Psychology · Ch 1 — Variations in Psychological Attributes
Intelligence: Interplay of Nature and Nurture
Intelligence: Interplay of Nature and Nurture
The Nature-Nurture Debate in Intelligence
The question of whether intelligence is primarily inherited or shaped by experience is one of the oldest in psychology. The evidence, drawn from twin studies and adoption research, shows that both heredity and environment matter — but not equally at all times, and not in a simple additive way.
Evidence from Twin Studies
Twin studies provide the strongest evidence for hereditary influence because they allow researchers to compare people who share the same genes (identical twins) with those who share only half their genes (fraternal twins), while controlling for environment.
The key findings are:
- Identical twins reared together show an intelligence correlation of almost 0.90 — the highest possible, indicating that when both genes and environment are shared, intelligence is extremely similar.
- Identical twins reared apart (separated early in childhood) still show a correlation of 0.72 — remarkably high, suggesting that genetic makeup exerts a powerful influence even when environments differ.
- Fraternal twins reared together correlate at about 0.60 — lower than identical twins, which points to the role of genetic similarity.
- Siblings (brothers and sisters) reared together correlate at about 0.50.
- Siblings reared apart correlate at about 0.25 — the lowest among biological relatives, showing that a shared environment also contributes.
The drop from 0.90 (identical twins together) to 0.72 (identical twins apart) is the contribution of a shared environment. The drop from 0.72 (identical twins apart) to 0.60 (fraternal twins together) is the contribution of genetic similarity. Both factors matter.
Evidence from Adoption Studies
Adoption studies separate the effects of biology and upbringing. The key finding is that children's intelligence is more similar to their biological parents than to their adoptive parents. This again points to a strong hereditary component.
However, environment also shows its effect in adoption research:
- As adopted children grow older, their intelligence scores tend to move closer to those of their adoptive parents — the environment they actually live in begins to shape their cognitive development.
- Children from disadvantaged homes who are adopted into families with higher socio-economic status show a large increase in their intelligence scores. This is direct evidence that a supportive environment can raise intelligence.
Environmental Factors That Influence Intelligence
Beyond adoption, research identifies several environmental conditions that affect intelligence:
- Environmental deprivation lowers intelligence.
- Rich nutrition supports cognitive development.
- Good family background (including parental education, stimulation, and emotional support) raises intelligence.
- Quality schooling consistently improves intelligence scores.
The Consensus: Interaction, Not Either-Or
Psychologists today agree that intelligence is not the product of nature alone or nurture alone. It is the result of a complex interaction between heredity and environment. …