It was discovered some time back that the 'raw score' for IQs was increasing over time - meaning the mean kept going higher and higher. This is called the Flynn Effect. [1] People were getting smarter. The section I linked to is about the fact this trend reversed some time around 1990 in the developed world. Wiki uses weaker language 'possible end' but it's been tested and verified repeatedly at this point - we're getting dumber, and quite rapidly. This remains true even after controlling for obvious explanations like immigration.
This is yet another reason why fertility is, in my opinion, the issue of the century. The world is going to look so absurdly different, and not in a good way, in 50 years, owing to fertility rates alone. We're very likely living on the tail end of a societal golden age.
[1] - https://en.wikipedia.org/wiki/Flynn_effect#Possible_end_of_p...
Have you ever seen Idiocracy?
This has also been observed in nature. For instance most people view the variation in Darwin's finches as something gradually happening over eons, but further studies on them showed a rapid shift in beak morphology following a single severe drought. [1] It's the same stuff - suddenly one beak became more likely to reproduce (or not), which drove a very rapid population level change.
[1] - https://www.sciencedaily.com/releases/2016/04/160421145759.h...
Yes and no, it's a lot more nuanced. There can be higher fertility and the very low high and very high end of wealth, but it's also not consistent nor a solid link.
This article looked at as an well illustrated one to me:
https://ifstudies.org/blog/more-money-more-babies-whats-the-...
Here is the percent of women in each income bracket who gave birth in the past year by ascending income per household member (their choice of data, not mine - I would prefer completed fertility):
7.96 7.51 6.39 5.14 4.47 3.78 3.18
Here is the percent by total household income in ascending order of income:
6.27 5.23 5.64 5.88 5.26 5.30 5.26 4.98 4.64 4.75
In both cases there is a practically linear, and sharp, inverse correlation between income and fertility. I have no idea how they derived their graph as that data does not seem to be directly provided, but there's no combination of the lines in their graphs that would yield these data as an average, so I suspect they made a mistake.
I would not dispute that there is a U curved shape to fertility, but it's misleading as the tail end is in extremely high incomes. And in any case, their graphs look more like some sort of messed up sine waves, which is obviously just wrong!
[1] - https://www.census.gov/data/tables/2022/demo/fertility/women...
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EDIT: Actually I have a hypothesis. The census tables give an extremely useless datum that's very easy to misinterpret. The column is simply labeled "percent". It waxes and wanes all over the place, very much like their graph does. But it's the percent of all births that came from a given income group. But that is completely meaningless, because what matters is the data I gave (and had to manually calculate - by adding a new column) which is the percent of each group that is having children. Otherwise you're graphing some bastardization of population size at each percentile (a bell curve) multiplied by a pseudo-randomizing linear decreasing factor (fertility). So you get a graph that looks weird and makes no sense.
In the end what is successful is high-functioning sociopaths, that we can find in all high-power places nowadays. Funnily enough, inequality has increased, and now the wealth of your parents determines your life trajectory much more than even 30 years ago. This is really not surprising, once the exploitative sociopath gets in power, they really don't like fair competition that much.
And this is why both IQ and fertility is going down, it is not a future oriented system. It has a lot to do with the capitalist mindset, where we exploit the earth to the point of making it inhabitable at some point...