Fertility Fall Still Puzzling

Bryan Caplan:

One popular story about the decline in family size over the last two centuries goes like this:  Back in the old days, having kids paid.  Children started working when they were quite young, and provided for their parents in their old age. … Ted Bergstrom summarizes evidence showing that even in pre-modern societies, kids did not pay.  Kids did not pay in hunter-gatherer societies. … Kids did not pay in agricultural societies.

Wikipedia lists five explanations for our having fewer kids than our ancestors:

  1. Parents … need not require so many children to be born to ensure a comfortable old age.
  2. Urban living … raises the cost of dependent children to a family.
  3. The cost of children to parents is exacerbated by the introduction of compulsory education …
  4. Lower … acceptance of childbearing and motherhood as measures of the status of women. Working women have less time to raise children.
  5. Improvements in contraceptive technology are now a major factor.

Bergstrom undercut #1, and fertility fell lots before contraception tech, saying #5 is minor.  #2 and #3 don’t obviously raise the cost of kids relative to income, or to kids’ value to parents.  That leaves #4, some unexplained feature of how kids give moms status.

A quick lit review finds a few recent suggestions, such as this:

Reproductive decision making might be driven by a human psychology designed by natural selection to maximize material wealth.


Advice and comment on reproduction that passes among kin is more likely to encourage the creation of families than that which passes among nonkin and (b) this advice and comment influence the social norms.

and this:

Mortality reductions affect the incentives of individuals to invest in human capital and to have children.

But the basic question remains open.  While we have some good clues to proximate causes, we just don’t understand how or why natural selection gave us preferences that, in our modern environment, produce such unadaptive behavior.

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  • Jef Allbright

    Evolved organisms, including humans, are not fitness maximizers, but rather, adaptation executors. With the current environment offering much more variety, what’s to explain?

  • Eric Johnson

    #2 and #3 don’t obviously raise the cost of kids relative to income

    I’m not convinced. Are you including opportunity costs?

    Also, on a farm pre-1800, you got most of your food outside the market and aside from your money income. Food was not accounted for in your money income, but it was a huge chunk of your income in the broad sense – and much of it came from your children’s labor.

    As I wrote on Caplan’s post, it doesn’t make any sense for him to ask whether kids were “profitable” financially, or “paid” (he even asks if they are more profitable than other uses of capital). What matters is just whether the value of their labor made them a lot cheaper – not necessarily free. Obviously, I don’t have to cut the price of pizza down to $0 a slice or $(-1) a slice to increase demand. We desire pizza and children inherently and so we buy at a price well above zero.

    • http://hanson.gmu.edu Robin Hanson

      In a rich society, kids are more expensive, but not clear they are more expensive as a fraction of income. When land is cheap, the land cost for kids is cheap, but then income is low, so the price may still be relatively high. The main cost of kids today is time, and not clear that is a larger fraction of time than long ago.

  • Zukov

    r/K selection theory might provide some clue as to ultimate causation.

    Mammals in general seem to be more K-selected, with human beings arguably most K-selected of all.

    • fburnaby

      I’m not sure I see what you’re trying to point out yet: Are you arguing that we have become increasingly K-selected over the past 200 years? Or are you just pointing out that given we are K-selected, our tendency to prioritize things other than reproduction is less of a surprise?

      • Zukov

        We’ve become increasingly K-selected over millions of years as we evolved into mammals and hominids.

        And yes, I am “just pointing out that given we are K-selected, our tendency to prioritize things other than reproduction is less of a surprise.” I think it is the ultimate cause that Robin is seeking.

    • http://timtyler.org/ Tim Tyler

      r/K selection theory is probably only a small part of the answer – most of the rest of the answer lies with the hijacking of human brains in the service of memes.

      • Zukov

        the hijacking of human brains in the service of memes.

        I agree, though I consider this to be more of a proximate cause than an ultimate one. These memes operate on already “vulnerable” organisms that are very K-selected with a tendency to prioritize things other than reproduction.

      • Zukov

        I can recall being a child during elementary school and listening to my older sister who was in middle/high school expressing quite anti-natalist attitudes that were inculcated in school and by the wider culture.

