Biology and Behavior

How Children Develop (3rd ed.) Siegler, DeLoache & Eisenberg

Chapter 3

Overview

I. Nature and Nurture

II. Brain Development

III. The Body: Physical Growth and Development

I. Nature and Nurture

A. Genetic and Environmental Forces

B. Behavior Genetics

A. Nature and Nurture

Both heredity and environment influence individuals’ characteristics.

When scientists first began to investigate the contributions of heredity and environment, they generally emphasized one factor or the other as the prime influence.

Recent efforts to map the human genome established that individuals differ from one another by only about 1 to1.5% of their genes.

1. Genetic and Environmental Influences

The interplay between genes and experience is very complex.

This model of hereditary and environmental influences can help to simplify this interplay.

Three Key Elements of the Model Genotype: the genetic material an

individual inherits

Phenotype: the observable expression of the genotype, including body characteristics and behavior

Environment: includes every aspect of the individual, and his or her surroundings, other than genes

Four Fundamental Relations

1. Parents’ genetic contribution to the child’s genotype

2. Contributions of the child’s genotype to his or her own phenotype

3. Contribution of the child’s environment to his or her own phenotype

4. Influence of the child’s phenotype on his or her environment

Four Fundamental Relations

Parents’Genotype

Parents’Genotype

Child’sGenotype

Child’sGenotype

Child’sEnvironment

Child’sEnvironment

Child’sPhenotype

Child’sPhenotype

Relation 1: Parents’ and Child’s Genotypes

Genetic material is passed on as chromosomes—long, threadlike molecules made up of DNA Carry all the biochemical

instructions involved in the formation and functioning of an organism

Genes are sections of chromosomes that are the basic units of heredity for all living things Karyotype

Sex Determination

Sex chromosomes determine an individual’s sex.

Females have two X chromosomes in the 23rd pair, whereas males have an X and a Y chromosome.

A gene on the Y chromosome encodes the protein that triggers the formation of the testes, which subsequently produce testosterone, which in turn takes over the molding of maleness.

Diversity and Individuality

Mutations: changes in sections of DNA caused by random or environmental factors

Random assortment: the shuffling of the 23 pairs of chromosomes in the sperm and egg; chance determines which member of the pair goes into the new sperm and egg

Crossing over: the process by which sections of DNA switch from one chromosome to another during meiosis, further increasing genetic variability

Relation 2: Child’s Genotype and Phenotype

Although every cell in your body contains copies of all the genes you received from your parents, only some of those genes are expressed.

Gene Expression: Developmental Changes

Regulator genes largely control the continuous switching on and off of genes that underlie development across the lifespan.

A given gene influences development and behavior only when it is turned on.

Gene Expression

About a third of human genes have two or more different forms, known as alleles.

The dominant allele is the form of the gene that is expressed if present.

The recessive allele is not expressed if a dominant allele is present.

A person who inherits two of the same alleles for a trait is described as homozygous.

A person who inherits two different alleles for a trait is described as heterozygous.

Mendelian Inheritance Patterns

Polygenic Inheritance

When traits are governed by more than one gene

Applies to most traits and behaviors of interest to behavioral scientists

Relation 3: Child’s Environment and Phenotype

As the model indicates, the child’s observable characteristics result from the interaction of environmental factors and the child’s genetic makeup.

Norm of Reaction

Refers to all the phenotypes that could theoretically result from a given genotype, in relation to all the environments in which it could survive and develop

PKU

Children with phenylketonuria (PKU)—a disorder that is related to a defective gene on chromosome 12—are unable to metabolize phenylalanine.

With early diagnosis and a properly restricted diet, however, mental retardation resulting from PKU can be avoided.

Genetic Transmission of Diseases and Disorders

Over 5,000 human diseases and disorders are presently known to have genetic origins.

Recessive gene: PKU, sickle-cell anemia, Tay-Sachs disease, cystic fibrosis

Single dominant gene: Huntington’s disease, neurofibromatosis

Polygenic inheritance: cancer, heart disease, asthma, psychiatric disorders, behavior disorders

Genetic Transmission of Diseases and Disorders

Sex-Linked inheritance: male-pattern baldness, red-green color blindness, hemophilia, Duchenne muscular distrophy, fragile-X syndrome

Chromosomal anomalies: Down syndrome (trisomy 21), Kleinfelter syndrome (XXY), Turner syndrome (XO)

Genetic Transmission of Diseases and Disorders

Regulator gene defects: genetic male with female genitalia

Unidentified genetic basis: autism spectrum disorder (ASD)

The Case of MAOA Young men who had

experienced severe maltreatment were in general more likely to engage in antisocial behavior than those who had experienced none.

