Children, Parents Drive Each Other to Early Graves
Monday, January 15, 2007
What exasperated or overworked parent hasn't declared to a child at least once: "You'll be the death of me!"
Now we know -- with unprecedented precision -- just how true that can be.
A pair of researchers, drawing on the experience of nearly 22,000 couples in the 19th century -- has measured the "fitness cost" of human reproduction. This is the price that parents pay in their own health and longevity for the privilege of having their genes live on in future generations. The findings, published last month in the Proceedings of the National Academy of Sciences, manage to be both predictable and surprising.
Not surprisingly, women paid a bigger price than men. Older mothers were four times as likely to die in the year after having a child than their mates. Having lots of children was especially risky. A mother of 12 had five times the risk of dying prematurely as a mother of three. Even after their child-bearing years came to an end, women who had had many children died earlier than women who had had few.
The price of parenthood wasn't trivial for men, either. Despite the obvious fact that men avoided the hazards of childbirth, fathering more children meant more risk of dying before their time, too.
And it wasn't only parents who paid the "fitness cost" of reproduction.
The later-born children in very large families had less chance than their older brothers and sisters of surviving into adulthood and having children themselves. Losing a mother raised every child's risk of dying young.
The findings begin to provide for human beings what's been learned about fruit flies, guppies and mice -- namely, a measure of the trade-offs between unchecked procreation and individual survival in evolution's calculus.
As raw material, the researchers, Dustin J. Penn of the Austrian Academy of Sciences in Vienna and Ken R. Smith of the University of Utah, used a database of genealogical information kept by the Church of Jesus Christ of Latter-day Saints in Salt Lake City. They examined the reproductive history and survival of 21,684 couples married between 1860 and 1895. Each person was married only once, and polygamists were excluded. (Polygamy was outlawed when Utah became a state in 1896).
Altogether, the couples bore nearly 175,000 children -- slightly more than eight each. The women's average age at the birth of the last child was 39 years. About 1,400 of them died within a year of delivering their last baby, and 2,400 within five years. For men, the corresponding numbers were about 600 and 1,700. About 18 percent of the children died by age 18.
The data sample is the largest used to estimate the cost of human procreation. It covers a wide spectrum of society -- most men in previous studies were aristocrats -- and a period largely before modern hygiene and medicine greatly reduced maternal and childhood mortality.
"These are basically pioneers, and the mortality is probably more like what it was before the Industrial Revolution," said Penn, who heads the Konrad Lorenz Institute for Ethology but had previously been at Utah.