        Those memes are definitely still with her today as she enters her 30s. At this point, the only thing I believe could potentially challenge or counteract those memes is the personal experience of actually bearing and raising a child. It’s conceivable that she finds it to be a positive, fulfilling experience and strongly desires to have more children. But of course at that point she would be near the close of her fertility window. This experience isn’t too uncommon among women these days, especially well educated, professional types.

  • improbable

    Perhaps it would help to clarify where & when we’re comparing to. I have in mind England, for which the data is better than most places.

    (1) In pre-industrial times, the population growth rate was pretty close to nil, so on average couples had close to 2 surviving children. They might have had more births, but they didn’t have big families. (4 children per family produces an enormous population explosion.)

    (2) The huge growth in population was essentially a 19th-C thing. So the kids weren’t useful for harvest time, if anything they were useful in factories, but that ended before the boom ended.

    Perhaps the question to be answered is what went wrong in the 19th C, with the 18th and 20th (or certainly 17th and 21st) on the norm of close to 2 children. Perhaps people hadn’t yet got used to a society in which they got married that young, and such a high proportion of their kids survived.

    This would line up with for instance Africa, where relatively stable populations have suddenly multiplied enormously, perhaps partly from using norms adjusted to the old times in a world which now has penicillin, and food aid, etc.

    I agree this isn’t an economic, or evolutionary, explanation.

    • http://www.webkist.com/ Mike

      Perhaps the question to be answered is what went wrong in the 19th C, with the 18th and 20th (or certainly 17th and 21st) on the norm of close to 2 children.

      I think this is going to become THE interesting question of economic history. I just listened to the EconTalk podcast interview with John Nye and basically the same question was central: the question is not, “why did fear of markets suddenly arise in the early 20th century?”, but rather, “why was the 19th century so market-friendly?”

    • MikeM

      You’ve come very close here.

      I thought this was all pretty well known? The Demographic Transition and all that. Death rates fall as a result of increased wealth and stability while the natal-norms that prevailed in a period of high infant mortality continue on for a while before people adjust. Old preferences of having a lot of pregnancies in order to have several surviving children continue into an age where many more infants make it to adulthood than before. Birth rates only adjust after a long and variable lag where people pick up on the drawbacks of large families and adjust their natal-norms to reflect the new environment.

  • Aaron Miller

    I agree with Eric. It’s not that the inherent cost of children is driving down fertility; it’s the opportunity cost. Women have more earning potential away from home and children than ever before, so they choose more often to have fewer or no children.

    I learned this in my American Economic History class about ten years ago. It’s not exactly a new insight. Someone should update that Wikipedia page.

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  • Eric Johnson

    Aaron, interesting point. Also, some might look at the woman’s income as a “bonus” – but actually it’s “required” if you want to keep up with the Jones’ positional goods, or stay in the bidding for fixed-supply raw goods like suburban land.

    • http://www.rationalmechanisms.com Richard Silliker

      The loss of the intuitive capacity to raise children, would in my mind lead to the drop in the birth rate. If you have not experienced a family why would you want one. This birth rate decline has a trajectory of its own and will continue until …..? In Canada, the government gives us up to $475.00 a month if we have a child, based on income. Scarcity of money is an impediment to having children. The implementation of the surreal is a factor as well. A surrealism is a license to decide who, what, where, when, and why gets. Layer upon layer of licenses have been put in place and to navigate your way through them requires a lot of effort. Who in their right mind would want to have children. Some even consider having children to be a PSYCHOTIC event.

  • Eric Johnson

    Robin, I would say many fewer person-hours are spent today watching children because they tend to be creched together. However, I suspect watching the kids cost farm wives rather little, since almost all the work of caring for kids out of diapers is simply supervising and conversing with them, and women could work indoors or out while doing those things. Also, for the later-born, older siblings could look after them, whereas today siblings are often 3 or 4 years apart or less.

  • http://www.weidai.com Wei Dai

    While we have some good clues to proximate causes, we just don’t understand how or why natural selection gave us preferences that, in our modern environment, produce such unadaptive behavior.

