Future Fertility

Many a young women has looked inside herself, decided that she just doesn’t want kids, and went on to live her life under that assumption.  But a decade or so later, her biological clock suddenly went off and she found herself very much wanting kids.  I’ve seen this happen several times.  None of us should be very confident about what introspection tells us we will later want.  Evolution has designed us to express different genes at different ages; we just can’t know what future genes we have been designed to express.

This caution should apply all the more to humanity and its descendants as a whole; we just can’t look inside ourselves to discern our future fertility.  Since the industrial revolution made us rich, we’ve seen the remarkable phenomena of the demographic transition, whereby after nations get rich population growth rates eventually fall dramatically, often below replacement levels.  This is not the sort of adaptive behavior that would have evolved had our ancestors repeatedly encountered such extended boom times; clearly we could now leave more descendants by using this opportunity to breed like crazy.

Our evolved instincts must be framing our novel situation as similar to some ancient situations where having fewer kids actually did make sense.  We still don’t understand this well, but we can see that those evolved instincts are mistaken.  At the moment contrarian subgroups who choose more kids (e.g. Hutterites, Hmongs, or Mormons) are slowly increasing as fraction of the population.  The more heritable, either genetically or culturally, is this fertility behavior, the more these groups will come to dominate future global population and fertility.

Of course we can’t say much with confidence about any one group; they may well fragment or become assimilated by larger groups.  But what we can say with more confidence is that if our society continues to be competitive, without strong central coordination, selection will be a powerful force influencing fertility over the long run, e.g. a thousand to a million years.

Yes it can take a long time for selection to favor behavior that starts out in only a small minority, and yes our descendants may have very different bodies whose designs are encoded not in DNA but in computer files.  And yes, for a time individual regions or nations may forcibly limit the fertility allowed there.

But as long as enough people are free to choose their fertility, at near enough to the real cost of fertility, with anything near the current range of genes, cultures, and other heritable influences on fertility, then in the long run we should expect to see a substantial fraction of population with an heritable inclination to double their population at least every century.  So if overall economic growth doubles less than every century, as I’ve argued it simply must in the long run, income per capital must fall over the long run, a fall whose only fundamental limit is subsistence; we can’t have kids if we can’t afford them.

Added 8p: Does anyone doubt that the main reason humans stayed near subsistence level for a million years until about 1800 was selection for behavior giving maximum sustainable population levels?   What exactly is going to be so different over the next million years?

Added 24Sept: Bryan responds here.

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  • Re: if our society continues to be competitive, without strong central coordination […]]

    Looking at the current state of the emerging global super-organism, that may be quite an “if”.

  • Does your thesis apply to the notion of “memes”?

    If you’re unfamiliar with the idea of memes, it’s an analogy. Ideas are somewhat similar to organisms (their environment is organisms, so they would be most similar to viruses, parasites, or symbiotes). They might undergo selective pressures.

    If our descendants might be encoded in computer files, then they might be peers of ideas.

  • Your argument completely collapses due to two major assumption which are wrong:
    * You assume lack of centralized influence over fertility, where such influence already exists right now in form of nation states. Government of China alone extremely strongly influences fertility of 20% of human population, by explicit policy. Other governments tend to use taxation and other indirect measures for the same end.
    * You assume that fertility is hereditary, where everything we know shows that it’s cultural, and culture can and does spread horizontally, so it doesn’t follow this marginal evolutionary arguments.

    There’s also obvious problem that all empirical evidence points to fertility decreasing. And there’s little evidence that people want children as such. They want sex, pair bonding, cute things to look after etc., all that are really well provided by substitutes such as sex with birth control, childless relationships, keeping pets, and so on. It can be as well argued that market will keep providing better and better substitutes for these natural urges, and as a consequence fertility must keep falling.

    • TGGP

      Behavior is heritable, you know

      Robin also explicitly allowed for culture playing a role in fertility. He also acknowledged that currently the demographic transition is causing lower fertility but claims that over the long run (thousand to million years) that won’t last. Did you actually read his post?

      Why the market likely won’t forever keep fertility down: the market supplies demands that people have. As people who demand “children as such” become a larger fraction of the population, the market will have less effect.

      • Curt Adams

        Specifically fertility has shown up as genetically heritable in the long-running Danish Twin Study There are two important conclusions: first, we are in a very different selection environment than our ancestors, as otherwise the heritability for realized fertility would have to be very low. Second, fertility will increase rapidly under current condition – there would be a substantial increase every generation with numbers like that.

