Why Teenage Girls Are Hitting Puberty At an Earlier Age

The onset of puberty may be an individual decision.

By Robert SapolskyJun 1, 1992 5:00 AM


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Oh, what an unmitigated horror puberty was. For girls, there was the inevitable first menstruation that came right in the middle of gym class: the bloody shorts, the panic--no one had told you you’d bleed that much--and the conviction that you were about to die of either embarrassment or hemorrhage. Training bras, pubic hair, acne; there were no end of disquieting changes.

For boys, there was the first wet dream, inevitably occurring at sleep-away camp--sneaking off to the woods to bury your disgusting soiled pajamas--the visible erection with a mind of its own in the swimming trunks, the voice that cracked whenever you spoke to someone of the opposite sex. Humiliation, agonies, pimply insecurities; you wondered if life would ever return to normal, and the answer, you knew, was no.

Most of all, there was the heartfelt prayer of every pubescent child: “Please, please, if I have to go through this, don’t let it happen to me one minute before or after it happens to everyone else. Please don’t let me be different.”

For the peer-pressured adolescent, the timing of puberty is everything. Indeed, for all mammals going about the business of being fit enough to survive and reproduce, the timing of puberty is pretty important. But some mammals do something about it. As social or environmental conditions shift--when there’s a change in the availability of food, for example, or in the number of animals competing for mates--many animals find that it sometimes pays to reach puberty at an earlier age, and it’s sometimes better to delay the process.

Now there’s evidence that humans, too, manipulate the timing of puberty’s onset. In a controversial paper published last fall, a team of researchers proposed that girls reach puberty at an earlier age if they are raised in homes filled with parental strife or where the father is absent because of divorce or abandonment.

The notion that our bodies can engage in such weighty deliberation shouldn’t be surprising. After all, we’re comfortable with the idea that some of our lives’ landmark events are decided strategically. We do it constantly. For example: “Oh, let’s not. What a terrible time this would be for me to get pregnant--you’re out of work, my dissertation research is bogged down, the Nazis are marching on Paris, and we’ll have to flee.” That sort of thing.

But such strategies are conscious. It’s harder to think about the timing of puberty as a strategy. We do not think, “The time is ripe--I’ve decided to start ovulating,” any more than a bamboo plant decides, “The rainfall’s been fabulous--I think I’ll start flowering,” or a deer decides, “Ah, spring is in the air--time to grow me some antlers.” The “thinking” and “deciding” do not really occur. Instead, a more complex evolutionary process is taking place: organisms that have the biological means to time an important life-history event (such as puberty or flowering or growing antlers) so that it occurs when the environment is best suited for it have an advantage over organisms that cannot do so; therefore more of them will survive, more will reproduce, and more will leave copies of their genes to future generations. And thus their adaptive timing mechanism will become more common in their species over the millennia.

When does it best make sense to go through puberty? Well, to start with, puberty is an energetically costly experience, so it probably makes sense to go through it early only if you’ve been fed well and you’re healthy and can afford the metabolic expense. It wouldn’t hurt to make sure that there is someone attractive around who is available to mate with, and it also helps if the environment is conducive to the survival of any children you may have. When is it logical to defer puberty? If things are bad enough that you need your energy to survive instead of to reproduce, if it makes sense to still be taken care of by Mom instead of becoming a mom yourself, if there’s no one around to mate with except close relatives.

Take, as an example, a female prepubescent mouse left on her own. Eventually, of course, she will reach puberty. But put an adult male into the cage with her and she will reach puberty earlier. This intriguing phenomenon, known as the Vandenbergh effect, is produced by pheromones-- chemicals that cause a behavioral response in other animals of the same species--in the male’s urine. These pheromones, when detected by a sensing organ in the roof of the female’s mouth, crank up her ovulatory machinery. The male’s urine does this trick even when the male isn’t present--just dab a smidgen of male urine on females and they reach puberty earlier. Moreover, the more male sexual hormones the mouse has in his bloodstream, the more effective he is at accelerating the onset of puberty.

Conversely, when a prepubescent female mouse is exposed to a large number of adult females, puberty is delayed. Once again the signal is a pheromone, but in this case it’s one found in the urine of the adult females. Block the young female’s sensing organ, and the puberty delay no longer occurs.

