This week, the World Health Organization released new guidelines for the amount of time children ought to be in front of electronic screens. The key finding: Children under age 2 should not be exposed to TV, computers and cellphones, and those ages 2 to 5 should be limited to under an hour a day. A few years ago, the American Academy of Pediatrics issued similar recommendations.

WHO’s report focuses largely on the increasingly sedentary lifestyle of Americans, especially children, and its correlation to obesity and the myriad adverse physical issues that result. The idea is that children who become addicted to video entertainment over the course of their childhood — and beyond — are less prone to be active, get outside and, perhaps, rely on their own imagination.

Given what we know about the effects of obesity and health, that ought to be enough to convince parents to rely less on screens as babysitters. There is also an opportunity cost. Children plunked in front of a screen are less likely to be interactively engaged. While there certainly are games and shows that can help kids learn, there’s lots of evidence that children learn better at some stages through physical activity. Plus, a child watching TV or a movie or playing a video game isn’t spending that time on social interaction or developing other skills. There are only so many hours in the day.

Further, there’s the question of just what those children are being exposed to on the screen. A parent might not object to educational offerings from PBS or developmental learning apps, but handing a toddler an iPad showing “The Lion King” or “The Secret Life of Pets” could be traumatic; never mind the superhero films or shoot-‘em-up video games many kids get exposed to far too early. And children’s minds are particularly susceptible to absorbing the messages they see on those screens, often a dangerous thing in itself.

There are potential physical effects, too. Adults are warned not to look at their phone or computer for too long at a stretch, lest they suffer eye strain, dry eyes or the effects of too much “blue light,” which can cause macular degeneration. That could be even worse for children’s developing eyes.

There’s more, but it’s less conclusive. WHO looked at thousands of studies on child development before making its recommendations. That includes not just obesity trends and sedentary lifestyles, but also motor and cognitive development. But there’s never been a large-enough study, over a long-enough period, to provide irrefutable evidence of developmental harm.

A “60 Minutes” report in December, based on a study by the National Institutes of Health, noted heavy screen use has been associated with lower scores on some aptitude tests and linked to accelerated “cortical thinning” — scientists believe intelligence is linked in general to the thickness of the cerebral cortex — in some children. But not in others. And there’s the rub. There’s so much variation in brain development that it’s difficult to make sweeping statements based on limited studies. The NIH study “aims to reveal how brain development is affected by a range of experiences,” The New York Times wrote. But it will be years before it arrives at any conclusions.

Some smaller-scale studies have found a correlation between screen time and brain development; others have not. The problem is that developing brains continue to adapt, to rewire themselves, well into adulthood. But since every brain has been exposed to different stimuli, each one rewires itself uniquely, making it harder to draw conclusions.

WHO, then, is setting its guidelines based on what it can derive: that time spent sitting in front of a screen is inherently harmful from the standpoint of contributing to a sedentary lifestyle. Therefore, less time in front of a screen is better.

We note in the same vein that WHO also recommends children get more exercise and sleep. All are necessary for not only proper development, but also lifelong health.

The organization’s sensible guidelines, then, might well apply to many adults, too.