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Older people who stay sharp tend to have a thicker cortex, which is the outermost region of the brain that includes the areas responsible for judgment and complex thought. But while preserving the cortex is important for successful aging, a new study suggests that childhood intelligence — not anything specific done in old age — largely accounts for why some elderly people have more cortical tissue and better cognition.

The study, which was published in Molecular Psychiatry, included nearly 600 Scots born in 1936 who had their IQs tested at age 11 and again at 70. When the participants were 73, they had their brains scanned to measure their cortical thickness. The research found that more than two-thirds of the association between cognitive ability in the elderly and cortical thickness was accounted for by differences in IQ decades earlier in childhood.

“It appears that aging well cognitively is not strictly due to something that one does or does not do in old age but rather, the end point of things that have been going on throughout one’s life,” says lead author Dr. Sherif Karama, an assistant professor of clinical psychiatry at McGill University in Montreal.

The study has critical implications for aging research. If people who fare better cognitively in old age are simply those who’ve been smarter throughout life — and presumably had relatively thicker cortices early on — it’s not a matter of other people losing more cortex. Instead, those people may have had less of it to lose.

“It’s not something that simply happens in old age and determines whether you’ll keep your rank order among people of the same age; it’s rather something that has been going on all your life,” says Karama. “This, to me, is an important point. We tend to view old age as if it’s something completely independent of earlier life history.”

Studies of the elderly rarely include data on childhood intelligence because it is not easy to get — and, of course, IQ tests are a controversial measure — but this research suggests that it may need to be taken into consideration.

Still, the relationship between higher IQ and greater cortical thickness is not well understood. It may be that genetic factors create a thicker cortex initially, but it is also known that brain development is exquisitely dependent upon experience. The more a region is used, the bigger it tends to become. This means that some people may become smarter by simply exercising their brains more throughout life. Perhaps the genes involved in intelligence work by making people want to learn; the act of learning then enlarges the brain. “The possibilities are endless,” says Karama.

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The research does imply that attempts to enhance cognition through mental exercise or “brain training” are better started earlier rather than later. “I wouldn’t say that these [later] efforts are necessarily misguided but at this point, the evidence for staving off dementia via these methods is unclear,” says Karama.

The study also supports the idea that smarter people have more “cognitive reserve” that can protect them from some of the intellectual decline that can come with age, although the research did not include people with diagnosable dementia.

But here too it gets complicated. It is not known whether having higher intelligence actually prevents cognitive decline and dementia or merely allows people with those conditions to function better because they can hide the most obvious signs. “Some data does indeed suggest that [higher intelligence] postpones its manifestations,” says Karama.

Depressingly, all of this means we have a long way to go before we can truly understand and prevent cognitive decline and dementia. But recognizing the role played by what happens early in life may help bring that goal closer.

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