No, I'm saying that both personality and material conditions have an effect on who has children. Or it might be that the personality traits that cause people t

No, I'm saying that both personality and material conditions have an effect on who has children. Or it might be that the personality traits that cause people to have children in 21st century America are different than those that caused people to have children in 19th century America. But in any event the traits that seem to cause people to have more children in current US conditions are substantially heritable. How many generations will it take? Quite possibly a lot but it'll happen eventually baring further external shocks. And there are lots of other factors involved as well. I think the differences in fertility between Germany and France are particularly illustrative.