Intelligence

Today, researchers generally agree that heredity and environment have an interactive influence on intelligence. Many researchers believe that there is a reaction range to IQ, which refers to the limits placed on IQ by heredity. Heredity places an upper and lower limit on the IQ that can be attained by a given person. The environment determines where within these limits the person’s IQ will lie.

Despite the prevailing view that both heredity and environment influence intelligence, researchers still have different opinions about how much each contributes and how they interact.

Hereditary Influences

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

• Family studies show that intelligence tends to run in families.
• Twin studies show a higher correlation between identical twins in IQ than between fraternal twins. This holds true even when identical twins reared apart are compared to fraternal twins reared together.
• Adoption studies show that adopted children somewhat resemble their biological parents in intelligence.

Family studies, twin studies, and adoption studies, however, are not without problems. See pages 36-–38 for more information about the drawbacks of such studies.

Heritability of Intelligence

Heritability is a mathematical estimate that indicates how much of a trait’s variation in a population can be attributed to genes. Estimates of the heritability of intelligence vary, depending on the methods used. Most researchers believe that heritability of intelligence is between 60 percent and 80 percent.

Heritability estimates apply only to groups on which the estimates are based. So far, heritability estimates have been based mostly on studies using white, middle-class subjects. Even if heritability of IQ is high, heredity does not necessarily account for differences between groups. Three important factors limit heritability estimates:

1. Heritability estimates don’t reveal anything about the extent to which genes influence a single person’s traits.
2. Heritability depends on how similar the environment is for a group of people.
3. 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:

• Fewer learning resources
• Less privacy for study
• Less parental assistance
• Poorer role models
• Lower-quality schools
• Less motivation to excel intellectually

Some researchers argue that IQ tests are biased against minority groups and thus cause the apparent cultural and ethnic differences.

However, not all minority groups score lower than whites on IQ tests. Asian Americans achieve a slightly higher IQ score, on average, than whites, and they also show better school performance. Researchers suggest that this difference is due to Asian American cultural values that encourage educational achievement.