact of heredity on

temperament: If one identical twin is active, the

other one usually is.

©
N

an
cy

S
he

eh
an

/
P

ho
to

Ed
it

03-W4803-RS1.indd 8903-W4803-RS1.indd 89 9/5/08 1:18:49 PM9/5/08 1:18:49 PM

SECOND REVISED PAGES

90 | CHAPTER 3

S
N

90

irritability, and persistence (Goldsmith, et al., 1999; Saudino & Cherny, 2001). If, for
example, one identical twin is temperamentally active then the other usually is, too.
However, the impact of heredity also depends on the temperamental dimension and
the child’s age. For example, negative aff ect is more infl uenced by heredity than the
other dimensions, and temperament in childhood is more infl uenced by heredity than
is temperament in infancy (Wachs & Bates, 2001).

The environment also contributes to children’s temperament. Positive emotional-
ity—youngsters who laugh often, seem to be generally happy, and express pleasure
often—seems to refl ect environmental infl uences (Goldsmith et al., 1997). Conversely,
infants more often develop intense, diffi cult temperaments when mothers are abrupt
in dealing with them and lack confi dence (Belsky, Fish, & Isabella, 1991). And some
temperamental characteristics are more common in some cultures than in others.
Asian babies tend to be less emotional than European American babies. For instance,
Asian babies cry less often and less intensely than European American babies, but
Russian infants are more fearful and emotionally negative (Gartstein, Slobodskaya, &
Kinsht, 2003; Kagan et al., 1994; Lewis, Ramsay, & Kawakami, 1993).

There’s no question that heredity and experience cause babies’ temperaments to
diff er, but how stable is temperament? …

S
N

126

4.1 T H E O N S E T O F T H I N K I N G :

P I AG E T ’ S AC C O U N T

Basic Principles of Cognitive Development

Sensorimotor Thinking

Preoperational Thinking

Evaluating Piaget’s Theory

Extending Piaget’s Account: Children’s Naive
Theories

4.2 I N F O R M AT I O N P R O C E S S I N G

D U R I N G I N FA N C Y A N D E A R LY

C H I L D H O O D

General Principles of Information Processing

Attention

Learning

Memory

❚ CURRENT CONTROVERSIES:

Preschoolers on the Witness Stand

Learning Number Skills

4.3 M I N D A N D C U LT U R E :

V Y G OT S K Y ’ S T H E O RY

The Zone of Proximal Development

Scaffolding

Private Speech

4.4 L A N G UAG E

The Road to Speech

First Words and Many More

Speaking in Sentences: Grammatical
Development

❚ SPOTLIGHT ON RESEARCH:

Infants Infer Grammatical Rules From
Speech

Communicating With Others

S U M M A RY

K E Y T E R M S

L E A R N M O R E A B O U T I T

04-W4803-MRB1.indd 12604-W4803-MRB1.indd 126 9/5/08 1:24:38 PM9/5/08 1:24:38 PM

SECOND REVISED PAGES

127

C H A P T E R 4

S
N

127

The Emergence of Thought
and Language

Cognitive Development in Infancy

and Early Childhood

O n the TV show Family Guy, Stewie is a 1-year-old who can’t stand his mother (Stewie: “Hey,
mother, I come bearing a gift. I’ll give you a hint. It’s in my diaper and it’s not a toaster.”) and hopes to

dominate the world. Much of the humor, of course, turns on the idea that babies are capable of so-

phisticated thinking and just can’t express it. But what thoughts do lurk in the mind of an infant who is

not yet speaking? How does cognition develop during infancy and early childhood? What makes these

changes possible?

These questions provide the focus of this chapter. We begin with what has long been considered

the definitive account of cognitive development, Jean Piaget’s theory. In this theory, thinking progresses

through four distinct stages between infancy and adulthood.

The next two sections of the chapter concern alternative accounts of cognitive development.

One account, the information-processing perspective, traces children’s emerging cognitive skills in

many specific domains, including memory skills. The other, Lev Vygotsky’s theory, emphasizes the cul-

tural origins of cognitive development and explains why children sometimes talk to themselves as

they play or work.

Throughout development, children express their thoughts in oral and written language. In the last

section of this chapter, you’ll see how children master the sounds, words, and grammar of their native

language.

