According to this Reuter’s article, women whose mothers live longer have more children and have them earlier. That would mean that bigger families happen to people whose moms survive a long time. Another thing that the article said is that grandmothers tend to die when their grandchildren start having children–when their role as helper is gone.
I think it is interesting that the article says they have findings for this. My mother’s mother died when my children were 2 and 3. I was the first of the grandchildren to have children. My father’s mother died long after the first grandchildren were having children. And her mother died when she was just two or three. Yet she had six children.
So it may be true, but it hasn’t held true in my family.
My mother’s grandmother was alive until I was through college, long after her daughter had children.
But I think the idea has impact on my novel that I am working on. Or, more truthfully, that I have been working on.