How much does iq change with age
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Changes in IQ with Age: A Comprehensive Overview
Heritability of IQ and Age: The Wilson Effect
The heritability of IQ, known as the Wilson Effect, increases with age. Studies involving twins and adoption scenarios have shown that the heritability of IQ reaches an asymptote of about 0.80 between the ages of 18 and 20, maintaining this level into adulthood. Concurrently, the influence of shared environmental factors decreases to approximately 0.10 by the same age range and remains stable thereafter.
IQ Decline in the General Population
Longitudinal studies indicate that IQ generally declines with age. Performance IQ tends to decline early and rapidly, while verbal IQ declines later and more slowly. This pattern is observed in the general population but varies significantly in individuals with intellectual disabilities, where verbal ability declines more and performance ability declines less compared to the general population.
The Flynn Effect and Age Variations
The Flynn Effect, which describes generational increases in IQ, does not uniformly apply across all ages and ability levels. In a study of 10,000 US adolescents, IQs increased by 2.3 points at age 13 but decreased by 1.6 points at age 18. Additionally, those with lower IQs (<70) experienced a decline of 4.9 points, while those with higher IQs (>130) saw an increase of 3.5 points. These findings suggest significant heterogeneity in IQ changes over time, challenging the generalization of the Flynn Effect across the entire population.
Age-Related Changes in IQ: WAIS-III Findings
Research using the WAIS-III intelligence test across a broad age range (16-89 years) has shown that intellectual abilities change with age. Cross-sectional and longitudinal studies indicate that while some cognitive abilities decline, others remain stable or even improve, depending on the generation and educational attainment.
Cortical Changes and IQ
Changes in cortical thickness and surface area are related to IQ. More intelligent children tend to have a thinner cortex at age 10, which thins faster over time. By young adulthood, this relationship reverses, with a thicker cortex being associated with higher intelligence. Additionally, the cortical surface area expands during adolescence and then decreases at a higher rate in more intelligent individuals. These findings suggest that intelligence is more related to the timing and magnitude of brain structure changes during development rather than the structure itself.
Stability of IQ Over Time
A study involving children aged 5 to 13 years showed that Full-scale IQ remained relatively stable over three consecutive years. The correlation between IQ scores and age increased over time, indicating a stable pattern of IQ changes with age. No significant sex differences were observed in the patterning of IQ changes.
Brain Aging and IQ
In older adults, brain anatomical measures such as grey matter and white matter volumes decrease with age, while lateral ventricle volumes and white matter hypointensities increase. Despite these neuroanatomical changes, verbal and non-verbal IQ measures remained stable over a four-year period, suggesting that cognitive decline is not necessarily linked to brain volume changes in healthy older adults.
Cognitive Reserve and Terminal Decline
The cognitive reserve hypothesis posits that individuals with higher IQs can better tolerate age-related brain pathologies, delaying the onset of terminal cognitive decline. Higher IQs are associated with a later onset of decline in perceptual and motor speed, as well as verbal ability, but once decline begins, it progresses more rapidly compared to those with lower IQs.
Longitudinal Stability of IQ
Data from the Scottish Mental Surveys of 1932 and 1947 show that IQ scores are stable from childhood to old age. The surveys also highlight the impact of childhood IQ on health and survival in later life, with higher childhood IQ being associated with better health outcomes and lower mortality rates.
Age Differentiation and Dedifferentiation in IQ
A longitudinal study from age 12 to 52 found that while the rank ordering of individuals' intelligence levels remains stable, the variance in general cognitive ability (g) increases over time. This increase in variance leads to a greater differentiation in cognitive abilities, supporting the age dedifferentiation hypothesis.
Conclusion
IQ changes with age in complex ways, influenced by genetic factors, brain structure changes, and environmental influences. While some cognitive abilities decline, others remain stable or improve, and the patterns of change can vary significantly across different populations and age groups. Understanding these dynamics is crucial for developing strategies to support cognitive health throughout the lifespan.
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