Is iq stable
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The Stability of IQ: Insights from Recent Research
Stability of IQ in Low Intellectual Ability
Research indicates that IQ stability in individuals with low intellectual ability (IQ < 80) is relatively high. A meta-analysis of commonly used IQ tests, such as those derived by Wechsler and Binet, found weighted-mean stability coefficients of .77 and .78 for Verbal IQ (V IQ) and Performance IQ (P IQ), respectively, and .82 for Full-Scale IQ (FS IQ) over an average test-retest interval of 2.8 years. However, it is noteworthy that while most Full-Scale IQs changed by less than 6 points, 14% of individuals experienced changes of 10 points or more, suggesting that IQ assessments should be approached with caution1.
Genetic and Environmental Influences on IQ Stability
IQ remains relatively stable throughout life, with genetic factors playing a significant role. Studies using bivariate genetic models on kinship pairs from longitudinal samples have shown that 66% to 83% of IQ stability can be attributed to genetic factors. Additionally, 43% to 69% of changes in IQ are also due to genetic influences. The remaining stability and changes are influenced by shared and nonshared environmental factors. Interestingly, there are notable race differences in the genetic and environmental contributions to IQ stability2.
Influence of Early IQ on Long-Term Stability
IQ stability from early childhood into adulthood varies depending on initial IQ scores. Research involving preterm and term-born individuals indicates that those with low IQ scores at preschool age exhibit more stable IQs from childhood into young adulthood compared to those with medium or high initial scores. This pattern suggests that individuals with initially high IQs may not maintain their relative advantage as others catch up over time3.
IQ Stability in Low-Birthweight Individuals
Studies on low-birthweight individuals show that IQ remains stable from childhood to adulthood. For example, stability quotients for Verbal, Performance, and Full-Scale IQs were found to be 0.86, 0.86, and 0.89, respectively, in one sample, and 0.66, 0.65, and 0.74 in another. These findings suggest that most low-birthweight children maintain stable intellectual development patterns from middle childhood to young adulthood4.
Differential Stability Based on Initial IQ Levels
Longitudinal studies have shown that IQ stability is generally higher for individuals with lower initial IQ scores compared to those with higher scores. For instance, children with initially high IQ scores tend to maintain their advantage into early adulthood, but the stability of their IQ scores is lower compared to those with initially low IQ scores. This trend highlights the importance of considering initial IQ levels when assessing long-term intellectual development5.
Verbal vs. Nonverbal IQ Stability
Research indicates that Verbal IQ scores tend to be more stable than Nonverbal IQ scores, especially in children under ten years of age. This finding underscores the need for caution when interpreting nonverbal IQ scores over long periods, as they may be less reliable indicators of cognitive stability6.
IQ Stability in Special Populations
In populations with specific conditions, such as developmental dyslexia, IQ stability can vary. For example, dyslexic individuals may experience a significant decrease in Verbal IQ over time, potentially due to less experience with reading and writing. Conversely, their Performance IQ may improve, possibly as a compensatory mechanism. These findings suggest that special education and tailored interventions are crucial for supporting the intellectual development of dyslexic individuals7.
Conclusion
Overall, IQ tends to be relatively stable across different populations and developmental periods, with genetic factors playing a significant role in this stability. However, initial IQ levels, environmental influences, and specific conditions can affect the degree of stability. These insights highlight the complexity of IQ development and the need for careful interpretation of IQ assessments over time.
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Most relevant research papers on this topic
The stability of IQ in people with low intellectual ability: an analysis of the literature.
Low IQ scores (IQ 80) show moderate stability, but 14% of Full-Scale IQ scores changed by 10 or more points over 2.8 years, suggesting caution in interpreting IQ assessment results.
Meta-AnalysisThe genetic and environmental architecture to the stability of IQ: Results from two independent samples of kinship pairs
IQ stability is primarily due to genetic factors, with race differences emerging in the relationship between genetic and environmental factors.
Observational StudyThe influence of preschool IQ on the individual‐order stability of intelligence into adulthood
Preschool IQ scores influence the individual-order stability of intelligence into adulthood, with those with low scores showing more stability into young adulthood than those with medium and high scores.
Observational StudyIQ stability: the relation between child and young adult intelligence test scores in low-birthweight samples.
Most low-birthweight children show stable intellectual development from middle childhood to young adulthood.
Observational StudyIntellectual development from early childhood to early adulthood: The impact of early IQ differences on stability and change over time
Higher initial IQ scores maintain advantages throughout the study period, with low-IQ children showing higher stability over time and more likely to attend higher educational tracks.
Observational Study
Highly CitedTen-Year Stability of Verbal and Nonverbal IQ Scores
Group verbal IQ scores show less stability than Stanford-Binet scores, and non-verbal IQ scores show less stability than verbal IQs.
Observational StudyHighly CitedStability of IQ measures in teenagers and young adults with developmental dyslexia.
Dyslexic teenagers and young adults may experience a decrease in verbal IQ over time, but performance IQ may improve due to compensatory processes or increased experience with reading and writing.
Observational StudyDevelopment of intelligence in early treated phenylketonuria
Early treatment of phenylketonuria (PKU) leads to stable IQ development after 10 years, with children having near normal outcomes.
Meta-AnalysisHighly CitedStability of IQS from Group-Administered Tests: Some Further Data
Group-administered intelligence tests in primary grades show less stability than individual-administered tests, requiring extreme caution when using them to predict later performance.
Observational StudyThe heritability of IQ
Maternal effects account for 20% of IQ covariance between twins and 5% between siblings, and understanding these effects may help identify ways to increase IQ.
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