The idea, culminating here:
Is that the 'dysgenics' discourse may have been focusing on the wrong topic - too much focus on fertility, and neglect
Natural selection depends on both fertility/ birth rates and mortality/ death rates. Babies are born, but reproductive success depends on how many of these babies reach sexual maturity and live long enough to have offspring.
Those interested in dysgenics have focused on the fact that the less intelligent (and those with less socially desirable personality traits) are more fertile than the more intelligent and more conscientious, self-controlled and altruistic.
But the primary fact is, I believe, reduced mortality - the situation that since 1800, more and more of the babies that are born will live to have offspring.
Every new baby is likely to have one or two new spontaneous deleterious gene mutations - some lethal, some only mildly harmful and others in between - they may also inherit mutations from their parents.
Why doesn't this genetic damage mount up, generation upon generation, to overwhelm and destroy the fitness of the species?
Essentially, under 'natural conditions' because the mutations are filtered-out by high child mortality rates.
Through most of evolutionary history, most babies and children (probably a large majority of them) especially those with the worst genetic damage - have died before reproducing. Thus mutation load is filtered by differential child mortality rates with each generation.
Those who have the least genetic damage are the healthiest and best adapted, and only they will (on average) be the parents of the next generation.
If it is correct that - up to about 1800 - in almost all situations almost all babies died (and maybe only the 'fittest' 15 percent or so survived - or the fittest thirty percent... the exact number makes little difference).
But since 1800, starting in England then incrementally spreading across the whole world with no exceptions, child mortality rates have got lower, and lower; the mutation filtering effect has got less and less complete - and the mutation load has got greater with each generation.
This must have happened. The only question is how much?
So, the primary mechanism of dysgenic change is mutation accumulation. But this is where more usual dysgenic topic of differential fertility comes in.
Theoretical calculations suggest that the effect of mutation accumulation under these circumstances would be quite slow - IF normal natural selection was in play and higher levels of mutation accumulation led to lower fertility.
But in the modern world, the direction of natural selection has reversed. In the modern world, higher levels of mutation accumulation lead to higher fertility (so long as the mutations are sub-lethal).
In historical times, natural selection filtered-out mutations; but in the modern world natural selection amplifies the carriers of damaging mutations!
(Up to a point, where pathology more-or-less prevents reproduction), the carriers of accumulated deleterious mutations ('mutants') - probably both between and also within populations, have the highest fitness, on average.
Indeed, in the modern world, natural selection actually filters-out undamaged genomes - since the people with the least damaged genomes are least likely to have offspring.
The best genetic specimens have sub-replacement fertility - the (ever fewer) least damaged genomes are being actively eliminated by reproductive choices: this applies especially to women more than men.
The above is why things are happening so very fast - e.g. why general intelligence is declining so rapidly (as revealed by simple reaction time slowing).
What is now needed is to gather more data to measure the long term rate of mutation accumulation.