The Puberty Puzzle

Time magazine considers a big important puzzle:

By the 1980s, the onset of puberty, if not actual menstruation, had gone into free fall–a change so sudden and pronounced that something more than normal evolution must have been at work. In a landmark 1997 study of 17,000 [US] girls … more than 10% of white girls and an astonishing 37.8% of black girls were showing early breast development by age 8. … Later studies, one in 1998 and another in 2010, included Hispanics and produced similar results. On average, 2 out of every 10 white girls, 3 out of 10 Latinas and 4 out of 10 black girls are showing breast development by age 8. (more)

They consider some possible explanations:

Obesity, a well-established puberty accelerant, is high on the list of suspects. … Data from China and India similarly indicate that race by itself isn’t a factor but general prosperity is. Onset of puberty is on a downward march in those countries too. … But even in Europe, where the standard of living has been high for decades and diets haven’t changed much, something strange is going on. A study of girls conducted in Denmark in 2008 found that the average age of breast development there is 8.86 years, which … is a full year earlier than it was for Danes as recently as 1993. … Some investigators are focusing on environmental contaminants like PBBs and … bisphenol A … A number of studies have found that overweight boys may, if anything, suffer from delayed puberty.

Oddly they don’t even mention divorce and out-of-wedlock birth, factors that some theory suggests are crucial:

Father absence is indicative of the degree of polygyny (simultaneous and serial) in society. Polygyny of both kinds creates a shortage of women in reproductive age, and thus, early puberty will be advantageous. Available comparative data indicate that the degree of polygyny is associated with a decrease in the mean age of menarche across societies, as is the divorce rate a presumptive index of serial polygyny, in strictly monogamous societies. (more)

This theory has some empirical support:

As specified by evolutionary causal theories, younger sisters had earlier menarche than their older sisters in biologically disrupted families (n = 68) but not biologically intact families (n = 93). This effect was superseded, however, by a large moderating effect of paternal dysfunction. Younger sisters from disrupted families who were exposed to serious paternal dysfunction in early childhood attained menarche 11 months earlier than either their older sisters or other younger sisters from disrupted families who were not exposed to such dysfunction. (more)

I heard of this theory a while ago, but until now I hadn’t realized its radical implication: humans may have evolved adaptations to make major body/life features conditional on our social environment! If girl brains can order hormones to induce early puberty after seeing lots of nearby polygyny, how else might our bodies be contingent what our brains see about our social world? Do young brains see the level of violence,  prosperity, or work complexity around, and adjust hormone-induced plans for body size, immune system strength, or brain resources? Could this adjustment explain recent trends in mortality, height, or intelligence? So many possibilities to consider!

Anthropologists often say that it is a mistake to look for “the” ancestral human environment or lifestyle, that what most defines humans is variety and adaptability. I’m going to take that view a lot more seriously from now on.

Added 4p: Why are people so much more willing to use strange chemicals to explain earlier puberty that other trens like increasing IQ, lifespan, and height? Is it because chemicals are bad, and therefore can only explain bad things?

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  • FredR

    James Chisholm, in “Death, Hope, and Sex”, argues that individual life-history strategies are enormously dependent on the social environment you experienced in the first few years.

  • MinibearRex

    This was a central argument in Robert Wright’s The Moral Animal. He argued that essentially, evolution programmed us with conditional responses to our social environment, and that many of the differences we see in people who grow up in different societies are due to different levers being pulled on our biological machinery.

  • Oddly they don’t even mention divorce and out-of-wedlock birth, factors that some theory suggests are crucial.

    Oddly, you don’t even mention the barrage of exotic chemicals we are now exposed to through as a consequence of the innovations and products of our modern industrial society.

    Divorce and out-of-wedlock birth?? How in hell is that anything novel to our species. We’ve had lots of times and settings in the past with such “polygynous” environments, but the early age of menarche and breasts and all the rest seems too drastic to be explained by psychological or social influences.

    A more likely explanation is that it’s some of the novel chemicals we are all now exposed to.

    There are even some who say that an insidious ruling elite class is actively trying to engineer a slave society more useful to them, à la Brave New World, through their seat of power atop industry and government.

    But your theories never seriously consider such things, do they?

    • Ilya Shpitser

      Robin also doesn’t seriously consider the theory that the Rand corporation, in conjunction with the saucer people — under the supervision of reverse vampires — is forcing people to go to bed early in a fiendish plot to eliminate the meal of dinner.


        Way to strawman.

      • The Brave New World hypothesis has a lot more pedigree to it than most people are willing to admit.

    • ShardPhoenix

      Doesn’t make much sense given the Flynn effect.

      • Anonymous

        The Flynn effect is compatible with the hypothesis that exotic chemicals are having significant effects on humans.

