Geoff Read is an associate professor of history at Huron University. Of all the motivating ideas of Donald Trump’s Republicans, their desire for Americans to have more children (i.e. their “pronatalism”) might seem benign. Who could object when Mr. Trump declared, “I want a baby boom”? Sure, the Republican fixation on this can get a little weird, such as when Elon Musk brags about his 13 (14?) children, but the idea of having more babies and growing the population seems mostly harmless. The fixation on the U.S. birth rate, however, is more troubling than it seems. Mr. Trump and his...