estimates don’t reveal anything about the extent to which genes
influence a single person’s traits.
depends on how similar the environment is for a group of people.
- Even with high
heritability, a trait can still be influenced by
Evidence for environmental influences on intelligence comes from the
- Adoption studies demonstrate that adopted children show some
similarity in IQ to their adoptive parents.
- Adoption studies also show that siblings reared together are more
similar in IQ than siblings reared apart. This is true even when identical
twins reared together are compared to identical twins reared apart.
- Biologically unrelated children raised together in the same home have
some similarity in IQ.
- IQ declines over time in children raised in deprived environments,
such as understaffed orphanages or circumstances of poverty and isolation.
Conversely, IQ improves in children who leave deprived environments and
enter enriched environments.
- People’s performance on IQ tests has improved over time in
industrialized countries. This strange phenomenon, which is known as the Flynn effect, is attributed to environmental influences. It
cannot be due to heredity, because the world’s gene pool could not have
changed in the seventy years or so since IQ testing began.
Cultural and Ethnic Differences
Studies have shown a discrepancy in average IQ scores between whites and
minority groups in the United States. Black, Native American, and Hispanic
people score lower, on average, than white people on standardized IQ tests.
Controversy exists about whether this difference is due to heredity or
A few well-known proponents support hereditary explanations for
cultural and ethnic differences in IQ:
- In the late 1960s, researcher Arthur Jensen created a storm of
controversy by proposing that ethnic differences in intelligence are due
to heredity. He based his argument on his own estimate of about 80
percent heritability for intelligence.
- In the 1990s, researchers Richard Herrnstein and Charles Murray
created a similar controversy with their book, The Bell
Curve. They also suggested that intelligence is largely
inherited and that heredity at least partly contributes to ethnic and
Many researchers believe that environmental factors primarily cause
cultural and ethnic differences. They argue that because of a history of
discrimination, minority groups comprise a disproportionately large part of
the lower social classes, and therefore cultural and ethnic differences in
intelligence are really differences among social classes. People in lower
social classes have a relatively deprived environment. Children may have: