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IQ and Age: Understanding the Relationship

Predictive Value of Early Childhood IQ Assessments

Ages and Stages Questionnaire (ASQ) at 36 Months

The Ages and Stages Questionnaire (ASQ) at 36 months has been shown to predict IQ scores at ages 5 to 6 years. A study involving 939 children found that a 36-month ASQ score threshold of 270 was optimal for identifying children at risk of having an IQ below 85 at ages 5 to 6 years. The sensitivity and specificity of this threshold were 0.77 and 0.68, respectively. Additionally, maternal education level and occupational activity were significant factors associated with lower IQ scores, independent of the ASQ score.

Longitudinal Stability of IQ

A longitudinal study involving 173 children aged 5 to 13 years, tested annually with the Wechsler Intelligence Scale for Children (WISC), found that IQ scores remained relatively stable over time. The study showed that the correlation between IQ scores increased with age, indicating a trend towards greater stability as children grow older. No significant sex differences were observed in the pattern of IQ changes.

IQ as a Predictor of Future Success

Childhood IQ and Adult Outcomes

Research has demonstrated that IQ scores between ages 3 and 18 can predict educational and occupational status in adulthood. By the second grade, the predictive correlations for occupational status approached those obtained with adult IQs. However, these correlations were not high enough for practical long-term predictions for individual children .

Age-Related Changes in IQ

WAIS-III and Age-Related IQ Changes

Studies using the Wechsler Adult Intelligence Scale-Third Edition (WAIS-III) have shown that IQ scores change with age. Cross-sectional and longitudinal analyses revealed that while Verbal IQ remains relatively stable across ages, Performance IQ and Full Scale IQ tend to decline with age. These findings support Horn's fluid-crystallized intelligence theory, which posits that fluid intelligence (problem-solving and reasoning) declines with age, while crystallized intelligence (knowledge and skills) remains stable .

The Wilson Effect: Increasing Heritability of IQ with Age

The Wilson Effect describes the phenomenon where the heritability of IQ increases with age, reaching an asymptote of about 0.80 by ages 18 to 20. Concurrently, the influence of shared environment decreases to approximately 0.10. This effect has been consistently observed in various twin and adoption studies across different settings.

Generational and Secular Trends in IQ

Flynn Effect and Age Variations

The Flynn Effect, which describes generational increases in IQ scores, does not uniformly apply across all ages and ability levels. A study of 10,000 US adolescents found that while IQs increased by 2.3 points at age 13, they decreased by 1.6 points at age 18. Additionally, IQs decreased significantly for those with lower ability levels but increased for those with higher ability levels. These findings suggest significant heterogeneity in IQ changes over time, challenging the generalization of the Flynn Effect across the entire population.

Age Differences in Secular IQ Trends

Longitudinal analyses of IQ scores from the Wechsler Intelligence Scale for Children (WISC) series revealed that the decline in test scores when new norms replace older ones was smaller for older children in earlier testings. However, these differences disappeared in more recent testings, indicating a complex relationship between age and IQ within secular trends.

Conclusion

The relationship between IQ and age is multifaceted, involving early childhood assessments, longitudinal stability, predictive value for future success, age-related changes, and generational trends. Early assessments like the ASQ can predict later IQ, while longitudinal studies show increasing stability of IQ with age. The Wilson Effect highlights the growing heritability of IQ over time, and the Flynn Effect's variability underscores the complexity of IQ trends across different ages and ability levels. Understanding these dynamics is crucial for educational planning and cognitive development strategies.