    The selfish meme explanation (which is given by one of the papers you quoted) seems to work just fine. Consider an analogy, where someone gets infected with HIV ad dies from AIDS before leaving any offspring. Would you also say that we don’t know how natural selection gave us such an unadaptive immune system?

  • mattmc

    Trying to think of a “status” based reason to appeal here…but it goes deeper. Thinking about #3, education, much of parental status is defined by the success of one’s children. This is pronounced in some cultures.

    On the other hand, how many successful children are necessary to convey the benefits? If you are well off and your children have a higher probability of success, you might be more comfortable with having fewer children. If there is a lower probability of success, you may want to increase the throws of the dice. At the welfare level, additional children are very low cost, but each has some chance of providing you success. So, the calculus is always changing, but culture takes time to react.

    The next thing that brings to mind is that the onset of children is coming much later in life- partially driven by education for women. While my grandmother had 13 children- she started when she was 17. My mother had her first child of three children at 27. My wife had her first of two at 29.

    Why did my grandmother have 13 kids? Perhaps religion encouraged her. We see the Mormon push for growth as very obvious today. There is also encouragement for large families in the Muslim world. The Catholic ideal of a large family was strong in her day. If you factor in benefits of eternal life, the children have a longer potential payback period in which to provide happiness.

    • Ilya Shpitser

      “Why did my grandmother have 13 kids?”

      Because if she didn’t, her descendant would be unlikely to write about her on overcomingbias :).

      /hattip to Scott Aaronson’s old contest.

  • tndal

    Probably most people who have lived had no need to take care of their parents because their parents died at a younger age. People live much longer today. Caring for aging parents is uncommon.

    And offhand I see no selection advantage to caring for aged parents.

  • jonathan

    There is a solid link between rising incomes and smaller families. I like to look at the factors that lead the poor to have more children as much as why the richer have fewer. I discount the idea that children earn for the family because even in primitive societies people understand the relative economics of feeding x many mouths. We’d have to assume that poorer people lack that basic capacity.

    Poor people have less, so inheritance is less of an issue. If you have something, even a position (as in a guild), then having more children can dilute that – thus the old upper crust conception that the first inherited, the second went to the military and the third to the church. The poor have little to pass on and so they are free to have children because they want to, because they can. One needn’t have a strong reason because removing the impediment to the urge to breed is enough.

    Another reason is the poor have their own culture. It may be more communal by necessity and that could provide an incentive to have a large family because then you are a clan and a clan has power or at least a potentially larger role in the mass community. We see this in many places around the world; more of a clan translates directly into political power, military power, economic power. So breeding among the poor has a reason.

    Yet another reason is that the poor may be influenced by traditional moral instructions that place value on having lots of children. I’m familiar with the social changes that happened to immigrants of various types to America. As they adapted their culture and religion to America, they observed, learned from, took aspirations from and thus emulated the better off. They were free to do that here but not back home. They were freed from religious inspirations to be fruitful and multiply that were imposed through the community’s social and religious mechanisms. It’s not far-fetched to say that though they were poor, they had hopes of moving up economically and socially and that also meant shifting to the culture of fewer children. We can see this in the contrasts between the devout and non-devout in a number of religious groups in America, with the former having many more children. We can also see this in the assimilation of some groups. A decent example is the Kennedy family, which was a typically large Irish Catholic family that has with each generation fit less to that model and more to the prevailing upper class cultural expectation.

  • FuzzyThinker

    Isn’t the fertility decline easily explained by hypergamy? Women’s evolutionary preference is to only have sex or children with men who are high status and powerful. As societies become richer, women’s power relative to men increases (because they no longer depend on men for material wealth), so fewer women will be able to find men of sufficiently greater power to cause them to want to have children.
    When societies are very poor, increasing wealth increases fertility (because then it’s physically possible to support more children). But as rich societies become even richer, the additional wealth primarily goes into allowing a greater percentage of women to be hypergamous and refuse sex or children with most men, which causes fertility to decrease.

  • http://timtyler.org/ Tim Tyler

    Perhaps see the extensive coverage of this question in “Not by Genes Alone”. It starts on page 169-181 and you can read most of it in Google Books – though there are a few missing pages.