©
M

as
te

rf
ile

04-W4803-MRB1.indd 12704-W4803-MRB1.indd 127 9/5/08 1:24:46 PM9/5/08 1:24:46 PM

SECOND REVISED PAGES

128 | CHAPTER 4

S
N

128

T
hree-year-old Jamila loves talking to her grandmother (“Gram”) on the telephone. Some-

times these conversations are not very successful because Gram asks questions and Jamila

replies by nodding her head “yes” or “no.” Jamila’s dad has explained that Gram (and others

on the phone) can’t see her nodding—that she needs to say “yes” or “no.” But Jamila invariably

returns to head-nodding. Her dad can’t see why such a bright and talkative child doesn’t realize

that nodding is meaningless over the phone.

W HY D O ES JAMILA IN SIST O N N O D D IN G H ER H EAD W H EN SH E’S T ALK IN G O N T H E PH O N E?
This behavior is quite typical, according to the famous Swiss psychologist Jean Piaget
(1896–1980). In Piaget’s theory, children’s thinking progresses through four qualita-
tively diff erent stages. In this section, we’ll begin by describing some of the general
features of Piaget’s theory, then examine Piaget’s account of thinking during infancy
and during the preschool years, and fi nally consider some of the strengths and weak-
nesses of the theory.

| Basic Principles of Cognitive Development

Piaget believed that children are naturally curious. They constantly want to make
sense of their experience and, in the process, construct their understanding of the
world. For Piaget, children at all ages are like scientists in that they create theories
about how the world works. Of course, children’s theories are often incomplete. Nev-
ertheless, children’s theories are valuable to them because they make the world seem
more predictable.

According to Piaget, children understand the world with schemes, psychological
structures that organize experience. Schemes are mental categories of related events,
objects, and knowledge. During infancy, most schemes are based on actions. That is,
infants group objects based on the actions they can perform on them. For example,
infants suck and grasp, and they use these actions to create categories of objects that
can be sucked and objects that can be grasped.

Schemes are just as important after infancy, but they are now based primarily
on functional or conceptual relationships, not action. For example, preschoolers learn
that forks, knives, and spoons form a functional category of “things I use to eat.” Or
they learn that dogs, cats, and goldfi sh form a conceptual category of “pets.”

Like preschoolers, older children and adolescents have schemes based on func-
tional and conceptual schemes. But they also have schemes that are based on increas-
ingly abstract properties. For example, an adolescent might put fascism, racism, and
sexism in a category of “ideologies I despise.”

Thus, schemes of related objects, events, and ideas are present throughout devel-
opment. But as children develop, their rules for creating schemes shift from physical
activity to functional, conceptual, and, later, abstract properties of objects, events, and
ideas.

L E A R N I N G O B J E C T I V E S

According to Piaget, how do schemes, assimilation, and ac- ❚
commodation provide the foundation for cognitive develop-

ment throughout the life span?

How does thinking become more advanced as infants prog- ❚
ress through the sensorimotor stage?

What are the distinguishing characteristics of thinking during ❚
the preoperational stage?

What are the strengths and weaknesses of Piaget’s theory? ❚

How have contemporary researchers extended Piaget’s ❚
theory?

4.1 THE ONSET OF THINKING: PIAGET’S ACCOUNT

scheme

according to Piaget, a mental structure

that organizes information and regulates

behavior

04-W4803-MRB1.indd 12804-W4803-MRB1.indd 128 9/5/08 1:24:49 PM9/5/08 1:24:49 PM

SECOND REVISED PAGES

THE EMERGENCE OF THOUGHT AND LANGUAGE | 129

S
N

129

Assimilation and Accommodation
Schemes change constantly, adapting to children’s experiences. In fact, intellectual
adaptation involves two processes working together: assimilation and accommoda-
tion. Assimilation occurs when new experiences are readily incorporated into existing
schemes. Imagine a baby who has the familiar grasping scheme. She will soon discover
that the grasping scheme also works well on blocks, toy cars, and other small ob-
jects. Extending the existing grasping scheme to new objects illustrates assimilation.
Accommodation occurs when schemes are modifi ed based on experience. Soon the
infant learns that some objects can only be lifted with two hands and that some can’t
be lifted at all. Changing the scheme so that it works for new objects (e.g., using two
hands to grasp heavy objects) illustrates accommodation.

Assimilation and accommodation are often easier to understand when you re-
member Piaget’s belief that infants, children, and adolescents create theories to try to
understand events and objects around them. The infant whose theory is that objects
can be lifted with one hand fi nds that her theory is confi rmed when she tries to pick
up small objects, but she’s in for a surprise when she tries to pick up a heavy book.
The unexpected result forces the infant, like a good scientist, to
revise her theory to include this new fi nding.

