There's a lot nitpickers and economists alike could find to complain about in the over-the-hill population. They continue to drive when their reflexes have deteriorated. They're stubborn and set in their ways. They're a tax on the health care system and the younger members of their family. Some of them dress funny. And others may have taken the youth-obsessed culture a little bit too much to heart and have dressed up like teenagers. And at worse, they regress back to the point of infantilism. Many of these reasons, though, can be thought of as fears about aging. People don't want to lose control. They don't want look into that mirror of the inevitable future.
But older people aren't sacks of potatoes with a mouth. They're people who have been around for a while. They're wiser and if not that, at least they have more experience. They're living bits of history--something a textbook or a video could never replicate. But is this really the reason that we go on living for a couple more decades after our reproductive fitness has declined to zero?
Other animals don't have the benefit of social security checks, but that doesn't explain why after they produce their last offspring, they soon die. In the extreme case, there are insects that emerge into their adult forms for one day. They mate like mad, lay the eggs, then die. There is no such thing as old age for our six-legged friends. One theory on why humans have a longer life much past the reproductive peak is that it confers survival advantage to an individuals offspring. This makes sense if one thinks about it--some insects don't take care of their offspring, instead they let their eggs fend for themselves. Humans on the other hand are more social animals and take care of their offspring for far longer. First hand evidence are college kids. Theoretically, they're old enough to go out into the world by themselves, but in reality most of them still have some sort of financial support from their parents.
In a recent Nature article by Lahdenpera et al. (PubMed abstract) this group explores the fitness benefits conferred to the offspring of post-reproductive women. The grandmother effect was first proposed by George C. Williams in 1957 (Evolution, Vol. 11, No. 4, pp. 398-411):
In the human male and in both sexes of other animals, reproductive decline is a gradual process, as is the senecence of other systems. In the human female, however, it is rather abrupt, and some special explanation is required. At some point during human evolution it may have become advantageous for a woman of forty-five or fifty to stop dividing her declining faculties between the care of extant offspring and the production of new ones. A termination of increasingly hazardous pregnancies would enable her to devote her whole remaining energy to the care of her living children, and would remove childbirth mortality as a possible cause for failure to raise these children. Menopause, although apparently a cessation of reproduction, may have arisen as a reproductive adaptation to a life-cycle already characterized by senescence, unusual hazards in pregnancy and childbrith, and a long period of juvenile dependence. If so, it is improper to regard menopause as a part of the aging syndrome.
Lahdenpera et al. propose that the grandmothers become "helpers" by both taking care of their children and grandchildren, thus giving both survival advantages. They give experimental evidence to the theory through statistical sampling of two pre-modern populations in Canada and Finland. Controls for various factors such as age, offspring sex, geography, socio-economic status were taken into account by using a general linear model. From the data, both sons and daughters who had mothers living past menopause were able to raise more children to adulthood independent of wealth. However, less grandchildren were born if the grandmother did not live in the same place and a grandmother's beneficial effect to survival did not kick in until after the child was weaned.
Well, what about men? By just eyeballing some numbers, we can immediately see that men don't live as long as women (although there are articles trumpeting that the lifespan gap is narrowing). Williams also mentions this in his paper. Unfortunately, there is no "grandmother effect" equivalent in males because they are exposed to more risks such as fighting each other and spending too much energy on courtship displays.
Cross-posted at Syaffolee.