  • Err

    In addition to the above reasons about people with college educations (in US/Europe, higher levels of education correlate with less religiosity, which means that GFAM doesn’t work), how about the fact that those who are comfortably middle class would not be with children. Without children, they’re able to afford a moderately comfortable lifestyle, but with, they drop down to a few steps above poverty.

    How about another one that hasn’t been mentioned: it’s hard work and it’s annoying. If you’ve already got a busy career with also sorts of pressures, why would you want to add more? Haven’t we evolved traits that cause to avoid things that we find obnoxious?

  • Vladimir

    Robin Hanson:

    Bergstrom undercut #1, and fertility fell lots before contraception tech, saying #5 is minor. #2 and #3 don’t obviously raise the cost of kids relative to income, or to kids’ value to parents. That leaves #4, some unexplained feature of how kids give moms status.

    Even if we accept the conclusions cited by Caplan (of which I’m highly skeptical), this list is far from exhaustive. Off the top of my head, here are a few reasons that aren’t covered by it, but seem highly relevant to me:

    (1) The opportunity cost of having children has increased immensely, and continues to increase. There’s more and more stuff you can do that’s much more fun for (most) young people than raising kids, and the oldest age at which hedonistic single life is still acceptable keeps increasing. (“30 is the new 20″ — though the exact direction of causality here is moot.)

    (2) The disappearance of the patriarchal role of elders. One of the main benefits of having a family was the expectation that in old age, one would become a patriarchal figure commanding vast respect and authority. (This was true for both men and women in traditional Western societies.) Nowadays, however, old people are commonly estranged from their offspring and viewed as an unpleasant burden. Partly this is because of purely cultural changes, and partly because the world is changing so rapidly that old people are viewed as being hopelessly behind the times, rather than as sources of valuable wisdom.

    (3) Even not counting the diminished esteem shown towards parents and grandparents, there’s the fact of increased workforce mobility. There’s a high probability that your kids will move far away, possibly as soon as they reach university age, and that they (and their kids) will remain faraway strangers that you see only once in a year or two. This of course greatly diminishes the expected enjoyment that your adult children and their offspring should bring.

    (4) The increasingly competitive nature of the class system and the increasingly brutal status hierarchy, where lower-status people are being treated with ever increasing contempt. This situation is whipping many parents into a mentality where having kids is almost criminally irresponsible if they can’t ensure them optimal conditions at great cost. Again, partly this is because of objective factors — the breakdown of civilized norms of living among the lower classes means that ensuring a civilized environment for one’s kids is becoming increasingly costly. To a large degree, however, this is also because of the unrealistic “Yale or jail” mentality.

    This are just a few passing observations, without any pretense at a comprehensive explanation of these trends.

  • http://entitledtoanopinion.wordpress.com TGGP

    improbable, there was population growth until the Malthusian limits were reached. There were periodic negative shocks to population such as wars, famines and epidemics. After the Black Death England’s population greatly declined, income per worker increased, and subsequently the population growth rate was significantly positive until again reaching carrying capacity.

    jonathan, it used to be the case that a higher income meant more children, as I was just saying to improbable. In the pre-industrial era, (at least in unequal societies like England) the poor did not have enough surviving children the reproduce themselves. They were replaced by the surplus children of the wealthier, who would experience downward mobility.

    FuzzyThinker, women having higher standards and refusing to settle could possibly explain a lower marriage rate. I don’t see how it explains a lower fertility rate for married women.

    • Zukov

      FuzzyThinker, women having higher standards and refusing to settle could possibly explain a lower marriage rate. I don’t see how it explains a lower fertility rate for married women.

      Is this controlling for age of marriage? If women have higher standards and are refusing to settle and get married until they’re older, all else equal fertility should drop.

    • improbable


      Yes I agree there was some growth; I was trying to make the point that it was slow, that 4-(surviving)-children families were never the norm. It didn’t go much above 2 to replace people after the black death, I don’t think. I point this out because I grew up thinking everyone had 5, 6 kid families before the 20th C, but that would lead to population doubling every decade or two, and this was unheard of. (I have England in mind.)

      Certainly as you say the rich had larger families, the very rich much larger. If this was simply about wealth (in isolation) then indeed there is a puzzle, of why later common people, though just as wealthy, did not have such large families.

