What Good Is Grandma?

David Barash:

These circumstances can be stressful for youngsters, their parents, and their grandparents. But there is another way of looking at it: Today’s grandparents are doing exactly what their biology has prepared them for.

In pretty much all other species, individuals reproduce until they can’t anymore, and nearly always, this coincides with the end of their lives. Human beings are extraordinary, by contrast, in that long after we’ve stopped reproducing, we just keep on living. In particular, we appear to be the only animals in which half the population loses the ability to reproduce, through menopause, while they still have roughly one-third of their lives ahead of them, much of it quite healthy.

Natural selection rewards reproduction, and the mathematics show that a woman who produces an additional child will be favored over one who stops ovulating. (Reproduction offers the biological equivalent of compound interest; having just one more offspring means the potential of a whole lot of enhanced fitness, so selection should always favor giving it one more try, even if the would-be mother dies in the attempt.) As a result, the existence of menopause as a species-wide trait presents a species-wide mystery. Why don’t people, like other perfectly good mammals, keep on reproducing until they die? Or, another way of looking at it, why do women stop reproducing in late middle age, and yet keep on living? The answer that is emerging from a productive confluence of evolutionary theory and anthropology is that these old people—too old to reproduce, too young and healthy to kick the bucket—are highly biologically relevant.3