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.

I think the car is a proxy/correlation for a level of wealth. If you make little enough the marginal cost of the next kid "seems" cheap because you basically make it back in state benefits in a lot of cases.