SHRINKING BRAIN SIZE, PAEDOMORPHIC SELECTION, OR FASTER REPRODUCTION OF THE UNDERCLASSES?
Given that (a) yes, brain size makes a demonstrable difference in current human intelligence.
And given that (b) it’s unclear whether our brains are getting smaller or whether more of the smaller humans are breeding in cities. (which appears to be the case since cities are genetic and IQ sinks).
And given that (c) we have been selecting for immaturity for 10K years (paedomorphic selection).
And given that (d) women generally possess superior verbal skills and weaker modeling skills despite smaller brains.
I would hypothesize that all of the above are driving down AVERAGE brain size.
I would suggest we will saw additional paedomorphic selection domestication up and until the period of ‘Peak Human’ which I suspect was around 1500 bc.
And that at present we are seeing a rapid expansion of dysgenic classes, tilting the data dramatically.
Unfortunately I suspect that the initial shift caused by the adoption of general rules (science) rather than rules of specific cases (recipes), as well as the reduction in the malnutrition at the bottom (left) of the curve, means that the downward shift in genetic IQ and brain size from underclass reproduction is being hidden by the Flynn effect. But in real terms, on average we are getting smaller, more verbal brains, and a lot more dumb people breeding.