However, the effect was much stronger for those individuals who had a relatively inactive MAOA gene.

Parental Contributions to the Child’s Environment

A highly salient and important part of a child’s environment is the parents’ relationship with the child.

Relation 4: Child’s Phenotype and Environment

Children are active creators of the environment in which they live.

By virtue of their nature and behavior, they evoke certain kinds of responses from others.

They also actively select surroundings and experiences that support their interests, talents, and personality characteristics.

B. Behavior Genetics

The science concerned with how variation in behavior and development results from the combination of genetic and environmental factors

Question: Why are people different from one another?

Answer: Behavioral traits are heritable—influenced by hereditary factors

B. Behavior Genetics

Behavioral geneticists believe that most traits of interest are multifactorial. They are affected by many environmental factors as

well as by many genes.

1. Behavior Genetics Research Designs

The family study is the mainstay of modern behavior-genetics research.

Measure trait of interest among people who vary in genetic relatedness

Correlations between the measure of the trait in individuals with different relationships are examined to see if they are higher for individuals who: Are genetically more

similar Share the same

environment

Types of Family Studies Twin-Study Designs:

Correlations for pairs of monozygotic twins on a trait of interest are compared to those of dizygotic twins.

Adoption Studies: Researchers examine whether adopted children are more like their biological or their adopted relatives.

Family Studies of Intelligence

Identical Twins Reared Apart

Studied twin siblings who have not met since they were infants The team of investigators were struck by

the similarities they found in traits like IQ, reaction to stress, and traditionalism.

These similarities may be influenced by selective placement and similarities in fostering environments as well as by genetic factors.

2. Heritability

A statistical estimate of the proportion of the measured variance on a given trait among individuals in a given population that is attributable to genetic differences among those individuals.

Limitations: They apply only to populations, not to individuals They apply only to a particular group living at a

particular time They can differ markedly for groups of people who

grow up in very different environments High heritability does not imply immutability They say nothing about differences between groups

3. Environmental Effects

Most obvious source of shared environment is growing up together in the same family. Behavioral geneticists, however, have found surprisingly little

effect of shared environment on some aspects of development. Nonshared environment effects include experiences unique

to the individual. Siblings may have quite different

experiences within the same family and their experiences outside the family may diverge sharply.

The primary effect of nonshared environmental factors is to increase the differences among family members.

II. Brain Development

A. Structures of the Brain

B. Developmental Processes

C. The Importance of Experience

D. Brain Damage and Recovery

A. Structures of the Brain

Neurons are specialized cells that are the basic units of the brain’s information system. Cell body: contains the basic biological

material that keeps the neuron functioning Dendrites: receives input from other cells

and conducts it toward the cell body Axon: conducts electrical signals to

connections with other neurons These connections are called synapses

The Neuron

2. Glial Cells

Glial cells are the brain’s white matter and outnumber neurons 10 to 1.

Cells in the brain that provide a variety of critical supportive functions For example, glial cells form a myelin sheath around certain

axons, providing insulation that increases the speed and efficiency of information transmission.

Play a role in communication within the brain.

3. Cerebral Cortex Lobes are major areas of the cortex that are

associated with different categories of behavior. Occipital lobe: primarily associated with processing visual

information Temporal lobe: involved in memory, visual recognition, and the

processing of emotion and auditory information Parietal lobe: governs spatial processing and integrates sensory

input with information in memory Frontal lobe: organizes behavior and is responsible for planning

Information from multiple sensory systems is processed and integrated in the association areas.

Cerebral Cortex

Cerebral Lateralization

The cortex is divided into two separate halves, called cerebral hemispheres, which communicate through a dense tract of fibers, the corpus callosum.

The two hemispheres are specialized for different modes of processing, a phenomenon referred to as cerebral lateralization.

B. Developmental Processes

Question: How does the structure of the human brain come into being?

Answer: It is a partnership between nature and nurture.

1. Neurogenesis and Neuron Development

Neurogenesis is the proliferation of neurons through cell division, is largely complete by about 18 weeks after conception.

Neurons migrate to their destinations, where they grow and differentiate. Axons elongate. Dendrites form spines that increase their

capacity to form connections with other neurons. In the cortex, the most intense period of growth

and differentiation occurs after birth.

Myelination

A fatty sheath of myelin that forms around some axons in order to speed and increase information-processing abilities.

Mapping the Mind

Techniques used to map the mind and its workings in children: Neuropsychological Approach: examining

effects of brain damage on behavior

Electrophysiological Recording

EEG (electroencephalographic) recordings of electrical activity generated by active neurons

ERPs (event-related potentials) record changes in the brain’s electrical activity in response to the presentation of a particular stimulus.

