But those are the visible afflictions, the ones that show up on expanding bellies and skyrocketing death rates. Out of sight, another epidemic is silently raging: myopia, or nearsightedness. Between the 1970s and the early aughts, the incidence of myopia in the US nearly doubled, to 42 percent. Myopia's rise has been the starkest in Asia; one survey in Korea found a rate as high as 96 percent among teenagers.

Clearly, something is going on. But scientists can't agree on exactly what. Being constantly tethered to devices and books indoors might be part of it: Based on a handful of large epidemiological studies on myopia, spending time outdoors—especially in early childhood—reduces the onset of myopia. (So nerds and glasses? It's true.) But what exactly about the outdoors helps? Is it the bright sunlight or how eyes focus on objects far away outside or something else entirely?