It would be interesting to see to what extent exactly this is accurate (of course to a certain extent it is) if the majority of the population in liberal democracies had the option of having more than 1-2 children without massively compromising their material wellbeing (or the future of their offspring due to the lack of resources).
There's a few confounding factors:
- larger percentage of population living in urban centers without the room for larger families
- full time employment for both spouses becomes necessary as cost of living increases
- delayed marriage and delayed age of first child means fewer children can be safely carried to birth
The "highly liberal democracies" is just pleasant coding for "consumerized".