      • There are a lot of “IQ” zealots out there however:

        * There are problems with defining intelligence in ways both subjective and objective. Facts, reality, and intelligence are all both subjectively and objectively disputed.

        * A ruling class would try to control the “intelligence” test to its benefit.

        * The “Delta’s” version of the IQ / “intelligence” test might be totally misleading for “certain” conclusions about what is intelligence.

        * We’re talking about sheep who engineer mutton. Don’t go off of the mainstream consensus.

        –>You’re going off of bad sources.

    • Dave

      You may be on to something. The corporate rulers are not interested in slave workers. Late maturing nerds are good workers at operating the few high tech machines that are needed. Their own children can take the high paying responsibility jobs upon graduation from Ivy League grad schools.

      But what the ruling class needs is to keep the operation profitable is hyper-mature little girls to buy I- Phones and other consumer junk.

      People in government benefit too and are undoubtedly involved,making big bucks off of proliferating family dysfunction as teachers,guidance counselors and prison guards. Heh heh!!The chemicals themselves can even be imbedded in the plastic they use to make the products!!

    • Todd

      Robin isn’t saying that divorce is the ONLY factor, just one of them. He clearly mentions increased obesity as a part of early menarche, and I’ve heard a bunch about lower childhood infectious disease being a contributor. The only chemicals involved as the antibiotics making sure kids don’t die of infectious disease. 🙂

  • carl

    On a non-crazy note, is it father absence or non-relative adult male presence?

    Imagine a single mother who starts a new relationship, so now there is a non-relative adult male around the house. What kind of biological signal does that send?

    What kind of signal would that send in a forager society? Or a farmer society?

    Could it be a signal that the girl was effectively being shopped around as a potential mate? And therefore the body ramps up puberty?

    The lack of adult male relatives around is also a signal of lack of protection — all the more incentive to mature in time to be offered to these strange males that are coming around her living quarters.

    I still think obesity makes sense. Don’t fat cells secrete estrogen? So being fat accelerates puberty in girls, but delays it in boys.

    • Finch

      My understanding was that the divorce rate, having peaked around 1980, had fallen considerably through 2010. Unless there’s a huge lag for some reason, that would suggest divorce and early puberty are not connected.

      My guess, which is little more than speculation, is that the pill is causing women to disproportionately marry and then reproduce with more effeminate guys, and we’re seeing an evolutionary effect that’s lagging the introduction of the pill by a generation or two. That would also explain anomalies such as Justin Bieber and the Twilight series.

      • Douglas Knight

        Yes, the divorce rate has fallen, though I don’t think the aggregate divorce rate is a good statistic because generations are different sizes. A better statistic is that marriages from the 80s last longer than marriages from the 70s. Marriages from the 90s might be even more stable, but not much.

        But divorce is not necessary to produce a single mother. Childbirth out of wedlock has kept going up.

      • Finch

        > Childbirth out of wedlock has kept going up.

        Ah, thank you. Good point. See the link for some numbers:

        Still the childbirth out-of-wedlock trend was halfway to the present state by 1980, so it’s funny that the effect is only obvious now.

      • Army1987

        I think that in industrialized countries the fraction of people who die without children is way too small for evolution to be that fast.

      • Finch

        > I think that in industrialized countries the fraction of people who die
        > without children is way too small for evolution to be that fast.

        I think in the US it’s around 20 percent of women and a higher percentage of men. And the number of children per person who had children is also highly variable.

        I called it a guess for a reason, but I don’t think “evolution can’t happen that fast” is the reason it’s implausible. The effect, preference for less masculine guys, is studied, but I’m not sure the magnitude is sufficient to drive change this fast. Also, I don’t see that it explains the racial and socioeconomic differential effects unless the pill has been differentially used by those communities. It might have been, but I don’t know that.

      • Over the vast, vast, vast majority of evolutionary time, the average number of descendents the average woman had (survive and reproduce) was 2. If it was even 2.05, then in 1000 generations the number of humans would have reached numbers we know did not happen. (1.025)^1000 = 5e10.

  • Tony

    In all seriousness, chemical factors are far more likely. How else do you get gigantic effects, shortening the period to breast development by more than 10% in a decade?

    This is not idle speculation. A large number of chemicals used in the production of plastics, some of which remain in these plastics, mimic estrogens. There are problems all over the world with feminized fish where runoff from cities and industrial areas. It even fits with the apparent delay in male puberty that is mentioned.

    The racial (more likely poverty) effect is most likely due to the fact that people living in poverty are probably living in areas that are exposed to more industrial effluent, and most likely eating more heavily processed food that is more likely to contain more of these chemicals.

    • Anonymous

      Additional evidence supporting the hypothesis.

    • If you are correct we should be able to tease out the culprit by examining populations in diverse areas.

    • Konkvistador

      The racial effect is much more likley to be due to biological differences between human groups considering such effects have been shown to be independent of socio-economic factors.

