Waiting to have a baby: study suggests gloomy implications

ANN ARBOR—As more women delay having babies to finish their educations and establish their careers, a University of Michigan researcher identifies an unexpected long-term implication of waiting: likely eventual extinction of the older mother's lineage.

With so much concern about the difficulty of adjusting the biological clock to bear a child later in life, the future of descendants may not seem important to many people. But for those interested in their family's end-game evolutionary success as well as near-term parenthood, reproducing late in life is apparently a no-win strategy.

"We older moms are going extinct," says Bobbi Low, a behavioral ecologist who has just been appointed to direct the Evolution and Human Adaptation Program at the U-M Institute for Social Research (ISR). With U-M collaborators Carl Simon and Kermyt Anderson, Low has been using statistical models to determine how much advantages in education and income compensate for delayed first births and lower lifetime fertility a few hundred years down the road. The analyses appear in a recent article in the American Journal of Human Biology and a chapter in the forthcoming book, "The Biodemography of Fertility."

"In any species, other things being equal, whoever keeps their family line going and growing, persists while others go extinct," explains Low, the author of "Why Sex Matters" and a professor at the U-M School of Natural Resources and Environment. "In many cases, including that of humans in the past, that is best accomplished by making the most babies. But in modern societies, the number of kids is no longer the name of the game. The environment is so competitive that only 'super kids' do well. So women, quite reasonably, have started to shift from offering just reproductive value in the 'mate market' to offering a combination of reproductive and resource value—not just youth and good looks, but a good education and a good job."

The problem is, Low's analyses suggest, modern women may have gone too far, trading off so much to amass resources that they have lost the long-term game—evolutionary survival of their descendants. For the analyses, Low, Simon and Anderson ran a series of complex calculations simulating the varying life paths of modern women who have the same number of children but start at different ages and with differing levels of social, human, and physical capital. In the model, the women are born into one of nine socioeconomic levels, and throughout their lives, may find themselves in one of more than 900 different conditions. In each round of the model, roughly analogous to a five-year period, women die or move to the next older age level. At each age level, they are assigned some probability of going to school, having a child, or working.

After running the model for a period of time equivalent to roughly 220 years, they found that wealthy, late-reproducing women declined as a proportion of the population from 11 percent to about 5 percent. The proportion of the poorest women also declined. But the proportion of lower middle class women increased dramatically, from about 33 percent to about 60 percent. "Their relatively early reproduction and good childhood and adult survivorship combine to produce this result," says Low. "Under almost all conditions in the models, we found that reproduction in the early 20s led to the greatest lineage success for women."

Low, who gave birth to her first and only child when she was in her mid-30s, emphasizes that lineage success may not be an important goal for many modern women. "If you want to see your line persist, then it's probably optimum to start reproducing in your early to mid-20s," she says. "But if you want to have a wonderful lifestyle, you're probably better off either not having children or having them as late as possible."