The heritability of fertility makes world population stabilization unlikely

One of the sources: https://journals.plos.org/plosone/article?id=10.1371/journal...

>> This finding implies that individuals with genetic predispositions for an earlier AFB had a reproductive advantage and that natural selection operated not only in historical, but also in contemporary populations. The observed postponement in the AFB across the past century in Europe contrasts with these findings, suggesting an evolutionary override by environmental effects and underscoring that evolutionary predictions in modern human societies are not straight forward. Yeah, so basically we got a simple model by two economists vs. decades of empirical data?

It'd be funny if it wouldn't sound like a scientists version of the "immigrants will have a lot of children and replace us" lie.

In rural societies, children are productive members of the workforce and the care of the aged depends on having surviving children. This encourages large families.

In urban societies, children are expensive to rear and educate until they can become productive, and the care of the aged depends on social institutions. This encourages small families.

However, if urban societies socialize the cost of rearing and educating children, then that disincentive will be removed and families may become larger. Ethnic subgroups with strong extended families and religious solidarity tend to have larger families, even in urban settings.

Some anecdotal data.

My father's parents had 8 surviving children.

My mother's parents had 8 surviving children.

My parents had 4 surviving children.

I have 4 children.

My siblings have, on average, 3 children, and will probably have 3.5 over time.

My cousin's have on average 3.5 children.

Grandparents: Asian.

Parents: Asian and lived in Middle East half their life.

Me and siblings: Living primarily in Middle East.

This study (https://www.pnas.org/content/114/5/E727) measured the speed at which selection against cognitive ability is occuring in the Icelandic population (they measured this genotypically rather than phenotypically). The rate at which it occurs is meaningful on the scale of our lifespans. Same is probably true for fertility rates, but as fertility is being selected for directly (rather than the indirect effect on cognitive ability) it's probably even more rapid.

Then this study more directly measured fertility's rate of evolution: https://www.pnas.org/content/115/1/151. They used UK Biobank data (genetic data from 157,807 female and 115,902 male unrelated samples). They found the 'Age at First Birth' is the most strongly selected trait in the British population. So in the future we can look forward to a world filled with teenage mums.

That this is not obvious to everyone is just a testiment to how little people understand about evolution.

Well, the finite nature of the world makes its population stabilization likely.