Equilibration and Stages of Cognitive Development
Assimilation and accommodation are usually in balance, or
equilibrium. Children fi nd that many experiences are readily
assimilated into their existing schemes but that they sometimes
need to accommodate their schemes to adjust to new experi-
ences. This balance between assimilation and accommodation
was illustrated by our infant with a theory about lifting objects.
Periodically, however, this balance is upset, and a state of dis-
equilibrium results. That is, children discover that their current
schemes are not adequate because they are spending too much
time accommodating and much less time assimilating. When
disequilibrium occurs, children reorganize their schemes to re-
turn to a state of equilibrium, a process that Piaget called equili-
bration. To restore the balance, current but outmoded ways of thinking are replaced
by a qualitatively diff erent, more advanced set of schemes.

One way to understand equilibration is to return to the metaphor of the child as a
scientist. As we discussed in Chapter 1, good scientifi c theories readily explain some
phenomena but usually must be revised to explain others. Children’s theories allow
them to understand many experiences by predicting, for example, what will happen
(“It’s morning, so it’s time for breakfast”) or who will do what (“Mom’s gone to work,
so Dad will take me to school”), but the theories must be modifi ed when predictions go
awry (“Dad thinks I’m old enough to walk to school, so he won’t take me”).

Sometimes scientists fi nd that their theories contain critical fl aws that can’t be
fi xed simply by revising; instead, they must create a new theory that draws upon the
older theory but is fundamentally diff erent. For example, when the astronomer Coper-
nicus realized that the earth-centered theory of the solar system was fundamentally
wrong, his new theory built on the assumption that the sun is the center of the solar
system. In much the same way, periodically children reach states in which their cur-
rent theories seem to be wrong much of the time, so they abandon these theories in
favor of more advanced ways of thinking about their physical and social worlds.

According to Piaget, these revolutionary changes in thought occur three times over
the life span, at approximately 2, 7, and 11 years of age. This divides cognitive develop-
ment into the following four stages:

Period of Development Age Range

Sensorimotor period Infancy (0–2 years)

Preoperational period Preschool and early elementary school years
(2–7 years)

assimilation

according to Piaget, taking in information

that is compatible with what one already

knows

accommodation

according to Piaget, changing existing

knowledge based on new knowledge

equilibration

according to Piaget, a process by which

children reorganize their schemes to

return to a state of equilibrium when

disequilibrium occurs

This baby will learn that many objects can be

grasped easily with one hand—illustrating

assimilation—but will also discover that bigger,

heavier objects can be grasped only with two

hands—illustrating accommodation.

©
L

au
ra

D
w

ig
ht

/
C

or
bi

s

04-W4803-MRB1.indd 12904-W4803-MRB1.indd 129 9/5/08 1:24:49 PM9/5/08 1:24:49 PM

SECOND REVISED PAGES

130 | CHAPTER 4

S
N

130

Concrete operational period Middle and late elementary school years
(7–11 years)

Formal operational period Adolescence and adulthood (11 years and up)

The ages listed are only approximate. Some youngsters move through the periods
more rapidly than others, depending on their ability and their experience. However,
the only route to formal operations—the most sophisticated type of thought—is
through the fi rst three periods, in sequence. Sensorimotor thinking always gives rise
to preoperational thinking; a child cannot “skip” preoperational thinking and move
directly from the sensorimotor to the concrete operational period.

In the next few pages of this chapter, we consider Piaget’s account of sensorimotor
and preoperational thinking, the periods from birth to approximately 7 years of age.
In Chapter 6, we will return to Piaget’s theory to examine his account of concrete and
formal operational thinking in older children and adolescents.

| Sensorimotor Thinking

Piaget (1951, 1952, 1954) believed that the fi rst 2 years of life form a distinct phase in
human development. The sensorimotor period, from birth to roughly 2 years of age,
is the fi rst of Piaget’s four periods of cognitive development. In the 24 months of this
stage, infants’ thinking progresses remarkably along three important fronts.

Adapting to and Exploring the Environment
Newborns respond refl exively to many stimuli, but between 1 and 4 months refl exes
are fi rst modifi ed by experience. An infant may inadvertently touch his lips with his
thumb, thereby initiating sucking and the pleasing sensations associated with sucking.
Later, the infant tries to re-create these sensations by guiding his thumb to his mouth.
Sucking no longer occurs only refl exively when a mother places a nipple at the infant’s
mouth; instead, the infant has found a way to initiate sucking himself.

