> You're assuming this is entirely genetic. Not at all. I'm merely assuming that there is a non-zero genetic component. > The drop in birth rate since the ind

> You're assuming this is entirely genetic. Not at all. I'm merely assuming that there is a non-zero genetic component. > The drop in birth rate since the industrial era has nothing to do with genes, and everything to do with urbanization, at least from where I'm standing. :) I completely agree. Well, urbanization and other, related environmental factors (even farmers have fewer kids than before). When you change environment for any species, there is a possibility that reproduction rates go down, sometimes radically. For instance, if you put cheetahs in a zoo, you're not likely to get many cubs. From a reproductive perspective, a zoo is a very hostile environment for a cheetah, even if they are guaranteed enough food, a qualified vet and have no risk of being killed by lions. Still, over time (and unless the population goes to zero), when facing an environmental change that is hostile in the short term, the species will adapt to the new environment. When this adaptation only requires a change in relative frequency of already existing genes in the genepool, this can go very quickly. (Which is perhaps the greatest benefit of sexual reproduction.) If it requires a string of mutation, it can take a very long time.