      What I would like to argue is that perhaps partly it wasn’t simply that their wealth let them have large families. Perhaps it was in part a response to those around them? (The rich spent a lot on signals like white gloves, too, which are now no longer a symbol.) I don”t think this can be the whole answer, but it might be part of it.

  • Douglas Knight

    Most of these comments don’t reflect the correct historical scale. In the 18th century, Hume and Franklin predicted the demographic transition by observing its beginnings in upper classes. I believe that their claim is that parents max out investment in the human capital of their children, or at least invest enough to expect their children not to fall in status. I don’t know if they gave a deeper reason for this proximate psychology.

    Similar theories (without the 18th century pedigree) were mentioned more often in the comments on Caplan’s post. It might match Vladimir’s #4, though he seems to be looking too recently. Perhaps it matches Zukov’s comment on K-selection, but I think Hume and Franklin would say not that the memetic environment has changed, but that larger investments in human capital are possible; or larger investments are necessary to keep up.

  • Joe

    “we just don’t understand how or why natural selection gave us preferences that, in our modern environment, produce such unadaptive behavior”

    What is “unadaptive” about it?

  • Rachel H

    @Wei Dai said
    “Would you also say that we don’t know how natural selection gave us such an unadaptive immune system?”

    It is my never humble opinion that our immune systems have adapted, but not for the better.

    An obsession with germ killing (freakin’ hand sanitizers and antibacterials EVERYWHERE) leaves us less exposure, thereby weakening the immune response.

    It doesn’t get enough exercise.

    And the germs have incredible adaptive immune systems – witness the ubiquitous “antibiotic resistant” bacteria and ever changing viruses (AIDS, for example).

  • Jason Malloy

    “But the basic question remains open. While we have some good clues to proximate causes, we just don’t understand how or why natural selection gave us preferences that, in our modern environment, produce such unadaptive behavior.”

    My theory for the demographic transition is here. In short, humans have a weak innate drive to reproduce, but strong innate drives to have sex and gain social status. In the past reproduction was largely mediated through these drives. Prior to female educational and economic integration, the female status drive was adaptive; it resulted in higher reproductive success — i.e more surviving offspring — because women would seek to elevate their status through the one channel available: by acquiring high status mates. In turn this made high reproductive success itself a desirable symbol of social status, since higher status women had more children.

    But post-women’s liberation, status drive would have the opposite effect. Now status motivated women have to marry well and participate in the workforce to remain competitive with other women in the adult status arena. The time and expenses necessary for raising children are instead invested in education, career-building, and conspicuous consumption. In turn this makes low reproductive success a symbol of social status, since higher status have fewer children.

    The link between demographic transitioning and status drive are supported by a number of newer papers in the economics literature.

    For example, one recent study from Brazil suggests that status imitation drove their demographic transition. As soon as different regions acquired access to Soap Operas (1960-2000) about small, middle-class Brazilian families, local birth rates would drop dramatically to the levels featured in the TV shows (from 6.3 to 2.3 children), and parents would name their children after the characters in those shows.

    A similar effect on fertility followed cable television introduction in India.

    In other words, as soon as women see higher class women adopt low fertility behaviors, they rapidly follow suit.

    • http://hanson.gmu.edu Robin Hanson

      Those are very relevant and interesting articles.

  • Jason Malloy

    I should have said, “because women would seek to elevate their status through the one channel available: by acquiring high status mates. And higher status males could subsidize more children.”

  • http://lessertruth.wordpress.com/ Marcio Baraco

    I have also long pondered this odd puzzle of the modern world. Rousseau said that you could judge whether a government was good or bad according to whether the population was increasing or decreasing, and this metric would say weird things about the present world — or would it?

    The comment about K/r is (IMHO) the only one who seems to be on track, but it fails to mention exactly how we should apply it to our case.

    My present line of reasoning goes along the lines: in contemporary affluent societies human beings are competing not with environmental factors but with other humans, whereas in more traditional societies the weight of this intra-species competition was less relevant.

    In any case, i think the phenomenon underlines how much what we think as “material wealth” is actually not very relevant to survival but exclusively as status signaling.

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