The Biology of Behavior
PowerPoint®
Presentation
by Jim Foley
© 2013 Worth
Publishers
Module 5: Genetics, Evolutionary
Psychology, and Behavior
Topics we were born to learn about
Behavior Genetics and Individual Differences
 Genes: Molecules that code for life
 Learning about heredity from Twin and
Adoption Studies
 Gene-Environment Interaction
Evolutionary Psychology: Adaptive Success
 Artificial and Natural Selection
 Critiques of Evolutionary Thinking
Behavior Genetics:
Predicting Individual Differences
More ways of exploring the
origins of the biology of
behavior:
1. Understanding genes
2. Twin and adoption
studies
3. Gene/environment
interactions
4. Evolutionary
Psychology
Behavior geneticists
study how heredity
and environment
contribute to
human differences.
Let’s start by looking
at GENES.
GENES:
The Building Blocks of
Heredity and
Development
Genes are
parts of DNA
molecules,
which are
found in
chromosomes
in the nuclei of
cells.
DNA
(Deoxyribonucleic Acid)
Chromosomes are made of DNA,
which are made of genes.
Chromosome:
threadlike structure
made largely of DNA
molecules
DNA:
a spiraling, complex
molecule containing
genes
}
Chromosomes and Inheritance
 The human genome includes 46
chromosomes in 23 sets
matched sets; each
chromosome has the same
gene locations.
 This includes the X and Y
chromosomes, not a matched
set in males, who are missing
some genes on the Y.
 A biological parent donates half
his/her set of chromosomes to
his/her offspring.
 We received half a set of
chromosomes from each
biological parent.
The Human Genome:
20,000 to 25,000 Genes
 Human genomes are so nearly
identical that we can speak of
one universal human genome.
 Yet tiny genetic differences
make a difference. If there is a:
 .001 percent difference in
genome, your DNA would
not match the crime
scene/you are not the
baby’s father.
 0.5 to 4 percent difference
in genome, you may be a
chimpanzee.
 50 percent difference in
genome, you may be a
banana.
The genome: an
organism’s entire
collection of genes
How Genes Work
 Genes are not blueprints;
they are molecules.
 These molecules have
the ability to direct the
assembly of proteins that
build the body.
 This genetic protein
assembly can be turned
on and off by the
environment, or by other
genes.
 Any trait we see is a
result of the complex
interactions of many
genes and countless
other molecules.
Next step for behavior geneticists:
Controlling Variables
Can we design an
experiment to keep genes
constant and vary the
environment and see what
happens?
Or vary the genes in the
same environment?
Fraternal and Identical Twins
Fraternal “twins” from separate eggs
are not any more genetically alike than
other siblings.
Identical twin: Same sex only
Fraternal twin: Same or opposite sex
Twin and
Adoption Studies
To assess the
impact of nature
and nurture, how
do we examine
how genes make a
difference within
the same
environment?
 study traits of
siblings vs.
identical twins
 see if the
siblings vary
more than
twins
Identical vs. Fraternal Twins
Studies of twins in adulthood show
that identical twins are more alike than
fraternal twins in:
 personality traits such as
extraversion (sociability) and
neuroticism (emotional instability).
 behaviors/outcomes such as the
rate of divorce.
 abilities such as overall Intelligence
test scores.
Twin and
Adoption Studies
How do we find
out how the same
genes express
themselves in
different
environments?
We can study the
traits of identical
twins as they grow
up, or if they were
raised separately
(e.g., the
Minnesota Twin
Family Study).
Studies of Identical Twins Raised Apart
Similarities found in
identical twins despite
being raised in different
homes:
 personality, styles of
thinking and relating
 abilities/intelligence
test scores
 attitudes
 interests, tastes
 specific fears
 brain waves, heart
rate
Critiques of Twin Studies
1. In the more recent years of the
Minnesota Twin Family Study, twins
have known about each other and
may influence each other to be
more similar.
2. Coincidences happen; some
randomly chosen pairs of people will
have similar traits.
3. Environments may be similar;
adoptive families tend to be more
similar than randomly selected
families in education, income, and
values.
BUT none of these factors explains,
better than the genetic explanation,
why fraternal twins have more
differences than identical twins.
Searching for Parenting Effects:
Biological vs. Adoptive Relatives
Studies have been performed with adopted children for
whom the biological relatives are known.
Findings: Adopted children seem to be more similar to
their genetic relatives than their environmental/nurture
relatives.
Given the evidence of genetic impact
on how a person turns out,
does parenting/nurture
make any difference?
Does the home environment have any
impact?
Parenting Does Matter
Despite the strong impact of
genetics on personality,
parenting has an influence on:
 religious beliefs
 values
 manners
 attitudes
 politics
 habits
How does the interaction of genes
and environment work?
 Gene-Environment
Interaction: genes turn
each other on and off in
response to
environmental
conditions
Example in
animals:
shortened daylight
triggers animals to
change fur color or
to hibernate
 Epigenetics: The study
of how this happens:
The environment acts
on the surface of genes
to alter their activity
Example in
humans:
obesity in adults
can turn off weight
regulation genes in
offspring
Evolutionary Psychology:
Understanding Human Nature
Some topics:
 Natural selection and
adaptation
 Evolutionary success may help
explain similarities
Evolutionary
psychology is the
study of how
evolutionary
principles help explain
the origin and
function of the human
mind, traits, and
behaviors.
We have been talking so far about
human differences; let’s now
seek insight into the ways in
which humans are alike.
17
Evolutionary Psychology:
Natural Selection: How it Works
Begin with a
species’ genome,
which contains a
variety of
versions of genes
that shape traits.
Conditions make it
difficult for
individuals with
some traits (some
versions of those
genes) to survive
long enough to
reproduce.
Other individuals
thus have their
traits and genes
“selected” to
spread in the
population.
Artificial Selection
The Domesticated Silver Foxes
 Dmitri Balyaev and
Lyudmila Trut spent
40 years selecting
the most gentle,
friendly, and tame
foxes from a fox
population, and
having those
reproduce.
 As a result, they
were able to shape
avoidant and
aggressive creatures
into social ones, just
as wolves were once
shaped into dogs.
19
How might evolution have shaped
the human species?
Example:
 Why does “stranger anxiety”
develop between the ages of 9
and 13 months?
Hint: in evolutionary/survival
terms, humans are learning to
walk at that time.
Infants who used their new
ability to walk by walking away
from family and toward a lion
might not have survived to
reproduce as well as those who
decided to stay with parents
around the time they learned
to walk.
20
Evolutionary Psychology’s Explanation of
Biologically Driven Phobias
 Why do people so easily
acquire a phobia of
snakes, more easily than a
phobia of cars?
 An evolutionary
psychologist would note
that snakes are often
poisonous…
…so, those who more
readily learned to fear
them were more likely to
survive and reproduce.
21
Critiquing Evolutionary Psychology
“You’re just
taking current
reality and
constructing a
way you could
have predicted
it.” This is
hindsight
reasoning and
unscientific.
Response: yes, but there
are predictions made
about future behavior
using this reasoning.
“You’re
attributing too
much to genes
rather than the
human ability to
make choices
about social
behavior.”
Response: yes, but our
evolutionary past does not
prevent our ability to act
differently; “is” does not
equal “ought.”
Evolution: Theory
 Evolution is a scientific theory
(NOT a “guess” and not a
hypothesis, but something
more): a coherent set of
principles that fits very well
with the accumulated evidence.
 Parts of the evolutionary story
may conflict with other stories
of origins and change over
time.
 Is there room for overlap and
agreement?
Possible areas of
consensus, with or
without evolution:
 The human mind
and body seems
almost “designed,”
by evolution or
other forces, to
have certain traits
and abilities.
 Nurture may shape
us, but we seem to
start out with some
sort of human
nature.