What Explains Differences In
Intelligence?
Thesis
• A large portion of differences in IQ scores can
be explained by environmental differences
even though there is a substantial genetic
component
About IQ
• IQ scores are not the same thing as
intelligence
• They are estimates of intelligence based on a
particular way of defining it and a particular
standardized test
How is Intelligence Measured?
• Standardized Intelligence Tests
– WAIS
– Stanford-Binet
• Examples of subtests
– Digit span
– Picture arrangement
– Information
A Paradox About IQ
• IQ scores keep going up
– Flynn Effect
– Average IQ in U.S. increased about 22 points
between 1932 and 2002
• Behavioral genetics research shows that IQ is
highly heritable
• The increases in IQ happened too quickly to
be explained by genetic changes
Another Paradox
• Mean IQ scores for African Americans are
about 10-15 points lower than for European
Americans
• The environments would have to be
extraordinarily dissimilar to account for the
difference
• The difference is decreasing more rapidly than
could be explained by genetic changes
Genetic Influence
• Twin studies
• Identical twins who are adopted and reared
apart share all of their genes but not their
environments
• Fraternal twins or siblings who are adopted
and reared about share only half of their
genes
Is Intelligence Genetic?
• The genetic contribution depends on
differences in environments
• For individuals in similar environments, much
of the variability is explained by genetics
Resolving the Paradox
• In the real world, genetic and environmental
influences are not separate
• Genetic influences can cause environmental
differences, which then cause differences in
the trait (e.g., IQ)
• These influences are counted as genetic but
they are also environmental
Social Multipliers
• Social changes can cause rapid changes in a
trait
• These changes (e.g., increases in IQ) create a
need for individuals to keep up
• Changes in environment that increase average
IQ lead to even more increases in IQ
• Differences between individuals are still
explained largely by genetics
Examples of Social Multipliers
• Smaller families, leading to more time to talk
with children about abstract issues
• More leisure time, leading to more mentally
challenging leisure activities
• Technology, leading to less emphasis on rote
learning and more emphasis on problem
solving
What About the Race Issue?
• Reliable group differences in IQ scores
• Greater variability within groups than
between groups
• These group differences can’t be accounted
for by test bias
What About the Race Issue?
• Racial groups are defined sociologically, not
biologically
• Twin studies show that individual differences
in IQ are heritable, not that group differences
are heritable
What About the Race Issue?
• Recall that the changes in group IQ differences
can’t be explained by genetic changes
• Could differences in social multipliers account
for the group differences in IQ?
A Few Cautions About IQ
• An IQ score is an estimate
• Intelligence is defined culturally
• IQ scores are predictive of measures of
success, but this may be a self-fulfilling
prophecy