Bobbi Low, who has just been appointed to direct the Evolution and Human Adaptation Program at Michigan's Institute for Social Research, suggested that the longer a woman postpones childbearing past her 20s, the more likely she is to wind up with no direct descendents.

Low had her only child at 36 and said she has a five-year gap in her curriculum vitae.

"We older moms are going extinct," she told United Press International.

Low explained the relative costs and benefits of child-bearing strategies of women under various circumstances.

"In traditional societies, women's value to men was reproductive," she said. "Basically, a 14-year-old woman is worth more than a 15-year-old woman, who is worth more than a 16-year-old woman. ... And men's value to women was basically resources."

In her studies of parish records in 19th-century Sweden, Low found that land ownership was the best predictor of marriage among men — even among brothers.

"Men who owned land were 95 percent likely to get married, and non-landowning men were about 40 percent likely to get married," she said.

"Northern Europe in the 19th century had a lot of unmarried adults of both sexes," she added.

"Men didn't get married earlier depending on their status or wealth or occupation. But wealthy men married younger women — about 2.5 years younger than everybody else. Those women had the same inter-birth interval and age at last birth as other women, but they came out with one-and-a half more kids."

Transition

Things changed, however, when Sweden went through its "demographic transition." This is a change from high birth and death rates to low birth and death rates. At the end of the transition, total population grows slowly if at all.

In Western Europe and North America, it began in the 18th century and accelerated in the 19th century, when family size fell dramatically. Parts of the developing world are undergoing the transition today.

In the competitive environment of a post-transition society, the number of children is no longer the name of the game.

"You have to make superbabies," Low said. "Once the average family size goes from eight or 10 down to two, a woman's reproductive value is a fairly trivial concern. But the resources a woman can bring into a marriage become important."

Low said that in the short term it makes complete sense for a woman to switch from marrying at 16, having her first child at 18, and "cranking 'em out" till she's 40, to: "Let me become established; I can help get us set up; we'll have two kids."

But this might not make "biological sense" in terms of having descendants in succeeding generations. If a woman waits until her mid-30s or 40s to have her one or two children, Low said, "you really do lose out if there's any mortality in lineages at all. And there really are more physical, physiological problems in pulling off healthy kids when you're 40."

"Gambler’s ruin"

If a woman does not reproduce until late in her childbearing years, it increases the risk of what statisticians call "gambler's ruin." Just as too much bad luck can cost a casino's customer all his money, driving him permanently out of the game, a woman might end up with no direct descendents at all.

The researcher said that life history theorists have always known that there's an optimal time in any organism's life to give birth.

"If you have your kids too early, it takes away from your own ability to develop and get what you need. And you may not have enough to invest in them. So they may not get enough to get a good start in life. But if you wait too long, you lose out reproductively."

Low said it makes sense for poor, inner-city black women to have fairly early pregnancies.

"If they wait till they're 25, they have very few potential mates, their own health has deteriorated, their mother is likely to be dead — so they don't have built-in child care."

But it also means they have a difficult time breaking out of the cycle of poverty and ill health, and their long-term reproductive success may not be high.

She and colleagues Carl Simon and Kermyt Anderson made a model simulating the varying life paths of modern women who have the same number of children but start at different ages and with different levels of social, human and physical capital.

All women in the model had 1.05 daughters. The only difference was whether they had them between ages 16 and 20, 21 and 25, 26 and 30, or 31 and 35.

The researchers calculated how the lineages would grow, by applying published rates of sickness and death. The women who finish high school and have a daughter in their early 20s have the most great-great-great-great grandchildren.

"Let's make a situation where no babies ever die," Low said. "We're going to watch how many grandchildren the women have and how many great-grandchildren. It's the fact of exponential growth.

"In what I call the stark power of generation time, early-reproducing women swamped later-reproducing women."

This is independent of both the number of children a woman has and of mortality rates, she said.

Low uses the word "extinction" because when she plots the lineages of such women as herself, they disappear after a while.