At about 8 months, infants reach a watershed: the onset of deliberate, intentional
behavior. For the fi rst time, the “means” and “end” of activities are distinct. If, for ex-
ample, a father places his hand in front of a toy, an infant will move his father’s hand
to be able to play with the toy. “The moving the hand” scheme is the means to achieve
the goal of “grasping the toy.” Using one action as a means to achieve another end is
the fi rst indication of purposeful, goal-directed behavior during infancy.

Beginning at about 12 months, infants become active experimenters. An infant
may deliberately shake a number of diff erent objects trying to discover which produce
sounds and which do not. Or an infant may decide to drop diff erent objects to see
what happens. An infant will discover that stuff ed animals land quietly whereas big-
ger toys often make a more satisfying “clunk” when they hit the ground. These actions
represent a signifi cant extension of intentional behavior; now babies repeat actions
with diff erent objects solely for the purpose of seeing what will happen.

Understanding Objects
Objects fi ll the world. Some, including dogs, spiders, and college students, are animate;
others, including cheeseburgers, socks, and this textbook, are inanimate. But they all
share a fundamental property—they exist independently of our actions and thoughts
toward them. Much as we may dislike spiders, they still exist when we close our eyes
or wish they would go away. Piaget’s term for this understanding that objects exist
independently is object permanence. And Piaget made the astonishing claim that in-
fants lacked this understanding for much of the fi rst year. That is, he proposed that an
infant’s understanding of objects could be summarized as “out of sight, out of mind.”
For infants, objects exist when in sight and no longer exist when out of sight.

Piaget concluded that infants have little understanding of objects. If a tempting
object such as an attractive toy is placed in front of a 4- to 8-month-old, the infant will
probably reach and grasp the object. If, however, the object is subsequently hidden by
a barrier or covered with a cloth, the infant will neither reach nor search. Instead, the

sensorimotor period

fi rst of Piaget’s four stages of cognitive

development, which lasts from birth to

approximately 2 years

object permanence

understanding, acquired in infancy, that

objects exist independently of oneself

04-W4803-MRB1.indd 13004-W4803-MRB1.indd 130 9/5/08 1:24:51 PM9/5/08 1:24:51 PM

SECOND REVISED PAGES

THE EMERGENCE OF THOUGHT AND LANGUAGE | 131

S
N

131

infant seems to lose all interest in the object, as if the now hid-
den object no longer exists. Paraphrasing the familiar phrase,
“out of sight, out of existence!”

Beginning at about 8 months, infants search for an object
that an experimenter has covered with a cloth. In fact, many
8- to 12-month-olds love to play this game: an adult covers the
object and the infant sweeps away the cover, laughing and smil-
ing all the while! But despite this accomplishment, their under-
standing of object permanence remains incomplete, according
to Piaget. If 8- to 10-month-olds see an object hidden under one
container several times and then see it hidden under a second
container, they usually reach for the toy under the fi rst container.
Piaget claimed that this behavior shows only a fragmentary un-
derstanding of objects because infants do not distinguish the
object from the actions they use to locate it, such as reaching for
a particular container.

Piaget argued that not until approximately 18 months do infants have full under-
standing of object permanence. However, in a few pages, we’ll see that infants know
more about objects than Piaget claimed.

Using Symbols
By 18 months, most infants have begun to talk and gesture, evidence of their
emerging capacity to use symbols. Words and gestures are symbols that stand for
something else. When a baby waves, it’s a symbol that’s just as eff ective as saying
“good-bye” to bid farewell. Children also begin to engage in pretend play, another
use of symbols. A 20-month-old may move her hand back and forth in front of
her mouth, pretending to brush her teeth.

Once infants can use symbols, they can begin to anticipate the consequences
of actions mentally instead of having to perform them. Imagine that an infant
and parent construct a tower of blocks next to an open door. Leaving the room,
a 12- to 18-month-old might close the door, knocking over the tower because he
cannot foresee this outcome of closing the door. But an 18- to 24-month-old can
anticipate the consequence of closing the door and move the tower beforehand.

In just 2 years, the infant progresses from refl exive responding to actively
exploring the world, understanding objects, and using symbols. These achieve-
ments are remarkable and set the stage for preoperational thinking, which we’ll
examine next.

| Preoperational Thinking

Once they have crossed into preoperational thinking, the magical power of symbols
is available to young children. Of course, mastering this power is a lifelong process;
the preschool child’s eff orts are tentative and sometimes incorrect (DeLoache, 1995).
Piaget identifi ed a number of characteristic shortcomings in preschoolers’ fl edgling
symbolic skills. Let’s look at three.

