Faced with the dilemma of the high number of girls in their early teens who are becoming pregnant, experts have come forward with a theory that these young mothers are responding to a pattern in human evolution that induces people growing up in extremely stressful circumstances to bear children early and often.
The theory has received considerable attention and criticism.
Drawing on sociobiology, the theory holds that teen-age mothers -- especially in America's inner cities -- are implementing a reproductive strategy that from an evolutionary viewpoint is a smart bet.
Children who grow up in dangerous conditions, the theory holds, are primed to boost the chances of having their genes survive into the next generation by choosing earlier sex, earlier motherhood and more children. In Baltimore, for example, teen mothers account for nearly a quarter of the city's births, one of the highest percentages for teen births in the nation.
One prediction of the theory is that girls who grow up in householdswhere there is great emotional stress, and especially where the father is absent, will undergo puberty at an earlier age than other girls.
"We propose that the time of puberty is regulated and influenced to some extent by these earlier experiences, rather than being a fixed biological given," said Dr. Jay Belsky, a psychologist at Pennsylvania State University and the leading formulator of the theory.
The main factors in determining the age of menarche, which is when a girl begins to menstruate, are health and nutrition. Dr. John Tanner, a pediatrician at the University of London, estimates that in 1840 the average age of menarche in England was about 17 years.
By 1900, as health and nutrition improved in advanced countries, it had dropped to 15, and by 1960 to 13. In countries like the United States today, the average age at menarche is between 12 and 13 years.
While factors such as genetics, health and nutrition can account for differences of years in the age of menarche, childhood experiences -- such as fights between divorcing parents -- might account for just a few months' difference, in Dr. Belsky's view.
Still, the notion that such experiences should make any difference at all is a radical one.
"It's a small effect biologically, but quite meaningful, since the timing of menarche is so highly biologically fixed that no one had thought there was room for psychological influences at all," said Dr. Laurence Steinberg, a psychologist at Temple University and colleague of Dr. Belsky. "Even a small psychological effect on a biological system like this is surprising."
The theory is receiving wide attention among psychologists and sociologists. Next month, it will be the lead article in Child Development, the most prestigious journal among developmental psychologists.
The theory is already under attack. "A lot of us don't agree with the theory, even though it's gotten lots of play," said Dr. Jean Brooks-Gunn, a professor of child development at Columbia University.
Central to the new theory is the notion that there are two grand strategies to reproductive success in humans.
In stressful conditions, Dr. Belsky said, evolution has primed humans to pursue a "quantity, rather than quality, reproductive strategy. Earlier puberty is part of this plan."
He added, "It's not a conscious, deliberate strategy. It's a biological response. From an evolutionary perspective, life is about dispersing your genes into the next generation."
On the other hand, when such stress is absent, it may be advantageous in evolutionary terms "to take more time and search out a better mate, have fewer kids and invest more in each one," he said.
In the Child Development article, Dr. Belsky and his colleagues cite dozens of studies that support these broad links, which are generally accepted among experts on child development.
"It's part of a reproductive strategy that was useful in evolution, but is no longer so," Dr. Belsky said.
"When a child learns that the world is insecure and risky, the biological response is to get into reproduction sooner. Because the danger is that if you don't, you won't reproduce at all."
In an article to be published later this year in Child Development, Terrie Moffit, a psychologist at the University of Wisconsin, with Dr. Belsky and others, will report data that fit the theory's predictions.
In analyzing data from a study of 501 girls in New Zealand, the psychologist found that of the 81 girls who had the onset of puberty at 12 or earlier, 40 had grown up in households where the father was absent for many years, most because of divorce. Of the 82 girls who did not undergo puberty until age 14 or later, only 20 had fathers who were absent.