Researchers have speculated that these signals might play some role in regulating population density. For example, a large number of adult females probably indicate a dense population. The puberty-delaying pheromones serve as a brake on fecundity: if the population density is high enough, there will soon be food shortages, and why should a starving female waste energy on ovulating and getting pregnant when the odds of her carrying through a pregnancy are pretty small?

Such speculation is clearly supported by a study conducted by zoologists Adrianne Massey at the North Carolina Biotechnology Center and John Vandenbergh (of Vandenbergh effect fame) at North Carolina State University. The two researchers examined mouse populations in the acre or so of grass found in each loop of a highway cloverleaf out in the hills of North Carolina. Patches like these constitute “biogeographic islands,” closed mini-ecosystems where there are few mice immigrants (the ones who try to switch to a different cloverleaf patch usually get squashed by the cars on the road in between). Massey and Vandenbergh studied the fluctuation of mouse populations in each patch. When the population of a particular patch became large enough, the females began to make the puberty-delaying pheromone. And when the population dipped, the females stopped making this pheromone. Males indiscriminately made the puberty-accelerating odorant all the time, so when population density dropped and the delaying signals from the females were absent, puberty was accelerated. The bodies of prepubescent females were able to monitor the environment around them and choose wisely as to when to leap into the reproductive business.

Male mammals often show a similar savvy. Some antelopes, for example, such as gazelles and impalas, grow up in social groups in which a number of females and their offspring live with a single breeding male. Other mature males live either as wandering loners or in all-male bands, butting heads and honing their fighting skills for the moment when they stage a coup d’etat against the breeding male.

For a male growing up with his mother and the rest of the breeding group, puberty carries a stiff price. When an adolescent male begins to sprout pubescent “gender badges” such as horns, the breeding male perceives the youngster as a sexual rival and begins harassing him, driving him out of the group. The apron string is cut rather abruptly, with consequences that are not trivial--when males go out on their own, their risk of being eaten by predators soars. (It’s been theorized that the danger of a young male’s showing signs of maturity is what led females to sprout horns also: the female headgear could be an evolutionary dodge developed by protective mothers to draw attention away from their growing male offspring.)

What is a prepubescent male antelope to do? By reaching puberty too early, he is subject to the harassment of the dominant male when he may not yet be ready to survive the rigors of being driven from hearth and home. But by delaying puberty too long, he forgoes reproductive potential.

Clearly he needs to make a carefully considered decision--and apparently he does. Richard Estes of Harvard’s Museum of Comparative Zoology has examined the timing of puberty in antelopes and found that if the outside world looks pretty challenging and dangerous--a lot of other antelopes around to fight over food, not much food out there anyway, and a lot of lurking predators--the males delay the onset of puberty.

Similar strategizing occurs in primates. If you have ever watched orangutans in a zoo, you may have noted that the males seem to come in two varieties. One is short-haired, limber, and agile. The other is much heavier and lumbering, with odd, fatty cheek flanges, thick long hair, and a large muscular throat pouch. Primatologists studying orangutans in the wild always assumed that the more gracile form was simply an adolescent, the latter being the adult version. However, in zoo populations, some males retain the gracile form well into adulthood, even a decade later than normal. Invariably these are males living near an older, more socially dominant male. Some sort of signal given off by the dominant male makes the body of the subordinate delay the development of the adult secondary sexual features that give the heavier form its appearance. Take away the dominant male, and the eternally youthful subordinate rapidly develops all the mature traits.

None of this strategizing has yet been shown to occur in male humans, but psychologist Jay Belsky and anthropologist Patricia Draper of Penn State and psychologist Laurence Steinberg of Temple University have found something very much like it in females. Their argument is actually the flip side of the delaying tactic used by antelopes and orangutans. In a nutshell, their premiseis: When times are tough and life is unstable, it makes sense to hurry up, reach puberty, and start reproducing sooner, or it may be too late and you’ll die childless.