      Differences in onset of certain diseases, signs of ageing, age of first menarch, how common twins are clearly established between some ethnic and racial groups.

      An extreme example of this may be the Pygmy’s

      “I found cases of women 28 years old who said they no longer experienced menstruation. Very few births were occurring to women more than 30 years of age,” he said

      But naturally such explanations aren’t popular even when the effects are as dramatic as they are.

      Migliano’s theory will no doubt be controversial, particularly among creationists and proponents of intelligent design, because it proposes that pygmies are proof of how our species, Homo sapiens, continues to evolve.

  • One possible theory: The hormone melatonin is produced during the night and the level of it falls just before puberty. If melatonin can delay puberty and if staying up in the evening under artificial light (especially while staring into glowing screens) interferes with melatonin we might expect earlier puberty.

  • Anonymous

    Whole networks of genes can be expressed conditional on external input, including social input, by the release of associated transcription factors, with huge impacts on phenotypes. This mechanism is not big news in biology.

    To assume that it can create correlations such as the observed one is a plausible hypothesis.

  • Robert Koslover

    In regard to the above comments, I’m skeptical about assuming tat this is caused by increased exposure to generic exotic chemicals. Surely common exposure to at least many of the important environmentally-harmful substances has significantly decreased during the last couple of decades, due to improved emission control systems, elimination of lead paint, etc. Air pollution is also much reduced in many metropolitan areas, for example. People smoke much, much less too. Now, of course, perhaps there are links to soybean consumption, or many other foods. But in general terms, it is pretty undeniable that our environment is much “cleaner” than it used to be (for better or worse!).

    • Tony

      Exposure to overt toxins like lead, hydrocarbons in the air, etc. has been decreased. Meanwhile, less overtly toxic substances which nonetheless have more subtle effects on endocrine and signaling systems have mushroomed. These generally don’t poison you, but saying they have no effect is ignoring something quite important.

      • Meanwhile, less overtly toxic substances which nonetheless have more subtle effects on endocrine and signaling systems have mushroomed.

        Is this known or are you speculating?

  • Abelard Lindsey

    The problem with social environmental theories for this kind of stuff is that they lack intellectual rigor in that there is no postulated reaction mechanism on the molecular biological level to explain the effect. I think the reason for this is because the people making up these theories all have liberal arts backgrounds and, thus, lack the technical knowledge to come up with legitimate theories.

    • Anonymous

      Do you mean lack of understanding of the specific reaction mechanism for social environment -> puberty onset, or of reaction mechanisms in general? Because biologist already have such answers for various types of interactions, e.g.:

      • Abelard Lindsey

        I mean a defined bio-chemical link between social environment and puberty. If someone can present a credible case here, rooted in molecular biology, I will consider it. Otherwise, it remains one of those “soft” subject things. The soft fields are not real science. They are bogosity.

  • sabril

    I don’t buy it. Puberty has been coming earlier for girls in educated, affluent neighborhoods where the rate of out-of-wedock births is still where it was in the 50s.

    • the rate of out-of-wedock births is still where it was in the 50s

      Completely false. Finch has already linked to a study showing that the fraction of births which are to unmarried women has grown from ~4% to ~40% between 1955 and 2007.

      • Vaniver

        Did you miss the part where he said “educated, affluent neighborhoods”? The pdf linked doesn’t break things down by socioeconomic status.

      • Did you miss the part where he said “educated, affluent neighborhoods”?

        In fact, I did. Sorry, my mistake, sabril.

        Does anyone have this data?

  • Tim Wright

    Another theory similar to the chemical one, is all the hormones we have in our food. I just had a part time farmer and horse rancher tell me about all the hormones that they feed to animals and how it is affecting humans. I have no idea if these speculations make any sense. Besides the one link for chemicals, is there anything about growth hormone in our food? I personally think this idea is a great sound byte and will spread easily but who knows?

  • Abelard Lindsey

    The cause of the reduced age of puberty is known. It is due to the use of hormones in cattle and pork. Such hormones are banned, by law, from being used in poultry. This law resulted from a bunch of Puerto Rican girls going into puberty at age of 3 due to eating poultry fed with hormones in the 1980’s.

  • It is the hygiene hypothesis.

    It is due to the loss of commensal bacteria through bathing.

    It started ~1850, with the advent of the germ theory of disease and the availability of soap and piped water. Bathing washes off the ammonia oxidizing bacteria which metabolize ammonia in sweat into NO and nitrite and “set” the basal NO level. That is critically important in all endocrine functions. Low NO causes hyperandrogenic effects which accelerates growth, efficiency in converting feed into biomass and accelerates sexual maturity. The same reasons that antibiotics are given to farm animals.

    You don’t need anything “toxic” or xenobiotic to affect these things; screw up NO physiology and you screw up everything that depends on NO, including hormone synthesis.

