The Flynn effect has its own little nurture vs nature debate within it.

Was it better medicine and food that stopped both your height and your brain from being stunted?

Or was it people being trained from birth for a world where doing abstract brain teaser tests was important.

Notably both cause problems for the typical racist's use of IQs. If you can improve the scores with such interventions it makes a lot of their genocidal policy recommendations seem less scientifically sound, so they put a lot of effort into denying that IQ scores can be improved by interventions. Even though they have been, for decades.