Many people who pride themselves on being tough-minded acknowledge the reality of differences in human intelligence, and have speculated on the evolutionary causes.
Such speculations are usually framed in terms of the reproductive advantages of high intelligence; yet the actual historical reality was probably more a matter of the lethality of low intelligence.
The best worked-out example of differential intelligence is probably Cochran, Hardy and Harpending's theory of Ashkenazim high intelligence
In a positive sense, this was a matter of those with highest intelligence having the highest reproductive success due to the economic benefits of working as money lenders and the like.
But, on a second look, what they are mostly saying is that the selection pressure of medieval central Europe was such that Jews who were of low intelligence (and unable to do the job of money lender) were eliminated.
It is quite likely the same story wherever high intelligence was selected: the low intelligence people mostly died early from lethal accidents in societies too complex for them to understand
Or they died as unintelligent infants because their unintelligent mothers were unable to rear them in complex societies:
Or, in more general terms, they died of starvation and disease because they were unable to make the transition between low-tech nomadic hunter-gatherer lifeways and the more complex and elaborately-tooled high latitude H-G or later agricultural societies.
So, it is probable that high intelligence was mostly a consequence of natural selection culling those who were of lowest intelligence by accidents, diseases and starvation.
This apparently implies that the substantial decline of intelligence since the industrial revolution
is most likely (as a first hypothesis) due to to lower mortality rates among the least intelligent, resulting in cessation of the medieval societies 'culling' of lowest IQ persons.
If so, then this presents a much harsher view of matters than generally acknowledged.
It is reasonable to suppose, on current evidence, that the only sure and certain way that general intelligence could have been maintained at pre-1800 levels, was by humans having continued to live under the kind of high mortality rate selection pressure which killed off many or most of those with the lowest intelligence.
When society became less harsh, and mortality rates of the unintelligent declined from their near-total levels of medieval society, a substantial decline in general intelligence was inevitable.
It is plausible further to suppose that one consequence of this decline from pre-1800 levels of 'g' will inevitably be the decline of that post-industrial revolution civilization; since innovation will slow, stop, then go into reverse.
So medieval selection against low intelligence led to industrialization, which relaxed seleection against low intelligence, which relaxation will destroy industrialization.
If so, then modernity was a self-limiting 'blip' in world history; and we should anticipate a return either to the kind of harsh selection for higher intelligence; or else to go back to the simple hunter-gatherer organization; when average intelligence was lower than in agricultural societies, and (what we would think of as) very low general intelligence was not a overall disadvantage to reproductive success.