    • kevin

      Fertility is falling due to the very, very recent introduction of reliable contraception. In other words, the environment changed (it now had latex condoms and birth control pills) and most humans had low genetic fitness in the new environment. However, some humans (anabaptists, fundamentalist Mormons) had protective factors that allowed them to thrive (genetically!) in the new environment.

      These groups will fill the niche previously occupied by lineages with the maladaptive traits.

      • The demographic transition is actually prior to the recent introduction of more reliable contraception.

    • Some governments influence fertility lots, but most have only weak influence, not enough to overcome the selection effects I discuss.

      • Most governments don’t need to bother with fertility controls because fertility is falling without any government help. So far every government that seriously tried fertility control was successful, and all attempts at fertility increase even just to get back to replacement fertility were rather unsuccessful.

        If we ever get to the point where fertility will endanger per capita incomes, governments will look at limiting it seriously. The last bothered doing so in 1970s, without even significant need for it.

        And extrapolating to “the long run, e.g. a thousand to a million years” is just ridiculous, as human fertility now already looks drastically unlike what it looked just one hundred years ago, and is likely to change even more in the future, away from the natural state towards more artificially manipulated procreation (starting with genetic screening of embryos for diseases, and assisted procreation late in life right now, but it’s likely to get a lot further).

      • In the long sweep of human history, most folks near subsistence level, and yet most governments did little to prevent fertility.

      • It could be argued that it doesn’t matter, but is this even true? How close to Malthusian levels people really lived through history?

        One good piece of counter-evidence I’m aware of is demographic history of medieval Europe.

        If Malthusian hypothesis was true, population would stay stable and slowly increasing with agricultural productivity increases, and most of all, it would very rapidly recover from population shocks like wars and diseases.

        What we really see is – rapid decline in population in late antiquity, no recovery during 400-1000 in spite of ample available land (against Malthusian hypothesis), more or less doubling of population 1000-1350 without advances in agricultural productivity strong enough to explain it (against Malthusian hypothesis), and supposedly with increased quality of life, and increased wages etc. relative to previous period (against Malthusian hypothesis), 1350-1400 huge crash due to Black Death, no rapid recovery (against Malthusian hypothesis again).

        I’m no expert in history, but I’ve heard some pretty good explanations for low populations in late antiquity and early Medieval period – in particular widespread slavery, with slaves not being allowed to procreate, what obviously reduced fertility (and fresh non-slaves being captured into slavery with every generation, by external raiders like Vikings, Hungarians, Muslims and so on). End of slavery and external raids around year 1000 corresponds with rapid population growth, so it sounds plausible. Availability of food might have not been the primary constraint of growth at all.

  • Joe

    “Our evolved instincts must be framing our novel situation as similar to some ancient situations where having fewer kids actually did make sense.”

    This seems unfounded. There was (AFAIK) no common ancient situation in which sex and offspring were easily separable concerns.

    That’s leaving aside the fact that children were pretty valuable for most people, at least for most of recorded history. And the downside risks of children, for men at least, were pretty minimal when families were much more free to determine their own affairs (e.g. discipline and distribution of goods/food to children). In the modern Western welfare state: parents do not own their children, are required to provide them with a minimal standard of support, do not by and large control their education, and can not legally force much upon them in adulthood. Children generally need not rely on parents for survival — the state will step in whenever it sees that threatened. This situation is completely unlike any historical society I know of.

  • I don’t know if this is inherent, but it seems that every such super-fertile group holds values that make them — GENERALLY speaking — resistant to technological advance: either causing it, or using its fruits. So it seems that if push comes to shove, these groups will find themselves outgunned, even if they do eventually outnumber everyone else.

    The less fertile, more scientific groups will make up for their smaller numbers with superior productivity and superior militaries, giving them superior access to resources and putting Malthusian constrains on the more fertile, less scientific groups.

    • Curt Adams

      Hutterites and Amish limit modern technology but the Mormons are right in the thick of modernity. And far-right evangelical Christian groups like the quiverfull movement avoid science and education, but they love guns and the military.

      Also, a world in which many Amish-like groups are confined to squalid Malthusian reservations (or worse) by overwhelming military force is not a very nice one and I’d like to avoid it if there’s some less amoral approach to long-term control of human fertility.

