A scan of the paper seems to show that they don't consider the confounding influence of urban density. Humans don't seem to have innate "quorum sensing" the way bacteria do, but expensive housing is likely to have something to do with it
and between leftists dogpiling in big cities where their votes don't count and those cities making it close to impossible to build housing that has to be a factor, together with individualism, anti-natalism, etc.
https://www.pacificresearch.org/housing-costs-drove-the-majo...
not to mention the perception (if not reality) of an unsafe environment
https://ideas.repec.org/a/ebl/ecbull/eb-25-00236.html
and between leftists dogpiling in big cities where their votes don't count and those cities making it close to impossible to build housing that has to be a factor, together with individualism, anti-natalism, etc.