Genetics vs. Environment
Their Role in Intelligence
Nature vs. Nurture in IQ
• “Is a person’s intelligence due more to genes
or to environment?”
– Both genes & intelligence crucial for any
trait
– Environmental factors determines which
genes get switched on.
– Example of nutrition effecting you reaching
your genetic potential in height.
Heredity and Environment
• Heritability
– The variation within a group caused by genetic
factors
– Thought to be about 50%
• Environment
– Degree to which variation is due to environmental
rather than genetic differences
– Environmental Factors can be early intervention,
schooling and family life
Twin Studies & Family Influence
Thomas Bouchard & the Minnesota
Twins studies
• If trait genetic:
– closely related more similar than less closely related
• Many close relatives share environments too
• Types of studies to separate effects
–
–
–
–
–
monozygotic twins reared together
monozygotic twins reared apart
siblings/dizygotic reared together
siblings/dizygotic reared apart
adoptive siblings reared together
Twin Studies
Data Analysis
• The data show that both genetics and environment have
an effect on IQ scores.
• The more closely two individuals are related genetically,
the more similar their IQ scores:
– Identical twins reared together are more alike than are fraternal
twins reared together.
• Also show the importance of environmental influences:
– Identical twins reared together are more alike than are
identical twins reared apart
– siblings who are reared together are more alike than are
siblings reared in different homes.
Both genetics and environment have an effect
on IQ scores
Adoption Studies
Children's verbal ability scores became modestly more
like their biological parents' scores.
(Adapted from Plomin & DeFries, 1998.)
Group Differences in
Intelligence Test
Scores
Group Differences in Testing
• A number of studies show scoring
differences between different racial,
ethnic, and gender groups.
• Are these differences due to nature or to
nurture?
– Studies suggest environment is playing
a heavy role.
• Heredity and environment interact to
produce intelligence in individuals.
Why do Japanese and Chinese students
perform better than Americans on
intelligence and achievement tests?
• They score the same before school but
once school starts they begin to show
more gains. This due to:
– Spending more time in school
– More time doing homework
– Pressure and support from parents to
achieve academically
Group Differences
Australian aborigine & white scores on IQ tests
• Play “Intelligence and Culture” (4:02)
Module #4 from The Brain: Teaching
Modules (2nd edition).
• Are the differences in scores the result
of environment or genes?
• Which theory of intelligence does this
example prove? Gardner’s Multiple
Intelligences or Spearman’s g factor?
Racial Difference in IQ
• Racial difference in average IQ among different
racial groups can be measured
• More variation in IQ scores within a particular
group than between groups
• Stereotype Threat – When members of a particular
group fear they are being thought of or evaluated
according to a negative stereotype, they fear that
they may confirm the stereotype leading to selfdoubt and anxiety resulting in poor performance.
(see Box 7.4 on pg. 311)
Effects of Discrimination on
Intelligence
• Burakumin people of Japan are
severely discriminated against.
• Despite being genetically similar
to Japanese people…
• Their IQ scores are 10 to 15
points lower than “normal”
Japanese people.
Seed Analogy: Argument for Environment
Within and Between
Group Differences
• Each corn field planted from same package of
genetically diverse seeds
• One field is quite fertile, the other is not
• Within each field, differences due to genetics
• Between each field, differences due to environment
(fertility)
• Unless the environments of two racial groups are
totally identical, it is impossible to estimate the overall
genetic differences between the two groups.
Are IQ Tests Culturally Biased?
• If IQ test reflects white, middle-class cultural
knowledge and values it could result in poor scores by
minorities.
• Test will favor the people from the culture in which it
was developed.
• Factors like test-taking behavior, motivation, attitudes
towards the test and previous experiences with tests can
affect performance.
• Try some culturally biased tests yourself:
– Robert Williams IQ Test & the Dove Counterbalance Test
– Chitling Intelligence Test