Sometimes a society’s values change sharply with almost no one noticing, much less anticipating the consequences. In 1968, according to a Gallup survey, 70 percent of American adults said that a family of three or more children was “ideal”—about the same number as in Gallup surveys starting in 1938. That number helps explain the postwar baby boom that exploded after Americans were no longer constrained by depression and world war.

Those values and those numbers didn’t last. By 1978, Gallup reported that only 32 percent considered three or more children “ideal.” The numbers have hovered around there ever since, spiking to just 41 percent amid the late 1990s tech boom.