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.
Evidence for hereditary influences on intelligence comes from the following observations:
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 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:
Evidence for environmental influences on intelligence comes from the following observations:
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.
A few well-known proponents support hereditary explanations for cultural and ethnic differences in IQ:
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:
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.