Egocentrism
Preoperational children typically believe that others see the world—both literally and
fi guratively—exactly as they do. Egocentrism is diffi culty in seeing the world from
another’s outlook. When youngsters stubbornly cling to their own way, they are not
simply being contrary. Preoperational children simply do not comprehend that other
people diff er in their ideas, convictions, and emotions.

One of Piaget’s famous experiments, the three-mountains problem, demonstrates
preoperational children’s egocentrism (Piaget & Inhelder, 1956, chap. 8). Youngsters
were seated at a table like the one shown in ❚ Figure 4.1. When preoperational children
were asked to choose the photograph that corresponded to another person’s view of

When interesting toys are covered so that they

can’t be seen, young babies lose interest, as if

“out of sight” means “out of existence.”

Ja
n

M
ue

lle
r

Toddlers frequently gesture, a sign of their grow-

ing competence at using symbols.

©
T

om
S

te
w

ar
t

/
C

or
bi

s

egocentrism

diffi culty in seeing the world from an-

other’s point of view; typical of children in

the preoperational period

04-W4803-MRB1.indd 13104-W4803-MRB1.indd 131 9/5/08 1:24:51 PM9/5/08 1:24:51 PM

SECOND REVISED PAGES

132 | CHAPTER 4

S
N

132

the-mountains, they usually picked the photograph that showed their own view of the
mountains, not the other person’s. Preoperational youngsters evidently suppose that
the mountains are seen the same way by all; they presume that theirs is the only view,
not one of many conceivable views. According to Piaget, only concrete operational
children fully understand that all people do not experience an event in exactly the
same way.

Recall that, in the vignette, 3-year-old Jamila nods her head during phone conver-
sations with her grandmother. This, too, refl ects preoperational egocentrism. Jamila
assumes that, because she is aware that her head is moving up and down (or side-
to-side), her grandmother must be aware of it, too. Because of this egocentrism, pre-
operational youngsters often attribute their own thoughts and feelings to others. They
may even credit inanimate objects with life and lifelike properties, a phenomenon
known as animism (Piaget, 1929). A preschool child may think that the sun is unhappy
on a cloudy day or that a car hurts when it’s in an accident. Caught up in their ego-
centrism, preoperational youngsters believe that inanimate objects have feelings just
as they do.

Centration
A second characteristic of preoperational thinking is that children seem to have the
psychological equivalent of tunnel vision: They often concentrate on one aspect of a
problem but totally ignore other, equally relevant aspects. Centration is Piaget’s term
for this narrowly focused thought that characterizes preoperational youngsters.

Piaget demonstrated centration in his experiments involving conservation. In the
conservation experiments, Piaget wanted to determine when children realize that im-
portant characteristics of objects (or sets of objects) stay the same despite changes
in their physical appearance. Some tasks that Piaget used to study conservation are
shown in ❚ Figure 4.2. Each begins with identical objects (or sets of objects). Then one
of the objects (or sets) is transformed, and children are asked if the objects are the
same in terms of some important feature.

Figure 4.1 ❚
Egocentrism: When asked to select the

photograph that shows the mountains as the

adult sees them, preschool children often

select the photograph that shows how the

mountain looks to them.

animism

crediting inanimate objects with life and

lifelike properties such as feelings

centration

according to Piaget, narrowly focused type

of thought characteristic of preoperational

children

04-W4803-MRB1.indd 13204-W4803-MRB1.indd 132 9/5/08 1:24:54 PM9/5/08 1:24:54 PM

SECOND REVISED PAGES

THE EMERGENCE OF THOUGHT AND LANGUAGE | 133

S
N

133

A typical conservation problem involves conservation of liquid quantity. Children
are shown identical beakers fi lled with the same amount of juice. After children agree
that the two beakers have the same amount of juice, the juice is poured from one bea-
ker into a taller, thinner beaker. The juice looks diff erent in the tall, thin beaker—it
rises higher—but of course the amount is unchanged. Nevertheless, preoperational
children claim that the tall, thin beaker has more juice than the original beaker. (And,
if the juice is poured into a wider beaker, they believe it has less.)

What is happening here? According to Piaget, preoperational children center on the
level of the juice in the beaker. If the juice is higher after it is poured, preoperational

Liquid
quantity

Pour water from one
glass into a shorter,
wider glass.

Is there the same amount of
water in each glass?

Are there the same number
of pennies in each row?

Are these sticks the same length?

Does each ball have the same
amount of clay?