This thinking bears the mark of a certain style of ecology. One of the basic dichotomies made by ecologists is between “stable” species and “opportunistic” species. Stable species live in unchanging, predictable environments. They live long lives in large populations; they are large-bodied; and they lavish a lot of parental attention on relatively few offspring. They opt for quality versus quantity when it comes to reproduction; years later members of these species are still doing the kid’s laundry and paying tuition for chirping lessons. Compared with closely related species that don’t have as many of these traits, stable species tend to reach puberty later. For them there is no rush: they can afford to grow big and healthy before starting to invest all that energy in a few pregnancies. Think of elephants, with their 2-year pregnancies and 65-year life spans. Or ancient redwood trees. Or humans, a classic stable species.

In contrast are the opportunistic species, which live in ecosystems that are unstable and unpredictable--with long periods of severe weather, food shortages, and environments that are favorable only now and then. Animals that live near bodies of water that dry up seasonally or plants that grow best in soil just after a fire are opportunistic. They’re gamblers, living boom-or-bust existences. The population will be tiny for long periods. Then, when conditions suddenly become right, there’s tremendous pressure to take advantage of them fast, and wham--everyone starts mating or pollinating. Quantity over quality, reproduce as much as possible, don’t bother taking care of the kids, just flood the market with offspring while the going is good. Opportunistic species tend to be small-bodied and short-lived, their population size fluctuates wildly, and once conditions are favorable, they reach puberty rapidly. Think of rabbits reproducing like rabbits and invading some new region, or of some kind of scrubweed that pops up overnight in the disturbed soil on the edge of a construction site.

About a decade ago, Draper and her colleague Henry Harpending theorized that when girls are abandoned by their fathers during childhood, they grow up to become women who behave more like an opportunistic species. The girls have learned that males make unreliable parents, so they don’t bother searching for a reliable mate--they have sex at an earlier age, have more sexual partners, more kids, and are more likely to have rocky marriages.

When Draper and Harpending proposed their theory, it was already well known that women who grow up in unstable homes are more likely to behave this way--the novel thing about Draper and Harpending’s work was that they tried to explain this pattern from an ecological-evolutionary perspective rather than a psychological or socioeconomic one. Now Draper, Belsky, and Steinberg have built on this approach by making the controversial suggestion that in unstable settings it’s not only the behavior of such girls that becomes more opportunistic, it’s their bodies as well, and so the girls reach puberty earlier.

A handful of studies supports this idea. In two studies led by Belsky and one by Steinberg, girls who reported more strained relations with their parents, and girls raised in homes with persistent family conflict reached puberty earlier than girls from more contented families. In a detailed 1990 study, Michele Surbey of Mount Allison University in Canada (who at the time proposed a theory quite close to that of Belsky and colleagues) found that girls raised with the father absent because of divorce or abandonment also had earlier puberty. An Australian research group had found the same thing in 1972. Among all the various studies the effect was pretty small--on average, puberty was accelerated by five months.

The various authors wrestle a bit with potential physiological mechanisms for this effect (evolutionary theory is concerned with why puberty might be accelerated; physiological mechanisms, the nuts and bolts of hormones and nervous system, explain how puberty might be accelerated). Surbey, citing animal pheromone studies, hypothesizes that biological fathers might release a puberty-delaying chemical, perhaps an airborne one sensed by the nose--take the father and his pheromones away and puberty comes earlier. The problem here is how to explain Belsky’s results, in which the presumably pheromonally-rich father sticks around and fights constantly with the mother yet puberty is still accelerated in the girls.

But regardless of what the mechanism might be, the mere idea that social instability leads to earlier puberty has upset a lot of people. Some investigators who have tried to duplicate these studies haven’t gotten the same results. Other critics note that measures of “parental strife” or “strain” in girls’ relations with parents rely largely on the girls’ own reports, so there’s plenty of room for bias and inconsistency.

The biggest problem with the findings and the underlying theory is that they are exactly the opposite of much of what is known about the effect of stress on puberty. A lot of the research suggests that “when times are tough and life is unstable, hold off on reaching puberty--it’s a bad time to get pregnant and the kid won’t survive, so don’t waste energy ovulating.” And we’re not just talking about antelopes here. Girls who experience the stress of sustained physical exercise (for example, serious ballet dancers) reach puberty later than average, as do girls with anorexia nervosa.

