President Trump’s return to the White House has made pro-natalists more influential than ever. Vice President Vance says he wants “more babies” in America. Elon Musk has claimed that declining fertility could lead to “mass extinction.”

Their efforts are noble, but Vance and Musk both underappreciate the role of genetics in determining fertility. Without a proper understanding, their efforts will fail.

In his 1930 book “The Genetical Theory of Natural Selection,” seminal geneticist Ronald Fisher observed a substantial correlation between the fertility of parents and their descendants. Fisher noted that the granddaughters of large families tended to have more children than those from small families. Fisher concluded that “about 40 percent of the total variance” in fertility was attributable to genetics.

Importantly, Fisher didn’t just conclude that fertility varied between individuals because of different genetic abilities to have children. Instead, Fisher argued that the most important cause of variation was different genetic desire to have children.

In this, Fisher found reason for optimism. Observing the rate at which genetic inheritance led some families to have more descendants than others, Fisher theorized that “more fertile strains” with a greater desire for children could become more common "within a span of ten generations," or approximately 250 years.

In other words, cultural and economic features of modernity may lower birthrates. But the question of why modernity lowers birthrates is less salient than that genetic selection can counteract the parts of modern civilization responsible.

Why does this matter? Falling fertility is often attributed to social factors. Vance has mentioned car-seat mandates and high

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