Children, Parents Drive Each Other to Early Graves

When Penn and Smith examined their data, a clear and unmistakable trend stood out. The bigger the family, the smaller the chance that the parents would live into old age. Both mothers and fathers paid a price for having lots of children, with mothers always paying more, regardless of family size.

For example, 1.5 percent of mothers who bore one to three children were dead within a year of the last child, but only 0.4 percent of fathers were. Among women who had 12 or more babies, 6 percent were dead within a year of the last birth, compared with 2.5 percent of men.

Big families were hard on children, too. Twenty percent of children in the largest families died before age 18, compared with 10 percent in the smallest. About 15 percent of first-born children died by 18, compared with nearly 25 percent of 12th-borns.

Why would being in a large family, or being at the end of the birth order, be hazardous to a child's health? One reason is that those children are more likely to have their mother die -- and small children without mothers are more likely to die themselves.

Children who lost a mother before their fifth birthdays had a 78 percent higher chance of dying before they turned age 18 than children whose mothers survived. The same effect was seen -- again, less dramatically -- after the death of a father. Children who lost a father by age 5 had a 14 percent higher risk of dying in childhood.

The findings also provide an explanation for menopause, which ends a woman's reproductive capacity, but not her mate's.

Natural selection, the engine of evolution, favors traits that allow organisms to produce more offspring that survive to produce offspring of their own. For many species there is evolutionary "pressure" to reproduce early, have large and frequent batches of offspring, and to stay fecund for a long time.

That's the case with guppies, for instance.

David N. Reznick, a biologist at the University of California at Riverside, has shown that wild populations of guppies in Trinidad have different "reproductive life spans" depending on whether they live in water with lots of predators or few. The ones in high-predation environments have the genetic capacity to reproduce longer, apparently because the traits of wariness and agility that allow them to escape being eaten also happen make them more fertile. However, once fertility disappears, all guppies die quickly. They don't rear their young, and consequently there is no reason for natural selection to favor those that survive after they stop having offspring.

Not so for Homo sapiens.

Every human child has a stake in his or her mother's survival. Every mother has a stake in her children's survival. Menopause appears to be a way of protecting a mother's life and helping ensure she will live long enough to raise her last child to reproductive age.

So what might be the mechanism by which child-rearing erodes parents' longevity? The answer must involve basic physiology, because it occurs in both sexes and in women who survive childbirth.

One theory is that physical and psychological stress causes people to grow old before their time.

As cells age, chromosomes, where genetic information is stored, lose material from their ends, the DNA-protein structures called "telomeres." When telomeres get too short, a cell can't divide any more. It becomes senescent, or terminally old.

A study published in 2004 by Elissa S. Epel of the University of California at San Francisco measured telomere length in 39 mothers who were caring for children with chronic illnesses and 19 mothers raising healthy ones. She found that among the mothers of the sick children, the longer a woman had cared for her child, the shorter her telomeres. This was true even after adjusting for the telomere shortening that comes purely with age.

Between the women with the highest and lowest scores on a test of psychological stress, telomere lengths differed as much as between people 10 years apart in age.