Additional evidence that extreme physical stress suppresses reproductive physiology instead of stimulating it comes from studies of women who have already reached puberty. Consistently, physical stresses such as weight loss, heavy exercise, or illness delay or even block ovulation. Moreover, it’s been found that a variety of psychological stresses, rather than physical ones, can also trigger the cessation of ovulation. This latter finding is particularly relevant, given that the girls growing up in unstable home environments we reprobably being psychologically, but not physically, stressed.

The authors of the new theory try to reconcile these differences. If Surbey is right that the girls reach puberty earlier because some puberty-delaying pheromone from their fathers is missing, then this is no longer a contradictory story about stress and maturation but instead a story about pheromones and maturation. Her data also suggest this might not be a broad theory about “when life is unreliable, reach puberty earlier,” but merely about “when the primary male around you is unreliable, reach puberty earlier.” Surbey found that having a mother rather than a father absent didn’t accelerate puberty, yet growing up without a mother should be just as much of a lesson to a child that life is stressful.

Belsky, for his part, contends that there could be a difference between the effects of extreme, potentially life-threatening stress and milder forms of stress, such that a starving anorexic might delay puberty, while a stressed-out yet relatively healthy child of a broken home might reach it sooner.

One other important factor may explain the effect. The age of puberty onset is partially inherited--mothers who reach puberty early tend to have daughters who do so as well. Perhaps early sexual maturation is somehow related to having unstable marital relationships later. In that scenario, girls in unstable homesreach puberty earlier because they inherit that biological trait from their mother, not because the home environment is unstable. Surbey did find that the mothers of her early-maturing girls had indeed reached puberty early themselves. When the mother’s age of puberty onset was taken into account, the overall effect of home environment decreased.

The ideas of Belsky, Surbey, and their colleagues may or may not turn out to be valid; clearly much more research is needed before we’ll know, and the issue will in all likelihood depend on finely wrought details of scientific argument. So why have their various notions been the subject of heated discussion not just in academic circles but in newspapers around the country?

Because lurking behind the scientific arguments is a big public policy conflict over what to do about teenage pregnancy. The pregnancy rate for teens in the United States is astonishingly high for a Western country. It’s twice that of Canada or England, for example. Teen mothers come disproportionately from poor inner-city populations rife with unstable, broken families. Children of such mothers are more likely to have health and learning problems, and thus to underachieve and place a burden on social services for years to come.

Belsky thinks the theory he’s helped formulate has something to say about this mess. In his view, the unstable early environment of many an inner-city teenage girl produces earlier puberty and the whole range of attendant opportunistic behavior that ends in an increased risk of pregnancy. “These teenagers aren’t necessarily carrying out bad behavior,” he says. “Their bodies are just responding to forces that have emerged over the course of evolution.”

Belsky and Surbey’s critics, however, worry that the theory suggests that one can blame biology instead of society for the problem, and blaming biology implies that we can’t do much about teen pregnancy. Yet for a bloatedly wealthy country, they note, we expend very little effort to give a massive underclass any hope for jobs or education. Our mass media are permeated with a sexual hardsell that few adolescents have the means to respond to maturely. Our government makes war on sex education and access to contraceptives and makes it increasingly difficult for the young and the poor to have abortions. Are these the reasons that we have such an appallingly high rate of teen pregnancies among our poor? Or is it because early family conflict commands such girls to follow a biologically determined program of early pregnancy?

Belsky doesn’t think the explanations are mutually exclusive. He does not believe that his theory is about how biology can cause earlier pregnancy, but about how a biological drive can make a young person from an unstable home more likely to respond to various societal pressures. This is what he calls a “nature-based theory of nurture.” If true, it predicts that the effects on teen pregnancy rates of growing up in an unstable home would be stronger among poor kids than among middle-class kids.

And if he is right--if early stress and resulting biological changes do not form a direct link to more teen pregnancies but instead are one of the many factors influencing how teenagers respond to the seductive world around them--then perhaps the policy implications of his view are not so worrisome. Much of behavioral biology works through this sort of biology-environment interaction. These results are not an excuse for society to say, in effect, “There goes that darn biology again--nothing we can do about it,” and wash its hands of the mess. The findings are, if anything, a scientific rationale for why society has the responsibility to try to solve these social ills.

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