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Have any such brain differences been found?

Not convincingly. More than 100 scientific reports and surveys have studied screen habits and well-being in young people, looking for emotional and behavioral differences, as well as changes in attitude, such as in body image. In 2014, scientists from Queen’s University Belfast reviewed 43 of the best designed such studies. The studies found that social networking allows people to broaden their circle of social contacts in ways that could be both good and bad, for instance by exposing young people to abusive content. The review’s authors concluded that there was “an absence of robust causal research regarding the impact of social media on the mental well-being of young people.” In short: results have been mixed, and sometimes contradictory.

Psychologists have also examined whether playing violent video games is connected to aggressive behavior. More than 200 such studies have been carried out; some researchers found links, others have not. One challenge in studying this and other aspects of screen time is identifying the direction of causality: Do children who play a lot of violent video games become more aggressive as a result, or were they drawn to such content because they were more aggressive from the start?

Even if scientists found strong evidence of a single, measurable effect — if, say, three hours of daily screen time was associated with a heightened risk of being diagnosed with A.D.H.D. — such a clear association wouldn’t necessarily suggest there were any consistent, measurable differences in brain structure.

Individual variation is the rule in brain development. The size of specific brain regions such as the prefrontal cortex, the rate at which those regions edit and consolidate their networks, and the variations in these parameters from person to person make it very difficult to interpret findings. To address such obstacles, scientists need huge numbers of research subjects and a far better understanding of the brain.

Isn’t that what the N.I.H. study aims to address?

Yes. The ongoing A.B.C.D. study expects to follow 11,800 children through adolescence, with annual magnetic resonance imaging, to see if changes in the brain are linked to behavior or health. The study began in 2013, recruiting 21 academic research centers, and initially focused on the effects of drug and alcohol use on the adolescent brain. Since then, the project has expanded, and now includes other targets such as the effects of brain injury, screen time, genetics, and an array of “other environmental factors.”