Really, smartphones are just too new for us to have reliable data on how they affect developing kids. The first iPhone release was barely more than 10 years ago, and the sort of questions we are asking — about attention, social interactions, depression — are ones that can only be answered (reliably) by longitudinal studies that follow kids from childhood to adulthood. In the meantime plenty of ideas and folk wisdom will spread, but we won’t know for sure what the full effect of the this latest societal change will be.