Also, childless people, on average, will cost more to society than they contribute. To be conservative, pension programs (including social security) rely on there being at least as many people around in the future to pay the bills. If you don't have any kids, someone else's kids are paying the bills.

In a vacuum, if a couple has only one child, that child had better grow up to support those two people in retirement. If four grandparents only have one grandchild, it's even worse.

The current answers to that problem are immigration (in which case someone else is still paying for benefits) or incredibly low take-home pay for workers (lots of withholding for personal or public retirement funds).