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Intelligence heritability is a complex topic that has been extensively studied through twin studies, adoption studies, and family research. Current scientific consensus suggests that intelligence, as measured by IQ tests, has a heritability estimate of approximately 50-80% in adults, with most estimates clustering around 60-70%.
Key Research Findings:
Twin studies comparing identical twins (who share 100% of their genes) with fraternal twins (who share 50% of their genes) consistently show higher correlations in IQ scores among identical twins. Large meta-analyses have found IQ correlations of about 0.86 for identical twins and 0.60 for fraternal twins.
Adoption studies, which examine children raised apart from their biological parents, show that adopted children’s IQ scores correlate more strongly with their biological parents than their adoptive parents, supporting genetic influence.
Important Nuances:
Heritability changes with age. In early childhood, environmental factors play a larger role, with heritability estimates around 20-40%. By adolescence and adulthood, genetic factors become more prominent, reaching the 60-80% range.
Heritability varies by socioeconomic status. In disadvantaged environments, environmental factors constrain intellectual development more significantly, reducing apparent heritability. In more privileged environments where basic needs are met, genetic differences become more apparent.
Critical Limitations:
Heritability describes population-level variance, not individual determinism. A high heritability doesn’t mean intelligence is unchangeable or that environment doesn’t matter for any specific person.
IQ tests measure specific cognitive abilities valued in particular cultural contexts, not all forms of intelligence or human capability.
Gene-environment interactions are complex. Genes influence how individuals respond to environments, and environments can affect gene expression (epigenetics).
Environmental Factors:
Despite high heritability estimates, environmental factors significantly impact intelligence development, including:
The scientific evidence suggests intelligence has substantial genetic components while remaining significantly influenced by environmental factors, particularly in early development and disadvantaged circumstances.