Once teens get over the intelligence hump that makes eating Tide Pods seem like a good idea, human brains are well-equipped to handle the world. Still, one would hope they’d continue to gain intelligence through education and experience over the course of their lives. Unfortunately, a paper published Monday in PNAS indicates that brains as they are in their early 20s might indicate how sharp and dementia-prone they will be as we age.

There are three factors that have been shown to help improve the brain’s ability to stay sharp as we age, says lead study author William Kremen, Ph.D.: occupation complexity, education, and participation in cognitive intellectual activities. Kremen, a clinical psychologist at the University of California San Diego, believes these factors stave off age-related cognitive decline not because they’re miracle activities, but because the people who do all of these activities are pretty sharp to begin with. It’s a person’s initial cognitive ability — not these factors — that appear to predict how strong someone’s mind is years later.

“There is a difference between predicting and causing,” Kremen tells Inverse. “In other words, what looks like an effect of intellectual activities could really be due to differences in the people who choose to engage more in those activities.”

Intelligence at 20 = Intelligence at 62

To test his hypothesis directly, Kremen explains, you would have to take a group of people, randomize them to each activity, and then see how well they responded to tests of intelligence years down the road. His study doesn’t do that. Instead, it turns to the Vietnam Era Twin Study of Aging, which collected cognitive data on 1,009 American military service members who were recruited between 1965 and 1975.

Since they first enrolled in the study in their early 20s, the group of all-male twins have taken general cognitive ability (GCA) tests at crucial junctures in their lives. The test Kremen focused on was the one the twins took at age 62. In his analysis, he looked for signs that factors such as education or having a complex job might have influenced his GCA score. However, he found that those factors only explained one percent of the variations in scores between individuals.

A map of where the twins in Vietnam Era Twin Study of Aging live throughout the country. BU

But then, he compared the score of each individual at age 62 to the score they got at age 20. That, he found, could explain 40 percent of the variation in cognitive ability at age 62. In short, a person’s GCA in their early 20s was a better predictor of whether their brain would still be sharp 42 years later, regardless of how much education they had.

He also noted that individual GCA scores at age 20 were also correlated with cortical surface area (the area of the outer layer of the brain) at age 62. This suggests that having a higher GCA early in life is a good sign that all that physical brain material will still be around later on. However, Kremen adds, increasing the amount of cortical surface area as we get older — in an attempt to protect against aging — might not be as easy as it seems.

The Cognitive Plateau

Taking his data into account, Kremen suspects we may reach a “cognitive plateau” around the age of 20. That’s not to say that we can’t improve upon our GCA scores marginally as we get older, but at least when it comes to improving cognition or general measures of intelligence like IQ, what we’ve got around the age 20 is probably what we’ll be working with for good, regardless of how much education you get.

“Beyond that, our results suggest that more education does not seem to increase a person’s general cognitive ability,” Kremen says. “If [cognitive] gains plateau in early adulthood, it may also mean that improving cognition in later life will require a good deal of effort, and that gains may be small.”

This paper suggests that education won't improve cognition past a certain age, but it's still important. Pxhere

GCA Isn’t Everything

Kremen notes that the whole point of his paper isn’t to take a jab at college education (he is a professor, after all). Despite all this focus on GCA scores of “cognitive function,” this test simply measures abstract features of intelligence, like working memory, episodic memory, or verbal fluency. The verbal fluency task, for instance, tests how many words beginning with F, A, or S someone can say in 60 seconds.

This paper indicates that more education over the years won’t make you better at tasks like this. But that’s probably not the point of education anyway, Kremen adds.

“Increasing IQ or general cognitive ability is not the only thing to be gained by education. As university professors, we have no doubt that there is substantial value in further education,” he says. “More education beyond that still can expand and improve upon knowledge, expertise, and critical thinking.”