ERP Responses

These graphs show ERP waveforms in response to novel (red line) and familiar (yellow line) stimuli.

The infants who later recalled how to assemble a toy (left panel) had clearly discriminated between the familiar and novel items on an earlier recognition test.

The infants who did not recall the assembly sequence (right panel) had not discriminated between the components on the earlier test.

Brain Imaging Techniques

fMRI (functional magnetic resonance imaging) uses a powerful magnet to produce colorful images representing cerebral blood flow in different areas of the brain.

PET (positron emission tomography) uses a radioactive material injected into the brain for diagnostic purposes.

2. Synaptogenesis and Synapse Elimination

Synaptogenesis Each neuron forms synapses with thousands of

other neurons, resulting in the formation of trillions of connections.

Synaptic Pruning The extensive generation of neurons and

synapses results in an overabundance that must be eliminated.

Synaptic pruning occurs at different times in different areas of the brain and is not fully completed until adolescence.

Synapse Production and Elimination

The Adolescent Wave

Recent research indicates that the amount of gray matter increases dramatically in adolescence and then begins to decline. This second wave of synapse production

and pruning may be linked to the impulsive, irrational behavior, which is characteristic of adolescence.

Brain Maturation

The bluer the image, the more mature that part of the cortex is (i.e., the gray matter has been replaced with white matter).

C. The Importance of Experience

Plasticity is the capacity of the brain to be affected by experience.

Experience plays a central role in determining which of the brain’s excess synapses will be pruned and which will be maintained.

Synapses that are frequently activated are preserved, a process described as “neural Darwinism.”

1. Experience-Expectant Processes

Experience-expectant plasticity is the process through which the normal wiring of the brain occurs in part as a result of the kinds of general experiences that every human who inhabits any reasonably normal environment will have.

Is accompanied by vulnerability. If the expected experience is not available, as in the case

of congenital cataracts, development will be impaired.

Sensitive Periods

A key element in experience- expectant plasticity is timing.

There are a few sensitive periods when the human brain is particularly sensitive to particular kinds of external stimuli.

Experience-Dependent Processes

Experience-dependent plasticity is the process through which neural connections are created and reorganized throughout life as a function of an individual’s experience.

D. Brain Damage and Recovery

Timing and plasticity play important roles.

The worst time to suffer brain damage is when neurogenesis and neuron migration are occurring (during prenatal development and the first year after birth).

The greatest plasticity is observed when synapse generation and pruning are occurring during early childhood.

Emergent Effects of Early Brain Damage

At 6 years of age, children with congenital brain damage scored the same as normal children.

However, the children with brain damage failed to improve andfell progressively farther behind the normal children, so that by adolescence there were large differences between the two groups.

III. The Body: Physical Growth and Development

A. Growth and Maturation

B. Nutritional Behavior

A. Growth and Maturation

Compared with most other species, humans undergo a prolonged period of physical growth, which occurs during about 20% of the life span.

Growth is uneven across age, occurring most rapidly during the first 2 years of life and early adolescence.

Growth is also uneven over different parts of the body.

Growth Curves

Variability

Secular trends are marked changes in physical development that have occurred over generations, resulting from environmental changes such as improvement in health and nutrition.

One such change that occurs during puberty, especially among American girls, relates to how an individual perceives and feels about her or his physical appearance, or body image.

Failure-to-thrive (FTT)

A condition in which infants become malnourished and fail to grow for no apparent medical reason.

It is associated with disturbances in mother-child interaction that are thought to stem from characteristics of both child and mother.

B. Nutritional Behavior

1. Infant Feeding: Although breast milk is nutritionally superior, the majority of infants in the United States are exclusively or predominantly formula-fed.

2. Development of Food Preferences and Regulation of Eating

While some food preferences are innate, experience has a major influence on children’s food preferences and consumption from infancy onwards.

Children whose parents try to control their eating habits tend to be worse at regulating their food intake themselves than children of parents who let their children have more control over their eating.

3. Obesity

The proportion of U.S. children who are overweight has tripled in the past two decades.

Both genetic and environmental factors contribute to this increase.

Overweight children and teens suffer a variety of social problems.

Overweight—a growing problem

“Fat runs in families”

The overweight children and adults in this painting are all genetically related, and they are all overeating.

4. Undernutrition

Forty percent of the world’s children under age 5 are undernourished.

Undernutrition and malnutrition are almost always associated with poverty.

Malnutrition affects development directly and indirectly by leading children to withdraw from their environments to reduce energy expenditure.

Malnutrition and Cognitive Development