It would make sense why in Europe lower classes might leave fewer descendants; i.e. higher child mortality rate, or later on a desire not to have more children than they could feed.
Yet, the authors say child mortality wasn't a factor, or perhaps it tended to affect all the classes.
It's interesting that they say fertility issues were also a factor. We have a tendency to think fertility issues are a modern development, but perhaps they've always been with us, and in the past there was no access to shots to stimulate ovulation, or medicines to promote retention of the fetus, or in vitro fertilization.
I also wonder if there was a difference among the lower classes once welfare laws came into effect. Once people knew they could feed their children, did they then not want more, or some children, or did the fact that birth control was available around the same time just mean they decided to have fewer children and spend the money on themselves?
That isn't in fact what's happening in the U.S., where the poorest have the most children, i.e. often minority underclass groups.
For the higher classes or even just people who owned their own land, perhaps people knew of ways to limit the number of children they had, or maybe there was a desire not to divide up the patrimony or spend too much on dowries.
That happened in multiple generations of my family on both sides. The families were huge, but many didn't marry and stayed on the property basically as unpaid labor.
Yet, the authors seem to propose that the middle classes, the farmers, were the ones who left more descendants. That doesn't comport with what happened in my own family, although it makes sense that farmers would want more hands to work the farm. As I said, though, in our culture that meant some of the children didn't marry. Still, if you have 11 and 5 or 6 reproduce, you'll still leave more descendants then if you have 3 and only two or even 1 reproduce.