Car Seats as Contraception by Jordan Nickerson, David H. Solomon :: SSRN:
Since 1977, U.S. states have passed laws steadily raising the age for which a child must ride in a car safety seat. These laws significantly raise the cost of having a third child, as many regular-sized cars cannot fit three child seats in the back.
We had to sell our sedan and get a minivan to accommodate kid number three.
We estimate that these laws prevented only 57 car crash fatalities of children nationwide in 2017. Simultaneously, they led to a permanent reduction of approximately 8,000 births in the same year, and 145,000 fewer births since 1980, with 90% of this decline being since 2000.
I don’t know if this is true, but it’s certainly plausible. Still, you’ve seen declining birthrates throughout the industrialized world, and I don’t know if they have different car seat laws or not. What I can say with certainty is that society’s default family position for most things is two adults and two kids.