Does iq grow with age
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Does IQ Grow with Age?
Heritability of IQ and Age: The Wilson Effect
The heritability of IQ, which refers to the proportion of variance in intelligence attributable to genetic factors, increases with age. This phenomenon, known as the Wilson Effect, shows that the heritability of IQ reaches an asymptote of about 0.80 by the age of 18-20 years and remains at that level into adulthood. Concurrently, the influence of shared environmental factors decreases to approximately 0.10 by the same age range and continues at that level into adulthood.
Early Brain Growth and IQ Development
Infancy and Early Childhood
Research indicates that brain growth during infancy significantly influences IQ at later stages. For instance, head circumference at 8 months is a strong predictor of IQ at 3 years of age among very low birth weight infants, even when controlling for medical and sociodemographic variables. Similarly, head growth during infancy is associated with higher IQ at ages 4 and 8 years, with the brain volume achieved by the age of 1 year being a critical determinant of later intelligence.
Prenatal and Postnatal Growth
Studies have shown that both prenatal and postnatal growth impact cognitive performance. For example, greater size at birth and weight gain in infancy are associated with higher IQ at age 5 years in term-born children. Additionally, height at ages 9 and 13 years is a significant predictor of childhood IQ, suggesting that postnatal growth continues to influence cognitive development beyond early childhood.
Longitudinal Stability of IQ
Longitudinal studies reveal that IQ remains relatively stable from early childhood to early adulthood. The Munich Longitudinal Study on the Ontogenesis of Individual Competencies (LOGIC) found that individuals with higher initial IQ scores maintained their advantage throughout the study period, which spanned from preschool to early adulthood. This stability was more pronounced for shorter intervals between measurement points and increased with age.
Cortical Changes and IQ
Changes in cortical thickness and surface area over time are also related to intelligence. More intelligent children tend to have a thinner cortex at age 10, which thins faster over time. However, by age 42, a thicker cortex is associated with higher intelligence. Additionally, the cortical surface area is larger in more intelligent children at age 10, expands during adolescence, and then decreases at a higher rate once expansion is complete.
Growth in Low- and Middle-Income Countries
In low- and middle-income countries, linear growth in the first 1000 days of life is a predictor of schooling attainment and IQ in adulthood. However, growth in later childhood periods does not show a significant association with these outcomes. This underscores the importance of early life growth for long-term cognitive development.
Conclusion
In summary, while the heritability of IQ increases with age, early brain growth, particularly during infancy and early childhood, plays a crucial role in determining later intelligence. IQ remains relatively stable from early childhood to adulthood, with initial IQ levels being a strong predictor of future cognitive performance. Changes in cortical structure and early life growth are also significant factors influencing IQ development.
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