Nature, Nurture, and Intelligence: A Psychological Debate

Identical vs. Fraternal Twins

To determine if intelligence is primarily a genetic trait, scientists compare the correlation of intelligence scores between two types of twins:

1. Identical Twins: These twins come from the same egg and share 100% of their genetic material.
2. Fraternal Twins: These twins come from different eggs and share, on average, only about 50% of their genetic material—the same as regular siblings.

If intelligence is heavily influenced by genetics, we would expect identical twins to have much more similar intelligence scores than fraternal twins, because their genetic "blueprints" are nearly identical.

The data supports this hypothesis. When reared together, the correlation for identical twins is approximately $0.86$, whereas the correlation for fraternal twins is significantly lower at $0.60$.

The Role of Environment: Adoptive Children Studies

While twin studies provide strong evidence for the role of heredity, we must also account for the environment. If a child's intelligence is primarily shaped by their upbringing, then their performance on intelligence tests should more closely resemble that of their caregivers.

Adoptive children studies allow researchers to separate biological parents from environmental influences. In these studies, researchers compare the intelligence of an adopted child to their biological (natural) parents and their adoptive parents.

• The Heredity Hypothesis: If genetics are the dominant factor, the child's intelligence should correlate more strongly with their natural mother.
• The Environment Hypothesis: If the upbringing is the dominant factor, the child's intelligence should correlate more strongly with their adoptive mother.

The empirical evidence shows:

• Correlation with natural mother: $0.28$
• Correlation with adoptive mother: $0.17$

Because the correlation is higher with the natural mother, these findings suggest that heredity plays a significant role in determining intelligence-test performance.

Summary of Findings

The evidence from both twin studies and adoptive studies suggests that intelligence is not a product of one single factor, but rather a complex interaction. However, the data clearly indicates that genetics (nature) provides a powerful foundation for intelligence, while environment (nurture) continues to play a measurable role in how that potential is expressed.