      • Douglas Knight

        Mormons are right in the thick of modernity

        There’s even the Mormon Transhumanist Association

      • Pardon the cynicism, but isn’t that what we already have? International borders are enforced by national militaries, which are in turn supported by their respective nations’ technological and economic capabilities. The result is that many people today life in poor, high-fertility countries and are prevented from amplifying their access to resources via immigration, leaving them stuck with the Malthusian constraints of their home countries.

        Yeah, that sure would suck, wouldn’t it?

  • Eric Johnson

    Our evolved instincts must be framing our novel situation as similar to some ancient situations where having fewer kids actually did make sense.

    Very likely false. (By the way, since I mostly speak up when I disagree, I should note that I appreciate this blog and your thought very much.)

    First of all, I’ll bet no one can think of such an ancient situation that makes good sense. Second, it’s enough, almost, to simply have sexual desires, and then desire to take care of the kids once they appear. Actually desiring to have kids is somewhat redundant (not completely), and evolution tends not to build in redundant backups to the same degree that, say, humans do in designing a nuclear plant. Genes gradually mutate, and the mutations destroys functionality: there is a mutation pressure, as they say, and it takes selection pressure to maintain function against mutation pressure. If primary system A is 99% effective, the pressure to maintain the genes for backup system B is only 1% of the total pressure favoring that function. If B is nevertheless also 99% effective, then the pressure holding together backup system C is only .01 * .01 = 0.0001 times the total pressure.

    However, infertility is common enough that it’s odd one hasn’t evolved to have an overwhelming desire to leave a mate after say five childless years (as opposed to a mild desire which “merely” creates some difficulty in one’s relationship). Of course it might be you that’s infertile, but on the other hand it might be the mate. I wonder how other animals behave on this front, and I also wonder if it’s possible human infertility was far rarer in the primordial times.

    • kevin

      I wonder how other animals behave on this front, and I also wonder if it’s possible human infertility was far rarer in the primordial times.

      I suspect infertility is probably less common now, since being underfed causes amenorreah in women and lowers sperm count in men.

      • Eric Johnson

        Does it lower sperm count enough to substantially reduce fertility (assuming sperm competition is not a factor)?

        As for anovulation due to underfeeding, perhaps this doesn’t select per se for rejection of the mate. I’m assuming food is largely pooled between mates. If only one of the two mates is a poor economic performer, that alone might give sufficient signal for the other to reject them – leaving no role for a signal from the lack of children. After all, anovulation vs being just barely well fed enough to ovulate doesn’t matter all that much; in the latter situation you do at least have kids, but they’ll be non-vigorous because the family lacks food.

        On the other hand, if both mates are poor economic performers, they may not be in a situation to do any better by rejecting their mate as opposed to “clinging to hope.”

  • Eric Johnson

    I think the question for the long term is this. We are all, I think, assuming that fecundity desires are partially heritable, as most things are. Suppose fecundity desires become engineerable – which I think is rather likely. For the common good, will most people with high fecundity desires agree to engineer their children to have more modest desires (ie, to desire merely replacement level fecundity)?

    Will people, then, evolve the “meta” high fecundity trait of refusing to engineer their offspring for modest fecundity desires (or for the meta trait itself)? I guess: but it might take many many times longer.

    If genetic engineering becomes highly helpful for health, beauty, and IQ, will large numbers of people still refuse to ever start engineering their offspring at all, in the first place? I’m guessing yes, I guess.

    • I see little chance that people will want to en masse make their kids not want to have more kids. And many times a thousand years might be a hundred thousand years, still small in the long run.

  • Our evolved instincts must be framing our novel situation as similar to some ancient situations where having fewer kids actually did make sense. We still don’t understand this well, but we can see that those evolved instincts are mistaken.

    Or the human mind and body is more adaptable and less controlled by evolutionary instinct than you imagine: perhaps we have more power over mental constructs and considerations that dominate whatever “evolutionary” pressures remain.

    • Whatever urges or inclinations you think have overcome our evolutionary instincts, I can assure you those too evolved. Whatever part of you has the power, it evolved. And in a competitive world it will continue to be subject to evolutionary pressures.

  • Nuncio Salvage

    Existence over non-existence is always preferable. Ask anyone who exists.

    Octomon, unemployed, living at home, with her four previous children before her pregnancy, is the model of parenting we should all aspire to.

    Not producing children is immoral because you are denying people existence.

