By: Brian White
The fluctuation of Intelligence Quotient (IQ) scores between childhood and adulthood is one of the most intriguing questions in psychology. The reason isn’t a mysterious change in mental capacity, but rather the individual’s growth and maturation.
IQ is not an absolute measure, like measuring height in meters. It is a relative measure calculated in comparison to peers of the same age .
The formula is IQ = (15 x z) + 100, where the z-score is relative. This means that a child who is ahead of his or her peer in maturity may exhibit a higher IQ simply because of a statistical advantage in childhood. It is a notable fact that low-IQ groups tend to mature faster, while high-IQ groups mature more slowly.
The Rise of Genetics: The Wilson Effect
The real story behind IQ stability lies in its heritability. Intelligence, like height, is a trait largely determined by the DNA we inherit. As a child ages, the heritability of both traits increases. This means that IQ measurements become increasingly predictive of future outcomes in adulthood.
Heritability is traditionally analyzed through three components, known as ACE:
1. A (Additive Polygenic): This is the genetic influence. It increases continuously from early childhood to early adulthood, a phenomenon called the Wilson Effect.
2. C (Shared Environment): This is the influence of the family and shared environment. It decreases dramatically and tends to disappear around the age of 12.
3. E (Non-Shared Environment): The influence of each individual’s unique life experiences slowly diminishes.
In short, IQ becomes more stable in adulthood because genetic influence (A) predominates , while the influence of shared environment (C) becomes irrelevant.
Stability and Final Limits
By the time a person reaches adulthood, their cognitive development is stable, and the changes that transform a baby into an adult have already occurred.
Maximum Capacity: Intelligence is genetically determined and highly heritable. Questions about how a person can increase their intelligence in adulthood have the same answer as how they can increase their height: it cannot be increased.
Vulnerability: Although IQ cannot be increased, it can be stunted during development by negative environmental factors such as disease, toxins, or head trauma.
The Adjustment: If your tested IQ appears to “decrease” from childhood to adulthood, this doesn’t mean a decline in your ability. It means your statistical advantage over your peers has normalized. In adulthood, the test reflects your genetic stability, becoming a more accurate and less volatile measure of your potential.
IQ in adulthood is therefore the most faithful reflection of your genetic potential, as the influence of temporary environmental circumstances (the C) has disappeared.
