Knitting together strands from economic development, genealogy, assortative mating, and the work of Gregory Clark.
There is a perception that in modern societies we are seeing a rise in mental and psychological conditions such as autism, ADHD, depression, etc..
There is, however, also debate as to whether we are really seeing a rise in the incident rate of such conditions. It is possible that the rate has stayed the same all along but that in the past, before the industrial revolution, 95% of the population was always at the knife's edge of survival and that people with such conditions in the past simply died at young ages and therefore were not a great presence in the surviving adult population.
Alternatively, the rate may have also stayed the same and the only difference is that we now have the diagnostic infrastructure in place to identify that which was previously ignored.
Related, and I am inclined in this direction, the infrastructure that has developed for diagnosis has also generated incentives for diagnosis. I.e. there may be an increase in diagnoses but the difference between the new rate and the long term rate is primarily false-positives.
But let's assume into existence, for speculative argument, a real increase in psycho-neurological conditions. What might be the cause. We have the celebrity contingent arguing that it is a consequence of vaccines but the empirical evidence for that is weak or at best mixed.
Others assume that there is some tie to changes in societal structure, urbanization, or pollutants arising from the industrial revolution. All plausible but the evidence is, again, at best mixed.
I wonder, if the rate rise is real, whether it might be a result of recent and persistent assortative mating based on cognitive ability.
We know with some confidence that this has happened with Ashkenazi Jews (not all Jewry). Their own religious and cultural pressures strongly selected for high cognitive performance. In Europe in the Middle Ages, these pressures were accompanied by exogenous host culture pressures which precluded Jews from many economic activities and forcing them into high cognitive professions. The combination of internal and external selection pressures led to a standard deviation rise in cognitive capability (IQ 115 versus norm of 100).
However, this selection pressure also has been associated with an increase in a range of rare conditions such as Tay-Sachs disease, Familial Dysautonomia and others. There is dispute as to exactly why these range of conditions manifest at such higher rates among Ashkenazis. It might perhaps simply be a founder effect. Others speculate, though, that it is associated with the pressures driving the cognitive shift.
That is the first speculation - that rises in cognitive selection are associated with related neurological and biological consequences as by-products of that selection.
There are two greater path shifts in the past five hundred years. 1500 (1497 technically) marked the logistical revolution with the first hesitant and intermittent trans-oceanic, trans-continental movements of peoples and goods. Hesitant, but from that point forwards on-going and accelerating.
The second great path shift occurred roughly two hundred and fifty years later around 1750 with the emergence of the industrial revolution in northwestern Europe.
The logistical revolution and the industrial revolution in combination with one another had transformative impacts. Populations gradually departed from the Malthusian norm whereby population always remained just on the edge of catastrophe and any improvement in productivity led to an unsustainable population increase which led to famine which led to a winnowing of the population.
Population levels exploded, people's living conditions became dramatically more dense (movement from the country to towns to cities). With market economies, people specialized their labor. Overall, the speed of everything increased dramatically, the complexity of everything, the uncertainty of everything. All of sudden there was a disproportionate selective value placed on pre-existing capabilities such as memory, observation, abstraction capability (pattern recognition), fluid intelligence as well as crystallized intelligence, processing speed, behavioral skills, self-discipline and self-control, etc.
If we take 1750 as a generalized ground zero where virtually all (exaggerated) the population is agriculturally based and distributed randomly across country with few and small concentrations of towns and villages, one might imagine that pair-bonding and mating selection is driven essentially by proximity.
However, in the 250 years since then, ten generations, the great majority of the population has moved from agriculture and rural distributed populations to industrial and services in towns and cities (80% of the population). Within the cities, with labor specialization and close proximity, there is for the first time, an opportunity for mass assortative mating based on cognitive capacity.
If it is true that strong selective pressures on cognitive capability drive increased neurological and morbidity effects, then it becomes reasonable to assume that 250 years of selective pressure might lead to an increase in autism, depression, neuroticism, ADHD, etc.
We still have the issue of whether the rate effect is real or not, but if we ever get that nailed down, then the cognitive selective pressures of 250 years of assortative mating associated with urbanization and densification should be testable at a population level. By nation and by family line, those that industrialized and urbanized earliest should show the greatest increases in autism, depression, neuroticism, ADHD, etc.
Regardless of the speculation, I observe that I have seen very little research on the worldwide genetic consequences arising from rapid urbanization and densification in just 250 years and the consequent effects of cognitive based assortative mating.