As the father of two adolescents and one former adolescent, I am acutely aware that the teenage brain is a very complex developing organ. I am also aware that since the advent of the iPhone in 2007, the entire way that adolescents relate to each other and their world has changed. Currently teens spend, on average, four or five hours per day on their phones, which most experts believe is far too much.

But what is the exact impact of excess screen time (TVs, computers, smartphones, iPads, video games, etc.) on the developing brain? And is this impact based on separation from the physical world and exposure to a more artificial and self-serving one? Or is it the content absorbed from these screens that impacts adolescents the most? No one knows.

But one thing’s for sure – things are changing, and not necessarily for the better. One recent study from the University of Pennsylvania revealed that decreasing the amount of time spent on Snapchat, Instagram and Facebook to less than 30 minutes per day decreased the rate of depression by 30 percent. In recent years, emergency room visits for self-harm by young teenage girls has tripled and increased smartphone use appears to be a contributing factor.

Still, we need more answers. To this end a new study by the National Institutes of Health (NIH) – the Adolescent Brain Cognitive Development (ABCD) Study – is attempting to find out the impact of substance use, concussion and screen time on adolescents. The effects are being studied in over 11,000 adolescents, at 21 sites across the U.S., over the course of a decade, at a cost of over $300 million. The study starts at the age of 9 or 10 and the initial results of the first 4,500 participants are in.

Not surprisingly, more than two hours a day of screen time is correlating with lower scores on thinking and language tests. But even more disturbingly, greater than seven hours per day using smartphones, tablets or video games shows thinning of the cortex (outer surface) of the brain on MRI scans. This type of thinning is characteristic of how the brain changes as it matures.

So are teens maturing faster as a result of all the new knowledge acquired from smartphone use? Or are their brains maturing prematurely in a way that isn’t necessarily good for healthy social development? I think the latter explanation is more likely. Of course this is a preliminary result, and the true long-term significance is not yet known.

When it comes to much younger children, previous studies among toddlers have shown that using an iPad in infancy can lead to sleep disturbances and speech delay. Other studies have shown that the way in which iPads and smartphones are used with toddlers can affect their learning and social skills either positively or negatively. The more interactive the devise or program or app, the better the teaching tool it is.

As children get older, it becomes more difficult to generalize. For some, like my children, they are able to incorporate hours of screen time into their lives and still maintain complex social interactions. For others, smartphones are the path to social isolation, where two friends sit next to each other speechless while texting. Worse is the anger and divisiveness that is rampant on social media.

As a wise person recently cautioned me, when it comes to science, correlation is not causation. In other words, even if increased screen time is associated with depression, isolation, less sophisticated decision-making, or decreased sexuality, there could be other factors in the environment that are causing these trends.

We will all be watching as the NIH study results for ABCD come in over the next several years, to see how the adolescents in the study (and our own children) make out. However, by the time the final results are in, smartphones and their apps will have been modified, and we will be dealing with shifting impacts on an entirely new generation of teens.