Now does each piece have the same
amount of clay, or does one have more?

Does each cow have the same
amount of grass to eat?

Now does each cow have the same
amount to eat, or does one cow have more?

Now are the sticks the same length, or is
one longer?

Now are there the same number
of pennies in each row, or does
one row have more?

Now is there the same amount of water
in each glass, or does one glass have more?

Stretch out the top
row of pennies, push
together the bottom
row.

Move one stick to the
left and the other to
the right.

Roll one ball so that it
looks like a sausage.

Spread out the squares
in one field.

Number

Length

Mass

Area

Type of
conservation Starting configuration Final configurationTransformation

Figure 4.2 ❚
Children in the preoperational stage of development typically have difficulty solving conservation problems, in which important features of an

object (or objects) stay the same despite changes in physical appearance.

04-W4803-MRB1.indd 13304-W4803-MRB1.indd 133 9/5/08 1:24:56 PM9/5/08 1:24:56 PM

SECOND REVISED PAGES

134 | CHAPTER 4

S
N

134

children believe that there must be more juice now than before. Because preopera-
tional thinking is characterized by centration, these youngsters ignore the fact that the
change in the level of the juice is always accompanied by a change in the diameter of

the beaker.
In other conservation problems, preoperational children also tend

to focus on only one aspect of the problem. In conservation of num-
ber, for example, preoperational children concentrate on the fact that,
after the transformation, one row of objects is now longer than the
other. In conservation of length, preoperational children concentrate
on the fact that, after the transformation, the end of one stick is far-
ther to the right than the end of the other. Thus, preoperational chil-
dren’s “centered” thinking means that they overlook other parts of the
problem that would tell them the quantity is unchanged.

Appearance as Reality
A fi nal feature of preoperational thinking is that preschool children
believe that an object’s appearance tells what the object is really like.

For instance, many a 3-year-old has watched with quiet fascination as an older brother
or sister put on a ghoulish costume only to erupt in frightened tears when their sibling
put on scary makeup. The scary made-up face is reality, not just something that looks

frightening but really isn’t.
Confusion between appearance and reality is not limited to cos-

tumes and masks. It is a general characteristic of preoperational
thinking. Consider the following cases where appearances and reality
confl ict:

A boy is angry because a friend is being mean but smiles be- ■
cause he’s afraid the friend will leave if he reveals his anger.

A glass of milk looks brown when seen through sunglasses. ■

A piece of hard rubber looks like food (e.g., like a piece of ■
pizza).

Older children and adults know that the boy looks happy, the milk
looks brown, and the object looks like food but that the boy is really
angry, the milk is really white, and the object is really rubber. Preop-
erational children, however, confuse appearance and reality, thinking
the boy is happy, the milk is brown, and the piece of rubber is edible.

The defi ning characteristics of preoperational thought are sum-
marized in ● Table 4.1.T H I N K A B O U T I T

Children with low birth weight often

have delayed intellectual development.

According to Piaget, what form might

the delay take?

● TA B L E 4 . 1

Characteristics of Preoperational Thinking

Characteristic Definition Example

Egocentrism Child believes that all people see A child gestures during a telephone
the world as he or she does conversation, not realizing that the
listener cannot see the gestures

Centration Child focuses on one aspect of a In conservation of liquid quantity, child pays attention
problem or situation but ignores to the height of the liquid in the beaker but ignores
other relevant aspects the diameter of the beaker

Appearance as reality Child assumes that an object Child believes that a person smiling at another person
really is what it appears to be is really happy even though the other person is being mean

In conservation problems, preschool children

typically do not believe that the quantity of a

liquid remains the same when it is poured into a

taller, more slender beaker.

©
T

on
y

Fr
ee

m
an

/
P

ho
to

Ed
it

For young children, appearance is often reality

and so they are frightened when familiar people

wear scary masks.

©
T

on
y

Fr
ee

m
an

/
P

ho
to

Ed
it

04-W4803-MRB1.indd 13404-W4803-MRB1.indd 134 9/5/08 1:24:56 PM9/5/08 1:24:56 PM

SECOND REVISED PAGES

THE EMERGENCE OF THOUGHT AND LANGUAGE | 135

S
N

135

| Evaluating Piaget’s Theory

Because Piaget’s theory is so comprehensive, it has stimulated much research. Much
of this work supports Piaget’s view that children actively try to understand the world
around them and organize their knowledge and that cognitive development includes
major qualitative changes (Brainerd, 1996; Flavell, …

error: Content is protected !!