The systems theorist Niklas Luhmann defined modernity in terms of functional differentiation - that social functions become increasingly specialized and autonomous (yet coordinated) - for example the division of labour in a factory, or the economy; or the specialization of philosophy from theology, science from philosophy, biology from science, and zoology from biology.
When exactly this differentiation began is difficult to be sure, but clearly it accelerated from the industrial revolution.
However, shortly afterwards it seems that the biological specialization between human beings may have begun to undergo accelerating de-differentiation.
The theory runs like this: for almost all of human history, there was a very powerful selection pressure acting upon infants and children - far more babies reached advanced pregnancy and were born than survived to adulthood and themselves reproduced (this especially applied to males, among whom losses were greater at every stage - unless this pattern was specifically affected by contrary social practices).
But from the industrial revolution onwards, this stopped happening. A greater and greater proportion of infants reached adulthood, until in some parts of the world it approached 100 percent - and (probably) the major selection pressure of human history all-but ceased to operate.
Since each human is born with a few new and damaging genetic mutations, this implies a generation-upon-generation accumulation of deleterious genes.
What effect would this have? Well, as Geoffrey Miller has described - since about half of genes are involved in brain function, the brain is a huge 'mutational target' - so deleterious mutations probably affect brain function more than anything else.
This, I guess, is a major cause for the substantial decline in intelligence (measured objectively by simple reaction times) since the industrial revolution. (I am assuming that intelligence is something related to speed and efficiency of central nervous system processing; impaired by the same process that impairs reaction times).
But another affect would (I think) be a reduction in specialization; a reduction in the differences between people in terms of their specialized abilities - a reduction in high levels of specific excellence.
This would mean that differences between people in terms of functional ability would become more a matter of random variation (merely due to the deleterious effects of genetic mutations, pulling each person down from optimal functioning in different ways and by different degrees simply according to the actions of these mutations); and less a matter of different people having different specifically evolved adaptations.
So, it may be that the same broad period of human history - the modern era, post-industrial revolution - has been witnessing an increase in social complexity (differentiation) but a decrease in biological complexity (of differences between people related to specialized human abilities).
And perhaps that the reduction in biological complexity is now affecting social complexity - first causing a plateau in social complexity, then a decline in social complexity - a simplifying, a de-differentiation of society - for example in the imposition of crude political ideas over all functional social systems (ie political correctness/ New Leftism).