In the first days of my son’s life, during the fall of 2023, he spent much of the time when he wasn’t sleeping or eating engaged in what some cognitive scientists call “motor babbling.” His arms and legs wiggled; his eyes wandered and darted, almost mechanically. One night, as he was drifting off to sleep, he smiled for the first time. As I admired him, wondering what he might be thinking about, his expression suddenly went blank—and then, in quick succession, he looked upset, then surprised, an...