    • Psychohistorian

      How can a existing person know that existence is preferable to non-existence? I exist, and I have absolutely no idea what it would be like to not exist. If I didn’t exist, I wouldn’t even fail to have an opinion on the matter – “I” just wouldn’t even be meaningful.

      Show me that people who don’t exist would prefer to exist, and I’ll support you completely. Otherwise, preventing people from existing is quite literally a victimless crime.

      • Nuncio Salvage

        People get value out of being alive. If you do not want to commit suicide than you value being alive.

        If so, the consequence of having children, rather than not having children, is good; you create people whose lives are worth living. And thus the consequence of not having offspring rather than procreating is, by comparison, bad. So if you, like me, think your actions are more moral when you do more good for others, you should agree with me that having as many children as possible is moral, and abstinence, condoms, birth control, and abortion are immoral.

        For women, multiple in vitro pregnancy births should be the first option really, like Octomom, all her children have lives worth living. It’s okay if such mass pregnancies risk birth defects or retardation because those children will have lives worth living. The risk of stillborns or infants that can’t survive is outweighed by the value of lives from the many others born; besides people die eventually anyway. For men, rape is good, don’t torture the woman, just don’t confuse lack of kindness with cruelty; you’ll do more good by making lives with value than otherwise.

      • Psychohistorian

        To RH: Better threading, please!

        To Nuncio:

        First, we should have an unbelievably powerful norm and law against suicide. If everyone believes they’ll go straight to hell to be tortured for all eternity if they want to commit suicide, or tortured here and now if they fail, then no one will desire committing suicide, and everyone’s life will, as I understand your definition, be worth living, no matter how we abuse and enslave them. I admit the law claim may be faulty, but the hell claim seems valid.

        Second, do you actually live by what you say morality dictates? If not, why not?

        Third, who’s worse off when people fail to procreate? The not-born? The not born don’t exist – they don’t even not-have utility curves, they don’t even not-have anything. “People get value out of being alive” is generally true. But you can’t compare people to unpeople – “12 > (1/0)” isn’t true – it’s nonsense.

        Fourth, if you take existence as a prerequisite, high-pop poor world is much, much less preferable than low-pop rich world. Everyone born into rich world will be much more excited about it than everyone born into poor world. If everyone is happier about being in rich world, how is poor world preferable? If you don’t take existence as a prerequisite, there’s no “you” to ask the question to.

        Fifth, I do admit that if your terminal value is aggregate total utility, and you quantify everyone who is alive and non-suicidal as possessing positive utility by definition, the repugnant conclusion is not repugnant and is probably inescapable – assuming of course that that new life-worth-living does not negatively effect others enough to outweigh its existence. I’m assuming you value a million super-happy people over a million and one super-depressed but not quite suicidal people.

      • Nuncio Salvage

        Thanks for pursuing my comments Psychohistorian, slow for the slow reply.

        I can only agree with you as I was expressing a position was not my own.

        In a recent September post (Painless Meat) Robin referred to the piece he had written on the subject of the ethics of meat eating.


        Putting aside whether or not the age old philosophical debate on vegetarianism is right or wrong, I found the arguments Robin presented to be atrocious.

        In short, he creates a moral premise that is devoid of circumstance, intention and consequence. Even strict economics needs to take such matters into account. I would have let it go as a fluke since he wrote it in 2002, but this problematic thinking keeps rearing itself today in posts such as this one and others on nature and population.

        While I did ratchet up the rhetoric a notch to catch some attention, I didn’t stray far from his position. For the most part, I swapped out a few words and even kept much of the language structure. I didn’t even have to construct an analogy to animals since in Robin’s piece he established the good of bringing humans into existence as an unequivocal good in it’s own right.

  • RobQ

    I thought I learned in high school that in historical (not current) “rich situations” you would have had more kids reach healthy/productive adulthood by having less of them.

    I’ve heard similar issues raised by my racists acquaintances, “those people are just going to come here and outbreed us and then out vote us.”

  • anon

    I doubt it. The reason the population stayed so stable was the Malthusian limitations of animal power. Humans as well as their draft animals have very limited efficiency, 1-5% of the energy input comes out as productive work. The use of hydrocarbons and nuclear allowed humans to break the animal energy barrier and redefine agricultural output.

    It was not a better breed of man, but rather a better breed of capital that made the difference. That, of course, was always the primary goal of eugenics anyways, to explain away the achievements of capitalism by giving the credit to breeding.

