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.

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.

Even the supposed narrow ones can be a pain in the arse to actually get in the back.

Every family I see with at least 2 kids has a minivan, so maybe we can discuss if minivans are causal.

I didn't come across those narrow seats when we looked into solution for fitting two kids and an adult (grandma) in the back row.

So we went for (especially in Europe) rather limited subset of cars where all 3 of the 2nd row seats are proper sized, with Isofix on each of them.

Usually same makes/models that offer the option of additional 2 seats in the 3rd row.