A person’s general cognitive ability at age 20 is a better predictor of cognitive function and reserve in upper-middle age than other factors, such as higher education, occupational complexity or engaging in late-life intellectual activities.
The new study is published in the journal PNAS (Proceedings of the National Academy of Sciences).
General cognitive ability (GCA) refers to the diverse set of skills involved in thinking, such as reasoning, memory and perception.
Previous research has associated higher education and late-life intellectual pursuits — such as doing puzzles, reading or socializing — with reduced risk of dementia and sustained or improved cognitive reserve.
Cognitive reserve is defined as the brain’s ability to improvise and find alternate ways of getting a job done. It may help people compensate for other changes that occur with age.
In the new study, an international team of scientists, led by researchers at the University of California San Diego School of Medicine, sought to address a “chicken or egg” conundrum posed by these associations. For example, does being in a more complex job help maintain cognitive skills? Or do people with greater cognitive abilities tend to enter more complex occupations?
The study evaluated more than 1,000 men enrolled in the Vietnam Era Twin Study of Aging. Although all were veterans, nearly 80 percent reported no combat experience.
All of the participants, now in their mid-50s to mid-60s, had taken the Armed Forces Qualification Test — a measure of GCA — at an average age of 20. As part of the study, researchers evaluated participants’ performance in late midlife, using the same GCA measure, plus assessments in seven cognitive domains, such as memory, abstract reasoning and verbal fluency.
The findings reveal that GCA at age 20 accounted for 40 percent of the variance in the same measure at age 62, and approximately 10 percent of the variance in each of the seven cognitive domains.
In fact, after accounting for GCA at age 20, the authors concluded that the other factors had little effect. For example, lifetime education, complexity of job and engagement in intellectual activities each accounted for less than 1 percent of variance at average age 62.
“The findings suggest that the impact of education, occupational complexity and engagement in cognitive activities on later life cognitive function likely reflects reverse causation,” said first author William S. Kremen, Ph.D., professor in the Department of Psychiatry at UC San Diego School of Medicine.
“In other words, they are largely downstream effects of young adult intellectual capacity.”
In support of that theory, the researchers also found that age 20 GCA, but not education, correlated with the surface area of the cerebral cortex at age 62. The cerebral cortex is the thin, outer region of the brain (gray matter) responsible for thinking, perceiving, producing and understanding language.
Still, the researchers emphasize that education is clearly of great value and can enhance a person’s overall cognitive ability and life outcomes. Comparing the new results with other studies, the authors theorize that the role of education in increasing GCA takes place primarily during childhood and adolescence when there is still substantial brain development.
By early adulthood, education’s effect on GCA appears to level off, though it continues to produce other beneficial effects, such as broadening knowledge and expertise.