Heritability

Heritability of Intelligence

Heritability is perhaps the most debated and studied concept in the field of intelligence research. It refers to the proportion of the difference in IQ scores between people that can be attributed to genetic factors versus environmental factors.

Crucial Distinction: It is important to note that heritability describes populations, not individuals. A heritability of 50% does not mean that 50% of your intelligence comes from your genes; it means that 50% of the variation in intelligence across a group of people is due to genetic differences.

The Twin Studies

Most of what we know comes from studying twins.

• Identical Twins (Monozygotic): Share 100% of their DNA.
• Fraternal Twins (Dizygotic): Share 50% of their DNA.
• The Finding: Identical twins raised apart (different environments) still have remarkably similar IQs as adults (correlation ~0.75), much higher than fraternal twins raised together. This strongly suggests a genetic component.

The Wilson Effect (Age Matters)

Modern consensus suggests that intelligence is highly heritable, but the degree changes throughout life. This is the Wilson Effect.

• Childhood (20-40% Heritable): In young children, the home environment plays a massive role. Parents control the books, the food, and the stimulation.
• Adulthood (60-80% Heritable): As we age, our genetic predispositions assert themselves. We gain independence and select environments that match our genetic propensities (e.g., a bookish child goes to the library). By adulthood, genes are the single biggest predictor of IQ.

The Paradox: Heritability vs. Change (The Flynn Effect)

If IQ is 80% genetic, how can generational IQ scores rise (The Flynn Effect)?

• The Answer: Heritability explains differences between individuals within a group, not the average level of the group.
• Analogy: If you plant corn in bad soil, genetics determines which stalk is tallest. If you add fertilizer (better nutrition/education) to the whole field, all the corn grows taller, but the genetic differences between the stalks remain.

From Twin Studies to GWAS: The Molecular Evidence

For most of the 20th century, heritability estimates came almost exclusively from behavioral genetic methods — comparing identical twins to fraternal twins, or comparing adopted children to their biological and adoptive parents. These studies converged on consistent estimates: heritability of intelligence is roughly 50% in childhood and rises to 70–80% in adulthood.

In the 21st century, molecular genetics added a new layer of confirmation through Genome-Wide Association Studies (GWAS). These studies scan hundreds of thousands of genetic variants (SNPs) across the genomes of large populations and identify which variants are statistically associated with higher or lower test scores.

Key findings from GWAS on intelligence:

• Polygenic architecture: Intelligence is not controlled by a small number of “smart genes.” Instead, tens of thousands of common genetic variants each contribute tiny effects, collectively explaining a substantial portion of the variance in IQ scores.
• Polygenic scores: By combining all these small genetic signals into a single “polygenic score,” researchers can now explain about 10–15% of the variance in educational attainment and general cognitive ability from DNA alone.
• Biological pathways: The genes associated with intelligence are disproportionately active in neurons and involved in synaptic function, myelination, and brain development — confirming that the heritability of IQ reflects real biological differences in brain structure and function.

Gene-Environment Interaction: The Hidden Complexity

Gene-Environment Correlation (rGE): Genes don’t just affect traits directly — they also influence the environments people seek out and create for themselves. A child with genes predisposing to high intelligence is more likely to ask more questions, choose to read more books, attend better schools, and seek cognitively challenging careers. This means that what appears to be an “environmental” effect (e.g., the influence of reading books) is partly a consequence of genetic predispositions.

Gene-Environment Interaction (GxE): Some genetic predispositions only manifest under certain environmental conditions. The heritability of intelligence is higher in high-SES families than in low-SES families. When environment is adequate, genetics determines how far above the baseline you go. When environment is deficient, it caps development regardless of genetic potential.

Shared vs. Non-Shared Environment

Behavioral geneticists partition environmental influences into two categories:

• Shared environment (c²): Family-level factors that siblings share — same home, same parents, same neighborhood. Surprisingly, shared environment has little lasting effect on adult IQ. Siblings raised together end up no more similar in adult IQ than siblings raised apart.
• Non-shared environment (e²): Experiences unique to each individual — different teachers, different peer groups, different illnesses. These have a modest but real effect on IQ.

The implication is striking: being raised in the same household by the same parents has much less impact on adult intelligence than researchers originally assumed. What matters far more is the individual’s own genetic predispositions and their unique personal experiences.

What Heritability Does Not Mean

A high heritability estimate is routinely misinterpreted:

1. High heritability does not mean unmodifiable. Height is highly heritable (>80%) but has increased dramatically across generations due to improved nutrition. The same logic applies to IQ.
2. Heritability within groups says nothing about differences between groups. The fact that IQ is highly heritable within a population tells us nothing about why two different populations differ in average scores — those differences could be entirely environmental.
3. Heritability is not destiny. It describes what has happened in a given population under a given range of environments. It does not predict what would happen under radically different environmental conditions.

Conclusion

While environment matters — especially in cases of extreme deprivation (lead poisoning, malnutrition) — the scientific literature overwhelmingly supports the conclusion that general intelligence has a strong biological and genetic basis. Intelligence is polygenic, influenced by thousands of tiny genetic variations. But genes and environment are not in opposition: they interact, they correlate, and they co-create the intelligence we observe. The most accurate summary is not “IQ is genetic” or “IQ is environmental” — it is that genetic predispositions and environmental circumstances interact in complex, age-varying ways to produce the cognitive capacity each person expresses.