IQ and Genetics | Does Genes Shape Intelligence?
Abstract
IQ (intelligence quotient) is often discussed as if it were an inborn, fixed trait. Contemporary research, however, shows that intelligence is a complex characteristic shaped jointly by genetic and environmental influences. Thousands of genetic variants contribute small effects, while environmental factors within the normal range such as education, health, and living conditions can exert substantial influence. This article synthesizes evidence from twin and adoption studies (heritability research) and molecular genetics (genome-wide association studies, GWAS), and clarifies a common misconception: high heritability within a group does not justify attributing differences between groups to genetic causes.
Is IQ Determined by Genes?
The Scientific Answer: It’s More Complicated Than That
Across medical and genetic science sources, a consistent framework emerges: intelligence is a multifactorial trait, influenced by both genetics and environment. Education, nutrition, healthcare, family context, and learning opportunities all shape cognitive development.
Importantly, genetic research has not identified a single “IQ gene.” Instead, intelligence reflects the combined influence of many genes, each contributing only a very small effect.
Key points:
- Genes matter, but they do not “stamp” IQ in the way genes determine finger number.
- Environmental factors are powerful and measurable.
- Genes and environment interact continuously throughout development.

Genes and brains illustrating intelligence variation
What Is Heritability and Why Is It Often Misunderstood?
What Heritability Actually Measures
In behavioral genetics, heritability (often written as h²) refers to the proportion of variation in a trait within a specific population and environment that is statistically associated with genetic differences between individuals.
Heritability does not mean:
- “60% of a person’s IQ comes from genes.”
- “IQ cannot change.”
- “Environment is unimportant when heritability is high.”
These are all conceptual errors.
Heritability Changes With Age and Context
Meta-analyses show that heritability of cognitive ability tends to increase from childhood into adulthood. This pattern likely reflects two processes: genetic influences becoming more expressed over time, and environmental variation becoming more uniform in some life stages. Heritability is therefore context-dependent, not a biological constant.
From Twin Studies to GWAS: Intelligence Is Polygenic
What GWAS Tells Us About IQ
Genome-wide association studies examine large populations to identify genetic variants statistically linked to intelligence. Across studies, the findings are consistent:
- Many genetic loci are involved.
- Each variant has a very small effect.
- Aggregated into polygenic scores, these variants can predict part of individual differences but they do not determine outcomes.
Large-scale GWAS have identified numerous loci associated with intelligence, reinforcing the polygenic model.
From Genes to Brains to Cognition
Recent reviews emphasize that while genetic variation correlates with differences in intelligence and some brain-related markers, the biological mechanisms remain incompletely understood. Moving from genetic correlation to causal explanation requires caution.
How Important Is Environment and Why This Doesn’t Contradict Heritability
Even when heritability within a group is relatively high, environmental factors can still produce large differences in IQ scores and cognitive skills. Modern intelligence genetics consistently highlights the influence of: education quality and duration, childhood nutrition and health, family conditions and cognitive stimulation, chronic stress and deprivation.
In short, genetic influence and environmental influence are not competing explanations; they operate together.
A Persistent Error: From Within-Group Heritability to Between-Group Claims
A common mistake in public debates (notably in controversies surrounding The Bell Curve) is the following reasoning:
“If IQ is highly heritable within Group A, and Group A differs from Group B in average IQ, then the difference between A and B must be genetic.”
This inference is flawed because it confuses two meanings of “genetic”:
- Genetically determined (fixed by genes regardless of environment), and
- Heritable (variation within a group is statistically associated with genetic differences).
Heritability is a population- and context-specific statistic, not a universal explanation of group differences. Philosophical and scientific analyses of heritability have repeatedly emphasized that within-group heritability cannot explain between-group differences.
Guidelines for responsible discussion of IQ and genetics:
- Do not infer innate superiority or inferiority of social or racial groups from heritability estimates.
- Claims about group differences require distinct data, appropriate study designs, and extreme caution in interpretation.
What Should We Conclude?
- IQ has a genetic component, but it follows a polygenic model there is no single “intelligence gene.”
- Environmental factors exert strong effects on cognitive development, especially in childhood.
- Heritability is not destiny, nor is it a percentage of an individual’s intelligence.
- Using heritability within a group to explain differences between groups is a logical and scientific error.
Understanding these distinctions allows for more accurate science communication and helps prevent the misuse of genetics in social and political arguments.
Reference:
- Plomin R, von Stumm S. (2018). The new genetics of intelligence. Nature Reviews Genetics.
- Deary IJ, Cox SR, Hill WD. (2022). Genetic variation, brain, and intelligence differences. Molecular Psychiatry.
- Sniekers S, et al. (2017). GWAS meta-analysis… identifies loci influencing human intelligence. Nature Genetics.
- Haworth CMA, et al. (2010). Increasing heritability of general cognitive ability from childhood to young adulthood.
- Block N. (1995). How heritability misleads about race. Cognition.