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...
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...