Grandchildren and Evolutionary Fitness

This explains a lot:

To begin with, we need to see that achieving reproductive success is not just about maintaining high birth rates: it is no good producing children every year if they die in infancy because it is impossible to care for several of them at the same time. In fact, a better measure of an individual's reproductive success is not the number of children but the number of grandchildren. That is to say, one must not only produce children, but also ensure that they do not die in childhood. It is necessary to invest in their upbringing, so that they in turn become successful adults who have a high probability of reproducing their parents' success.

Steven Shennan, Genes, Memes, and Human History