A problem therefore arises when the emotionally resonant adult-child cueing system so essential to early learning is interrupted—by a text, for example, or a quick check-in on Instagram. Anyone who's been mowed down by a smartphone-impaired stroller operator can attest to the ubiquity of the phenomenon. One consequence of such scenarios has been noted by an economist who tracked a rise in children's injuries as smartphones became prevalent. (AT&T rolled out smartphone service at different times in different places, thereby creating an intriguing natural experiment. Area by area, as smartphone adoption rose, childhood ER visits increased.) These findings attracted a decent bit of media attention to the physical dangers posed by distracted parenting, but we have been slower to reckon with its impact on children's cognitive development. "Toddlers cannot learn when we break the flow of conversations by picking up our cellphones or looking at the text that whizzes by our screens," Hirsh-Pasek said.

In the early 2010s, researchers in Boston surreptitiously observed 55 caregivers eating with one or more children in fast-food restaurants. Forty of the adults were absorbed with their phones to varying degrees, some almost entirely ignoring the children (the researchers found that typing and swiping were bigger culprits in this regard than taking a call). Unsurprisingly, many of the children began to make bids for attention, which were frequently ignored. A follow-up study brought 225 mothers and their approximately 6-year-old children into a familiar setting and videotaped their interactions as each parent and child were given foods to try. During the observation period, a quarter of the mothers spontaneously used their phone, and those who did initiated substantially fewer verbal and nonverbal interactions with their child.

Yet another rigorously designed experiment, this one conducted in the Philadelphia area by Hirsh-Pasek, Golinkoff, and Temple's Jessa Reed, tested the impact of parental cellphone use on children's language learning. Thirty-eight mothers and their 2-year-olds were brought into a room. The mothers were then told that they would need to teach their children two new words (blicking, which was to mean "bouncing," and frepping, which was to mean "shaking") and were given a phone so that investigators could contact them from another room. When the mothers were interrupted by a call, the children did not learn the word, but otherwise they did. In an ironic coda to this study, the researchers had to exclude seven mothers from the analysis, because they didn't answer the phone, "failing to follow protocol." Good for them!

It has never been easy to balance adults' and children's needs, much less their desires, and it's naive to imagine that children could ever be the unwavering center of parental attention. Parents have always left kids to entertain themselves at times—"messing about in boats," in a memorable phrase from The Wind in the Willows, or just lounging aimlessly in playpens. In some respects, 21st-century children's screen time is not very different from the mother's helpers every generation of adults has relied on to keep children occupied. When parents lack playpens, real or proverbial, mayhem is rarely far behind. Caroline Fraser's recent biography of Laura Ingalls Wilder, the author of Little House on the Prairie, describes the exceptionally ad hoc parenting style of 19th-century frontier parents, who stashed babies on the open doors of ovens for warmth and otherwise left them vulnerable to "all manner of accidents as their mothers tried to cope with competing responsibilities." Wilder herself recounted a variety of near-calamities with her young daughter, Rose; at one point she looked up from her chores to see a pair of riding ponies leaping over the toddler's head.