TV is making you dumb — or is it the other way around?

We already know that sitting on the couch binge-watching TV is bad for the waistline, but a new study shows it could also damage long-term intelligence.

Scientists at the Northern California Institute for Research and Education analyzed data from a study that followed more than 3,200 people aged between 18 and 30 for 25 years.

The participants who reported watching more than three hours of TV a day more than four days a week, while exercising rarely, were twice as likely to score poorly on intelligence and cognition tests over the course of the study.

“We found that low levels of physical activity and high levels of television viewing during young to mid-adulthood were associated with worse cognitive performance in midlife,” says study leader Dr. Tina Hoang. “In particular, these behaviors were associated with slower processing speed and worse executive function, but not with verbal memory.”

The researchers believe their study is one of the first to establish a direct link between too much TV, no exercise and cognitive decline on young brains.

While they make the hypothesis that lack of exercise hurts neurogenesis (the development of nervous tissue), the results could also be caused by the fact that those with low cognitive functioning are more likely to sit around and watch a lot of TV.

If in doubt, just get off that couch and go for a walk — that definitely can’t hurt.