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:
