[stefl14]

For how long? Being recent (on evolutionary timescales), evolution has created selection pressures on previously irrelevant variables (the desire to have children, in particular). Without contraception or female labor force participation, people didn't decide how many children to have before urbanisation: everyone who didn't die just kept having kids. The poorest places are still like this. But in evolutionarily novel urban contexts, people choose how many kids to have; if this decision has a genetic component, that means the current urban millieu selects for people who want more kids. Basic genetics tells us that the number of people who "want more kids" will grow exponentially (so long as culture doesn't keep driving preferences for children down). The key question, of course, is "what's the exponent?". It depends on the heritability of fertility, and data from behavioural genetics indicates that it's high in urbanised societies. If your parents had more kids than average, you probably will (even if you were adopted at birth and didn't grow up with your parents). The effect is large enough that it would have a big impact on the UN population estimates by 2100. See https://www.sciencedirect.com/science/article/abs/pii/S10905...

The cultural offset is an important caveat, but it's possible that most of the cultural fertility decline due to urbanisation has been exhausted in urban socities, in which case the heritability effect will become more important.

I find it strange that natural selection is never part of the argument on debates like this. It's laughable that it's completely ignored by the UN. I suspect the tendency of humans to view themselves as separate from the animals meakes selection seem irrelevant. But we shouldn't forget its power: the types of people who deciding not to reproduce today are the types of people who won't be there to make that decision tomorrow.

All this to day, our contemporary perspective might seem very parochial in a few generations and while we can't predict future culture, the power of selection should never be ignored.