Correlation is not causation. Beyond just the fear of "cancellation", which certainly exists, any good scientist would want to find an airtight causal mechanism before publishing a socially and morally disturbing statistic that would hark back to unscientific eugenics movements from before the discovery of DNA.

And, call me a canceller myself, but somehow I doubt any such causal mechanism would actually be intrinsic to the human populations themselves. Such a strongly disadvantageous mutation as decreased cognitive ability would get weeded out by natural selection: you don't spend 20% of the body's calorie budget on an organ that isn't doing its job.