    In 1850, the average of menarche was nearly 17. Virtually no one has menarche at that age now. Any toxic chemical would have variable exposure, there would be some less exposed and the dispersion in age of puberty would increase. The dispersion has not increased.

    In less developed regions the age of menarche is still later than in more developed regions.

  • Radford Neal

    My guess is that it’s a generations-long effect of better nutrition (and maybe other aspects of general health). It’s known that non-genetic changes that affect gene expression can be inherited. It makes evolutionary sense that age of sexual maturity should be lower if there is a good indication that times will be good, and hence rapid reproduction will be possible. But it doesn’t seem very safe to base this on food supply (and other good things) having been good for the few years that the person has been alive to personally experience it. Better to base it on a longer time period going back to that person’s parents and grandparents.

    Of course, like most commenters, I’m putting forward this theory without citing any actual evidence…

  • Being a single mother is *also* indicative of poverty – and that surely brings us back to a much simpler and more general r/K-selection model based on percieved resource availability.

  • Mark M

    I’m going to go with stress, and posit significant childhood stress pushes us all out of childhood faster. From an evolutionary angle, it makes sense for a child growing up in a hostile (stressful) environment to mature earlier in order to reproduce or possibly to escape or help to improve the environment.

    Citing chemical causes is easy, can’t be disproved, and I would be surprised if doctors haven’t found chemicals (hormones, I’m guessing) that will cause early onset puberty.

    I’m just going to say that stress can be chemically, physically, or socially induced. Everyone wins!

    • All stress effects are mediated through a lowering of nitric oxide levels. Low NO is physiology’s signal of “stress”, and yes, it does accelerate maturation, but at a cost of degenerative disorders.

      What is very worrying to me (knowing what I know) is that stress also causes epigenetic programming of DNA in gametes. If you raise the stress level enough, people may program stress responses in their children’s DNA that make reproduction impossible. That is what John Calhoun found in mice.

      Give a breeding population of mice everything that they need, food, water, air and they soon go extinct. This is just due to psychosocial stress, no toxins are needed.

      • Dave

        It always worries me when someone seizes upon a single factor as an underlying cause of a multiplicity of seemingly unrelated changes.The NO hypothesis seems to be imbedded in a blizzard of untested assumptions and the observed outcomes have numerous alternative explanations. So basically you are promoting a belief system. (So is everyone else.)

        However it is true that there is vast ignorance concerning the normal bacterial inhabitants of the human body and the consequences of messing with them.

        I was reviewing a bunch of obscure lung conditions the have been recently described and their is nothing but the vaguest idea what causes them. Same for skin and intestinal diseases.Yet vast amounts of data continue to be amassed.Humility is the best position here.

      • But they are not unrelated. All of them are related in that they involve signaling with nitric oxide. The transfer of zinc from metallothionein (the main storage form of zinc) to zinc finger transcription factors (the largest class of transcription factors) occurs via oxidation of the Zn-thiol couple via NO species.

        Regulation of the cytochrome P450 enzymes (the ones that synthesize cholesterol and steroids) is via nitric oxide. Regulation of ATP production is via nitric oxide, including mitochondria biogenesis, oxygen reduction at cytochrome c oxidase, mitochondrial inhibition, glycolysis via HIF-alpha, erythropoiesis via EPO, blood flow via shear mediated vasodilation.

        All of the observed outcomes are known to occur via pathways that are known to be regulated by nitric oxide. There are not alternatives that don’t involve nitric oxide.

      • Dave

        Yes, thanks for pointing this out. I searched for references in the New England Journal of Medicine. Now I know what Zinc fingers are and see they are pervasive in protein transcription involving hematology and endocrinology.

        So they could be a portal though many environmental factors have an effect. I still think it is premature to judge how this all works out in the real world for reasons I have given.
        It does make a nice pet theory.

      • Dave, it would be premature for you to make conclusions on it because you haven’t read the literature. I have, and it is not premature for me to make such a conclusion.

        There are many more pathways that involve nitric oxide. The Zn finger proteins are regulated by NO. That means that the pathways that regulate the Zn finger proteins produce an NO signal. That means the products that Zn finger transcription factors cause (or block) expression of, produce an NO signal to block (or cause) expression of more (or less) of that product.

        Everything that Zn finger transcription factors couple to has to be coupled via NO signaling.

  • orthonormal

    > Why are people so much more willing to use strange chemicals to explain earlier puberty that other trends like increasing IQ, lifespan, and height? Is it because chemicals are bad, and therefore can only explain bad things?

    Generally, I’d expect a difference between traits caused by exposure to new chemicals vs. traits caused by adaptation to new social environments: the former should only rarely be adaptive, the latter often so. (I don’t know whether or not earlier puberty is adaptive.)