Intelligence: The Influence of Heredity and Environment

Heritability estimates don’t reveal anything about the extent to which genes influence a single person’s traits.
Heritability depends on how similar the environment is for a group of people.
Even with high heritability, a trait can still be influenced by environment.

Environmental Influences

Evidence for environmental influences on intelligence comes from the following observations:

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 environment.

Hereditary Explanations

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 cultural differences.

Environmental Explanations

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:

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