Download Behavior Genetics and Evolutionary Psychology Nature vs. Nurture

Modules 5:
Behavior Genetics and
Evolutionary Psychology
Source of similarities and differences
• Similarities with other people such as
developing a languag, showing similar
emotions, following similar patterns in life
• Individual differences such as ability to learn
math, response to a stressful situation
• Sources of variability in mental processes and
behaviors:
Nature (genes, biology, heredity....)
or
Nurture (experience & learning)
Nature vs. Nurture
• Nature:
• “In the actual race of life…the determining factor
is heredity” E.L. Thorndike (1903)
• Nurture:
• “Give me a dozen healthy infants and I
guarantee to take any one at random and train
him to become any type of a specialist I might
select – doctor, lawyer, artist, mechanic, even a
beggar-man and thief regardless of his talents,
penchants, tendencies, abilities, vocations, and
race of his ancestors.” J. B. Watson (1925)
The Nature-Nurture Question
How much of our
differences and similarities
are due to our genes?
How much of our differences
and similarities are due to
our environment?
Behavior Genetics:
Individual differences
Evolutionary Psychology:
Human similarities
Behavior Genetics
Nucleus
Chromosome
Cell
Gene
DNA
Every cell nucleus in the body contains the
genetic code for the entire body.
Genes: Our Biological Blueprint
• Biochemical units of heredity that make up the
chromosomes
• A segment of DNA that contains coded
instructions to create protein molecules
• Transfers genetic characteristics by way of coded
instructions for the structure of proteins.
• We have about 30,000-70,000 genes
• We share about 99.9% of our DNA
• We share about 60% of our genes with a fruit fly
Studying relative effects of nature &
nurture: Heritability
• A statistical estimate of the proportion of the
total variability in some trait that can be
attributed to genetic differences.
• Expressed as proportion (e.g., 0.60 or 60/100):
60 % of variance in trait X can be attributed to genetic
differences.
• Some variables such as height are highly
heritable, other variables such as musical
ability are moderately heritable.
Facts About Heritability
ƒ Heritability estimates do
not apply to individuals,
only to variations within a
group.
ƒ Even highly heritable traits
can be modified by
extreme stimulations in the
environment.
Behavior Genetics approach to
the nature/nurture issue?
• Semi - Experimental approach:
How can we hold genes constant?
Twins
Identical
twins
Fraternal
twins
• Identical Twins
ƒ develop from a single
fertilized egg that splits in
two, creating two
genetically identical
organisms
• Fraternal Twins
Same
sex only
Same or
opposite sex
ƒ develop from separate eggs
ƒ genetically no closer than
brothers and sisters, but
they share the fetal
environment
Holding genes constant –
Adopted Twins
ƒ Identical twins who were separated early in life
and reared apart
ƒ Similarities Æ Genetic influences
Identical vs. fraternal twins
• Varying genes in a
controlled way
• Holding environment
constant
• Allows the estimation
of the effects of
genes vs.
environment
How can we hold environment
constant? Adopted Children
• Compare the correlations between the traits
of adopted children and those of their biological
and adoptive parentsÆ influence of heredity vs.
environment
• Compare the traits of adopted siblings Æ
similarity indicates influence of the environment
Example: IQ
• Greater genetic
similarity Æ greater
IQ similarity
• A substantial
contribution of
genes to IQ
Nature & nurture interaction
• Genes (nature) and environment (nurture) work
together.
• Genes provide the code for certain traits (predispositons)
• They also select / react to the environment
• Indeed genes and environment interact
• Genetically influenced traits evoke significant responses
• Environment triggers gene activity.
• As we grow older, we select environments well suited to
our natures.
Evolutionary Psychology:
What makes humans alike?
• The study of the evolution of behavior
and the mind, using the principles of
evolutionary theory and natural
selection.
• Key question
• What are the similarities in social behavior
across cultures and historical periods
How Natural Selection Works
Environmental pressure (changes
in the environment)
Competition
(for resources)
Selection of fittest phenotype
(among a variety of phenotypes)
Reproductive success (genotype
corresponding to fittest phenotype
passed to next generation)
Frequency of that genotype (and
phenotype) increases (in next
generation)
Evolution results in variations
in genes
• Natural Selection
• Species with features suited to demands of
environment will survive better than those
with less well-adapted features.
• Those well-adapted species will reproduce
more successfully.
• Mutations
• Random or unexplained errors in gene
replication that lead to a change in the
sequence of nucleotides
• If adaptive, mutations will prevail
Evolutionary Psychology:
Goals
• To understand the genetic basis of
psychological traits.
• To trace the evolution of human mind
and brain
• To understand how some psychological
traits may contribute to reproductive
fitness and survival.
Evolutionary Psychology:
Key assumption
• Like physical traits, psychological traits are:
(a) heritable (inherited from our ancestors)
(b) can contribute to the survival and
reproductive success of individuals, and
therefore
(c) can be shaped naturally.
• E.g. avoiding poisonous food helped us
developing an innate dislike for bitter tastes.
Nature’s selection principle
• Nature selects behaviors that increase the
likelihood of sending genes into the future
• Males compete with other males for access to
females, and try to inseminate as many
females as possible.
• Females conceive and carry only a limited
number of pregnancies so they choose fewer
more dominant males with good resources
and high status.
Evolutionary Psychology’s
explanation of mating preferences
• Men preferred attractive physical features
suggesting youth and health and fertility
• Women preferred resources and social status
Critique of Evolutionary
Psychology
• Hindsight approach: Explaining sex
differences after observing what those
differences are.
• Are observed differences justified or
morally right?
• Does evolutionary psychology lead us to
justify male promiscuity?
• Does evolutionary psychology justify gender
inequality?