This is ridiculous.

The replacement fertility rate to maintain population before modern medicine could be closer to ten than two not so long ago.

War, famine, disease, accidents, and infant mortality made it so surviving until you were old enough to start having your own children was rare and women were often pregnant or nursing continuously as long as they were fertile.

The premise of heritable fertility driving population and reproduction rates in humans is really far off. We're not having litters of kittens.

If it were true fertility rates could not possibly have dropped so far in so few generations after modern medicine and the insane reduction in violent death rates seen in the last thousand years.