General intelligence does not appear to influence specific mental skills uniformly over an individual’s lifetime or across different levels of mental ability. A large-scale study published in the journal Psychology and Aging found that the connections between different cognitive skills fluctuate significantly with age, with those connections becoming significantly weaker in people with higher overall intelligence. These changing patterns suggest that a single overall IQ score may be less reliable as a measure of brain power for certain people, depending on age and baseline ability.
In the field of psychology, researchers have observed that cognitive abilities tend to be related. People who perform well on tests of memory are more likely to perform well on tests of spatial reasoning. This widespread pattern is known as a regular manifold.
Statisticians use a single variable called the general intelligence coefficient, or g-factor, to represent this phenomenon. This metric acts as a sort of umbrella, capturing common differences across different mental tasks.
Despite its widespread use in clinical and educational settings, experts still do not agree on the mechanisms underlying the G-factor. Some scientists argue that general intelligence represents biologically specific mental abilities. Others argue that it simply appears as a statistical byproduct of different areas of the brain developing at the same time.
To better understand general intelligence, scientists are looking at how the strength of the G factor varies among different types of people. This area of questioning is known as differentiation research. Specifically, researchers are investigating whether the biological and environmental links between different mental skills change as people age or as their overall ability level changes.
Psychologist Moritz Breit from the University of Trier led a team of researchers from institutions across Germany to investigate these differentiating effects across the human lifespan. Researchers note that previous studies have often focused on narrow age groups, making it difficult to identify trends across the lifespan. By analyzing data from a single population across a wide age range, the research team aimed to precisely map how the structure of human intelligence changes from early childhood to late adulthood.
For this large-scale study, Bright and colleagues examined data from 4,129 participants. The ages of these people ranged from 2 years old to nearly 90 years old. Participants were drawn from a German standardization sample for the Wechsler test.
The Wechsler Test is the most frequently used intelligence diagnostic tool worldwide. Candidates complete a series of specific exams designed to measure different types of mental output. These areas include fluid reasoning, visual processing, working memory, processing speed, and acquired knowledge.
The researchers applied sophisticated statistical models to these individual test scores to see how strongly each specific mental skill was tied to the overall g-factor. They tracked these relationships across different age groups and a range of overall intelligence.
The research team observed a striking phenomenon called ability differentiation across all age groups. Mathematical modeling has revealed that there is an inverse relationship between a person’s overall intelligence level and the strength of their G-factor. As humans’ general cognitive abilities have improved, the statistical associations between specific mental skills have weakened.
This means that for individuals with very high assessment scores, a single measure of general intelligence is less representative of a variety of actual mental abilities. The researchers note that this ability differentiation effect is relatively stable during childhood. It then increases rapidly from mid-adolescence to late 20s, and then declines again in late adulthood.
This study also revealed highly variable patterns of age differentiation. The researchers documented alternating stages in the strength of general intelligence, rather than a steady decline or increase over the lifespan.
From early childhood to early elementary school, the G factor weakens, meaning that certain intellectual skills become more distinct from each other. Then, from around the age of 8 to early adulthood, the process of dedifferentiation begins. During this period, the connections between different cognitive abilities are strengthened and mental skills become more integrated.
Once you reach your 30s, the trend reverses again. General intelligence begins to decline, and the process of differentiation continues until about age 60. Finally, in late adulthood, the data hint at another reversal in which cognitive abilities begin to coalesce again.
This study relies solely on data from the German population. Although these particular Wechsler tests are based on widely used European standardized kits, their regional focus may limit the extent to which these precise trends apply to populations from other cultural backgrounds. Future research could examine intelligence test data from different countries to see if these patterns hold globally.
Because the researchers focused entirely on mental skills measured by the Wechsler Assessment, some cognitive domains were excluded from the analysis. Human intelligence includes different mental domains such as auditory processing and retrieval fluency, which are not assessed by these specific diagnostic tools. By examining a wider variety of test batteries, we may be able to learn more about how specific brain skills develop and diverge.
These changing patterns in the structure of intelligence have practical implications for educators and clinicians. Many schools and diagnostic centers rely heavily on a single composite score, often referred to as full-scale IQ, to determine academic placement and clinical intervention.
The finding that general intelligence is dramatically lower in highly capable individuals means that the margin of error for a single score is much larger in that population. To account for this variation in accuracy, the researchers suggest that psychologists should apply specific margins of error based on an individual’s age and cognitive level, rather than using a one-size-fits-all standard.
The study, “Contribution of general intelligence to cognitive abilities across the lifespan: A differential analysis of the Wechsler test,” was authored by Moritz Breit, Martin Brunner, Julian Preuss, Monika Daseking, Franz Paulus, Franziska Walter, and Franzis Preckel.
