It probably isn’t news to most people that in recent years we have seen a decline in birth rates in the more developed parts of the world. The reason has been, in large part, attributed to the increase of opportunity costs of raising children, as well as generational mindset changes due to progress (e.g. the rapid reduction in infant mortality rate) and culture.

It makes sense intuitively. In the older, agricultural world, manual labor was the primary way of production, and kids, though will cost some resources initially to raise, could bring value to the family from a very young age. In the modern developed world however, kids started to become more of a pure net cost to the family. Families no longer needed the productivity contribution from their children. The raw cost of raising a child has increased due to the many years of education needed. The net cost is even higher, since raising kids means that the parents are not devoting as much time on other, purely productive activities (i.e. opportunity cost).

This is of course just a very simplified view of some of the main forces driving this change. There are many other related causes not mentioned here, but the point is that the trend is real. The more developed a region is, the lower the birthrate. As a region develop economically, the birthrate tend to decrease. Within the same region, the more financially successful families tend to have fewer children than those less financially successful.

This creates an interesting effect, which is the partial reversal of natural selection. Previously, financial success is a clear sign of reproductive fitness, and rich people tend to have very large families. Kings and emperors for example had massive number of children. But now the opposite happens. Financial success nowadays means higher opportunity cost of child bearing and leads to fewer offsprings. More broadly, status, wealth, and power, which used to correlate with reproductive success, are now starting to become negatively correlated with it.

It is difficult to see, or even imagine, what the potential long term consequences of this partial reversal of natural selection are. One possible effect is that it may cause the social elite to become an even smaller, more concentrated group. When royalties and rich powerful people had massive families, their wealth and power also had to be divided in more ways. Now when they only have a small number of children, their wealth and power can be transferred across the generational gap with a much lower loss.

The impact of this effect on social composition is probably not that large, at least for now. Yes, the elites used to have much larger families, but there were a lot more commoners, who had smaller but still respectably sized families. The fact that the small percent of the rich and powerful have reduced their number of offsprings probably didn’t change the composition of the population much.