Double-buggies on public transport and more than two kids on a typical cargo cycle aren't fun either. Granted the age-span that's necessary is a little shorter than car seats.

That said, have 3 kids aged within 5 years of one another and we never had to get a double buggy. The older ones would be OK to walk (3 year olds will walk a pretty long way if you're patient) by the time the youngest got too big to be sling-carried.

It comes down to, dealing with three under-5's single-handed while out and about is pretty hectic full stop. Most places with high birth rates "solve" this by not allowing mums the expectation to be away from the house much, and/or they're multigenerational households where grandma or an aunt can be home with some of the kids.

So to your point, I think it's less the requirement to own a car, more the expectation of a kind of lifestyle which often, though not always, in turn requires one. Childcare for 2 year olds here is often upwards of $2500/month, now that's a contraceptive.

If you aren't a slave to your car, you likely live in a walkable area where the cost of a 4-bedroom apartment or house is going to be pretty high. I'm not saying you can't raise kids in a 2 or 3 bedroom apartment, and when I lived in apartments many families had kids in a 1-bedroom apartment, but it's very tight and many people would consider it a significant hardship for both the kids and the parents.

I would also add as a car slave that the kinds of cars large enough to fit the kinds of car seats marketed in the US are tens of thousands more than a compact or mid-size sedan, and that in a mid-size sedan having a car seat in the rear-facing configuration significantly constrains how far back you can put the passenger or driver seat. This is true even for the narrower seats that are designed for three-across seating. And worse, you might not have the latch system or an appropriate kind of seat belt on that third seat.

"Wouldn't the real cause of the depressed birthrates be the requirement to own a car in order to have children?"

Yes. The one-time setup costs for "properly" raising kids are probably around $30k. All the kids stuff is extra expensive (in the west) and for the kids seats you need a large car (in the west) and there's social stigma against kids sharing a room (in the west), so you also need a larger apartment.

The research is about the falloff in family size from 2 children to 3 children.

> If you aren't a slave to your vehicle there's no problem with the available space for car seats.

The abstract says the effect is limited to households with a car.