Is IQ at the Age of 15 Predominantly Genetic or Environmental?

Traditional notions suggest that intelligence is a result of both genetic and environmental factors. However, research has shown that the genetic component significantly influences intelligence, even as early as the age of 15. This article explores the extent to which IQ is determined by genetics at this age, with a focus on the concept known as the Wilson Effect.

The Role of Genetics in IQ

IQ is a measure of intelligence and is inherently linked to genetic makeup. This connection is supported by numerous studies that have demonstrated the strong hereditary aspect of intelligence. According to one study, adult IQ is 85% heritable, meaning that genetic factors account for the vast majority of the variance in IQ, while environmental factors contribute the remainder.

While genetic influence is consistent across all ages, the environment also plays a role, particularly in the early years. Studies (Bouchard, 2013) have highlighted a shared environmental component below the age of 12, which diminishes around this age. This transition is a key point in understanding how genetic and environmental factors interact as individuals develop.

Understanding the Wilson Effect

The Wilson Effect refers to the increasing proportion of genetic influence on IQ with age. This phenomenon was first observed by Ronald Wilson, who presented data from twin studies and adoption studies to support his claims. The Wilson Effect is evident in various populations and environments, showing that the heritability of IQ reaches an asymptote at around 80% by the age of 18 to 20 years, stabilizing at an even higher level in adulthood.

The Wilson Effect can be illustrated diagrammatically, as shown in the graphic below from Bouchard, 2013. The graphic demonstrates the increasing role of genetics in shaping IQ from early childhood through to adulthood.

Skeptical Views on Genetic Influence

While the genetic influence on IQ is significant, skepticism remains regarding whether IQ at the age of 15 is predominantly determined by genetics. A popular myth suggests that the age of 15 is a critical turning point where genetic influences are overshadowed by new DNA and protein synthesis, leading to a transformation in intelligence.

However, this myth is based on a misunderstanding of cellular biology. At the age of 15, while one's body continues to develop, the genetic instructions for synthesizing proteins remain the same. The proteins that are synthesized still follow the genetic blueprint, indicating that genetic factors do not undergo a significant change at this age. The apparent transformation is more a result of the physical changes and developmental milestones than an alteration in genetic function.

The Importance of Genotype and Environment

Research consistently shows that both genetic and environmental factors play crucial roles in determining IQ. Genotype provides the baseline for intelligence, but environmental factors can modulate this baseline. Wilson (2013) documented key studies showing a decreasing shared environmental influence with age, approximating around 10% by the age of 18 to 20 years, and continuing at that level into adulthood.

The interaction between genetic and environmental factors is complex and dynamic. While genetics provide the foundation for intelligence, environmental factors such as education, nutrition, and social interactions can significantly influence its expression. This interplay is particularly critical during the formative years, but the genetic influence becomes more dominant as one reaches adulthood.

Conclusion

The age of 15 is a critical period in the development of intelligence, with both genetic and environmental factors playing significant roles. While the myth of a transformation based on new DNA synthesis is not supported by scientific evidence, the increasing genetic influence on IQ is well-documented. The Wilson Effect, which demonstrates the increasing heritability of IQ with age, offers valuable insights into the mechanisms underlying intelligence development.

The intricate relationship between genetics and environment underscores the importance of considering both in understanding human intelligence. As research continues to advance, our understanding of these factors will also deepen, potentially leading to more effective strategies for enhancing intellectual abilities.

References

Bouchard, T. J. (2013). The Wilson Effect: The Increase in Heritability of IQ With Age. Twin Research and Human Genetics, 16(4), 566-577.

Wilson, R. (2013). The Wilson Effect and the Nature-Nurture Debate: A Quantitative Genetic Perspective. Twin Research and Human Genetics, 16(4), 553-565.