    • Even with only animal power our ancestors could have been a lot richer with fewer kids. There was a very real effect of rising population making fishing and hunting harder, and pushing farmers into more marginal land.

      • anon

        Certainly for hunting and fishing, but agriculturalists tended to prefer larger families because the physical labor of the family and it’s animals drove the farm. Young males were the most prized object of reproduction because they would output the most physical work, yet children and adults of both sexes were used extensively (child labor was the cultural norm for thousands of years before the industrial revolution). The more strong sons you had, the more food you made and the richer you were.

        The economic reward for reproducing farm labor diminished significantly with hydrocarbons. A single tractor can output more energy than an entire clan pulling on a rope. A modern farm has no need of child labor, in fact, it requires expert adult labor. The modern farmer is not a store of animal energy, but rather an engineer and factory forman. He drives chemical and mechanical production using techniques that vastly outstrip his inbuilt hominid abilities to make plants grow.

        He has no need to reproduce to increase production, indeed, he could even choose not to reproduce at all and still operate the farm. Whether or not our modern farmer John reproduces is not a matter of capital, but of personal preference. If he does, it will probably be 2.2 times with a female he met well over ten years after he reached sexual maturity.

      • Psychohistorian

        Historically, this is simply false, at least once you have agriculture.

        Once you get agriculture, your kids become employees rather than liabilities quite quickly, and you need as many as you can to survive into adulthood so you can marry them off, expand your land holdings, and have someone to take care of you in your old age. Your kids may be worse, because all 9 of them only inherit your property and maybe their spouse’s, but you’re better off. The farmers getting pushed onto marginal land were likely sons that didn’t inherit much, not fathers who had too many kids.

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  • Stuart Armstrong

    This caution should apply all the more to humanity and its descendants as a whole

    And I feel the same sort of caution should apply also to the predictions in this post. There may not even be such a thing as individuals in the future; and even if there were, simply saying that there will be “competition” neglects the difference between leaveraged buyout, baby booms, and world war II. We’re remarkably good recently at redirecting most competition towards usefull channels, rather than babies or wars; and this was done with reletively low international competition.

    I don’t want to extrapolate much from that point either; I just feel that you long term points, though interesting, should only change my probability estimates by tiny amounts.

    • You assume babies are not a useful channel for directing competition.

      • Stuart Armstrong

        I do assume that, and have a (fully written down) population ethics somewhat between average and total utilitarianism – which has its own wierdnesses, of course, but not that many, and I can live with them. It also has the usual assymetry between “not creating a being” and “destroying a being”, so I see no net grain to bringing a barely-happy child into a world of happy people.

  • Blackadder

    As I understand it, Prof. Hanson’s argument against economic growth continuing indefinitely is that there just aren’t enough atoms in the universe for this to happen. Wouldn’t the same hold for population growth?

    • Stuart Armstrong

      His argument is that population growth will be limited only by survival, that future people will return to subsistence-level lives.

  • Maximum Liberty

    Robin seems to have missed the role of prices as a factor partially determining fertility. If a small group doubles every century, it makes little impact on prices. If it grew to be half the population, its next doubling would affect prices (mainly of the capital that every worker needs). Any decent model of fertility would acknowledge a role for economic context.


  • whereby after nations get rich population growth rates eventually fall dramatically

    Cuba 1.66 children born/woman (2006 est.)
    North Korea 2.09 children born/woman (2006 est.)
    USA 2.1 children born/woman (2008 est.)[52]

    Wealth is just one factor. Socialism and urbanization are also important factors.

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  • We’d be more confident that the ascendancy of memetic evolution will be long-term if we could name other cases where the blind urge to reproduce had been enduringly subliminated to the needs of other replicating systems. Here’s a list: http://knowinghumans.net/2009/09/all-bets-are-off-in-evolutionary.html

  • Dagon

    A _very_ important question is whether the inclination to have more children is heritable enough to overcome the cultural and wealth changes that will take place over evolutionary time periods.

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  • Douglas Knight

    This is tangential, but demographic transition is not the consequence of mere wealth or “boom times,” but something more specific in the industrial revolution. Colonists to North America responded to that form of wealth by rapid expansion.

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  • Amanda

    I think this article made some interesting points, I read a textbook directly related to this topic, its called The Hutterites in North America by , I found my used copy for less than the bookstores at http://www.belabooks.com/books/9780155029156.htm

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  • Dear Colleagues:

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