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:

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