Infertility accumulates over multiple generations because of recombination:
A population of modest size is stable over very long times.  Just about every little village recorded in the Domesday Book is still present today.  Australian Aborigines have evolved a system of interrelated roving bands that have persisted for tens of thousands of years.

The fertility of the whole world is declining, not over a single generation but over three or four generations as shown by the UN statistics.  The studies in Denmark and Iceland show the mating choice of couples affecting not only the number of their children but the number of their grandchildren.  The computer model shows infertility accumulating over many generations in a large population.  The survival ability of empires declines over a period of roughly 10 generations.

The computer program does not model recombination at this time.  But the program is grossly oversimplified.  And it does not faithfully predict everything that we can see in the real world.  It does an excellent job for the big picture, but perhaps it could do better.

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