India, Sept. 27 -- A mother told me recently that her seven-year-old had started narrating cartoons to himself when the tablet wasn't in front of him, reciting lines from a show, alone, in the voice of a character. At the same time, the rest of the family sat two feet away. She laughed when she described it, the way parents do when something is a little unsettling, and they haven't decided yet whether to worry. Then she asked, almost as an afterthought, whether his eyes seemed fine to me. She'd noticed him sitting closer to the television than he used to.

Those two observations arrived in the same breath, and I don't think that was a coincidence. When a child's world shrinks to the size of a screen, it tends to shrink in more than one d...