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How much of intelligence is hereditary?
Based on the provided sources, intelligence is a complex trait influenced by both genetic and environmental factors, with a significant hereditary component that changes over an individual’s lifespan [1, 2]. Scientific consensus, based on decades of research, places the heritability of intelligence at approximately 50% in adulthood, meaning that about half of the variation in intelligence scores within a population can be attributed to genetic differences among individuals [1, 2, 5].
Here is a more detailed breakdown of the key findings:
1. Heritability Estimates from Twin and Adoption Studies
The foundational evidence for the heritability of intelligence comes from classical behavioral genetics, primarily twin and adoption studies. These studies consistently show that identical twins, who share 100% of their genes, are significantly more similar in intelligence than fraternal twins, who share on average 50% of their genes [1].
A crucial finding is that the heritability of intelligence is not static; it increases with age. A large meta-analysis of longitudinal twin and adoption studies found that the heritability of cognitive ability is:
- Approximately 20% in infancy
- About 40% in childhood
- Around 60% in adulthood [4]
This increase is thought to be a result of “gene-environment correlation,” where individuals increasingly select, modify, and create environments that align with their genetic predispositions as they gain autonomy from their family environment [2, 4].
2. The Molecular Genetics of Intelligence
Modern research using genome-wide association studies (GWAS) has moved beyond estimating overall heritability to identifying specific genetic variants (single-nucleotide polymorphisms or SNPs) associated with intelligence. This research has established that:
- Intelligence is highly polygenic: There is no single “intelligence gene.” Instead, thousands of genetic variants, each with a very small effect, contribute to individual differences in intelligence [2, 5, 7].
- Specific genes have been identified: Large-scale GWAS, such as a 2018 meta-analysis involving nearly 270,000 individuals, have successfully identified hundreds of genetic loci and specific genes associated with intelligence. Many of these genes are highly expressed in brain tissue and are involved in neuronal development and function, such as synapse formation and axon guidance [3, 5].
3. The “Missing Heritability” and Predictive Power
While twin studies estimate heritability at 50% or more, the specific genetic variants identified so far through GWAS can only account for a fraction of this variance. For instance, the large 2018 study could account for about 7% of the variance in intelligence [3]. This gap is known as the “missing heritability” problem, though researchers are continually identifying more variants as study sample sizes increase [2, 7].
From these identified variants, researchers can create “polygenic scores” (PGS) to predict a person’s cognitive ability from their DNA. Current findings show:
- Polygenic scores can predict between 10-15% of the variance in intelligence and educational attainment, a figure that is statistically significant and practically meaningful [6, 8].
- Within-family studies show that even among siblings raised in the same home, the sibling who inherits more of the intelligence-associated genetic variants tends to have higher cognitive scores, providing strong evidence for a direct causal role of these genes, independent of the shared family environment [8].
Conclusion
Intelligence is one of the most heritable psychological traits studied. Twin studies suggest that genetic factors account for about 50% of the population variance in adulthood, a figure that increases throughout development [1, 4]. While modern genetics has confirmed that intelligence is a highly polygenic trait and has begun to identify the specific genes involved, these discoveries currently explain a smaller portion of the heritability estimated by twin studies [3, 7]. It is critical to remember that heritability is a population statistic and does not imply that intelligence is genetically determined at the individual level; environmental factors remain profoundly important throughout life [1, 2].
Sources
- Is Intelligence Hereditary? – Scientific American
- This article provides a general overview, explaining that while intelligence is clearly heritable (often cited around 50%), it is not purely genetic. It emphasizes the complex interplay between genes and environment, noting that heritability is a population measure, not a statement of individual destiny.
- https://www.scientificamerican.com/article/is-intelligence-hereditary/
- The new genetics of intelligence – Nature Reviews Genetics (PMC Open Access version)
- This review summarizes the shift from classical twin studies to modern molecular genetics (GWAS). It establishes that intelligence is highly polygenic and heritable. It also discusses the “missing heritability” problem and the importance of gene-environment correlations. (Note: This is the same paper as source #7).
- https://pmc.ncbi.nlm.nih.gov/articles/PMC5985927/
- Genome-wide association meta-analysis in 269,867 individuals identifies new genetic and functional links to intelligence – Nature Genetics
- This source is a primary research paper detailing a large-scale GWAS. Its authors identified 205 genomic loci and over 1,000 genes associated with intelligence, demonstrating a clear biological and genetic basis for cognitive function. The identified SNPs explained ~7% of the variance in intelligence.
- https://pubmed.ncbi.nlm.nih.gov/29942086/
- Explaining the Increasing Heritability of Cognitive Ability Across Development: A Meta-analysis of Longitudinal Twin and Adoption Studies – Psychological Science
- This meta-analysis demonstrates that the heritability of intelligence increases dramatically with age, from about 20% in infancy to 60% in adulthood. The authors argue this is due to gene-environment correlation, where individuals increasingly shape their own experiences based on their genetic makeup.
- https://pmc.ncbi.nlm.nih.gov/articles/PMC3954471/
- Genetic Variation, Brain, and Intelligence Differences – Molecular Psychiatry
- This review connects the genetic findings from GWAS to tangible biology. The authors explain that genes associated with intelligence differences influence brain structure and function, providing a mechanistic link between DNA and cognitive ability.
- https://www.nature.com/articles/s41380-021-01027-y
- DNA and IQ: Big Deal or Much Ado About Nothing? – A Meta-analysis – Intelligence
- This meta-analysis assesses the current state of DNA-based prediction of IQ using polygenic scores. It concludes that while the predictive power is not yet definitive (explaining ~10-15% of variance), it is statistically robust and growing, making it a “big deal” for science, though not yet a tool for widespread individual prediction.
- https://www.sciencedirect.com/science/article/abs/pii/S0160289624000655
- The new genetics of intelligence – Nature Reviews Genetics
- This is the publisher’s link for the same review paper listed as source #2. It provides a comprehensive overview of the genetic architecture of intelligence, emphasizing its polygenic nature and heritability.
- https://www.nature.com/articles/nrg.2017.104
- Polygenic score prediction within and between sibling pairs for intelligence, cognitive abilities, and educational traits from childhood to early adulthood