New research reports watching a lot of television in early adulthood is associated with poorer cognitive performance at midlife.

Future idiots. (Photo: Mark Roy/Flickr)

Remember your mother yelling at you to turn off that television because it will rot your brain?

It turns out she may have had a point.

A new, large-scale study finds a link between heavy TV viewing as a young adult and below-average thinking skills in middle-age.

Specifically, it reports that people who were glued to the set during their 20s and 30s were more likely to perform poorly on certain tests measuring cognitive function.

"Lifestyle behaviors in early adulthood ... could have an effect on the risk of cognitive impairment in midlife," concludes a research team led by Tina Hoang of the Northern California Institute for Research and Education and Kristine Yaffe of the University of California–San Francisco. Their findings are published in the journal JAMA Psychiatry.

Re-runs may be relaxing, but that third go-round of Seinfeld may come at a cognitive cost.

The study featured 3,247 people who were recruited from four American cities (Chicago, Minneapolis, Oakland, and Birmingham) in the late 1980s. All were between the ages of 18 and 30 at the study's outset. They were tracked for the next quarter century, with follow-up examinations every two to five years.

At each such visit, participants reported their level of vigorous exercise (such as running, bicycling, or swimming) and moderate-intensity exercise (such as walking, golfing, or bowling). In addition, once every five years, they indicated "the average number of hours per day (they) spent watching television in the past 12 months."

Those who reported watching an average of more than three hours per day during more than two-thirds of their visits were categorized as having a "long-term pattern of high television viewing."

At year 25, the participants took three well-known cognitive tests: the Digit Symbol Substitution Test, which measures speed of mental processing; the Stroop Test, which measures processing speed and attention; and the Rey Auditory Verbal Learning Test, which assesses how well we remember information delivered orally.

Participants who showed a long-term pattern of high TV viewing (approximately 11 percent of the total) scored significantly lower, on average, on two of the three cognitive tests (verbal memory being the exception). This held true after taking into consideration a range of factors, including age, race, education, alcohol use, and body mass index.

In addition, those who revealed a "long-term pattern of low physical activity" (approximately 16 percent of the total) were more likely to show poor cognitive performance on one of the three tests (digital symbol substitution). And for the 3.3 percent of participants who both watched a lot of TV and did little exercise, the results were particularly grim: They were nearly two times more likely than heavy exercisers/light viewers to perform poorly on the two non-verbal tests.

The link between exercise levels and brain function is fairly clear. "Physical activity during young adulthood may preserve cognitive function, and contribute to cognitive reserve," the researchers write. Why heavy television viewing might negatively impact our mental abilities is a more complicated question.

Previous studies have suggested that "sedentary behaviors, such as television viewing, adversely affect metabolic function by increasing blood pressure, as well as lipid and glucose levels," the researchers write. They add that heavy viewing has also been associated with depression and poor eating habits—either of which could influence cognitive functioning.

The study has its limitations. Since participants assessed their own levels of physical activity and television viewing, it's impossible to know if their answers were completely accurate. And, as always, correlation is not causation; it's at least possible that high television viewers were more cognitively challenged to begin with.

Nevertheless, given that today's young adults have more screens than ever to stare at, these findings are particularly troubling. Hoang and her colleagues conclude that "early adulthood may be a critical period to promote physical activity for healthy cognitive aging."

Re-runs may be relaxing, but that third go-round of Seinfeld may come at a cognitive cost.

Findings is a daily column by Pacific Standard staff writer Tom Jacobs, who scours the psychological-research journals to discover new insights into human behavior, ranging from the origins of our political beliefs to the cultivation of creativity.