The "reverse Flynn effect" is something that's been observed in a small number of Western countries that do IQ tests as part of their military conscription.

But for example, in Denmark, when plotting measured IQ against first name, we observe that there is cultural effect, that would explain that the "reverse Flynn effect" is simply an averaging down, caused by the import of migrants from lower IQ regions. The "reverse Flynn effect" is not observed in Singapore, for example, which has maintained consistent high IQ over many years.

https://emilkirkegaard.dk/en/2019/06/article-out-first-names...

Fascinating. I'm reminded of the 'conservation of ninjutsu' trope. We've known for a while that birth rates fall as education rates increase, but this almost suggests there's a 'conservation of iq' principle at a population level- you can have growing population or iq, but not both!