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The Primate Diaries


Notes on science, politics, and history from a primate in the human zoo.
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Out of the Mouth of Babes

The views expressed are those of the author and are not necessarily those of Scientific American.


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Extended breastfeeding is the norm in most human and primate societies. So why are we the weird ones?

"Attachment (with respect to Martin Schoeller)" by Nathaniel Gold

"Attachment (with respect to Martin Schoeller)" by Nathaniel Gold

My son will be three-years-old next month and is still breastfeeding. In other words, he is a typical primate. However, when I tell most people about this the reactions I receive run the gamut from mild confusion to serious discomfort. Their concerns are usually that extended breastfeeding could be stunting his independence and emotional development–the “Linus Blanket Syndrome” in the words of Michael Zollicoffer, a pediatrician at the Herman & Walter Samuelson Children’s Hospital at Sinai Hospital in Baltimore. Worse yet, they hint that it might even cause “destructive” psychosexual problems that he will be burdened with throughout his adult life. Could they be right? Was our choice “a prescription for psychological disaster” as Fox News psychiatrist Keith Ablow wrote in response to TIME magazine’s provocative cover article on attachment parenting? Just when is the natural age to stop breastfeeding?

One thing I’ve learned in my research on human evolution is that people are quick to assume that what they do is “natural” simply because they don’t know of other examples where things are done differently. The primate brain is a pattern recognition machine and is adapted to quickly identify regularities in our environment. But when we are presented with the same pattern over and over again it is easy to fall victim to what is known as confirmation bias, or coming to false conclusions because the evidence we use does not come from a broad enough sample. In order to avoid falling for this bias on the question of extended breastfeeding the best way forward would be to draw from the largest sample possible: the entire primate lineage.

In their classic paper, “Life History Variation in Primates” published in the premier scientific journal Evolution, the British zoologists Paul H. Harvey at Oxford and Tim Clutton-Brock at Cambridge published the most comprehensive data then available on the world’s primates. The variables they measured included everything from litter size and age at weaning to adult female body weight and length of the estrous cycle among 135 primate species (including humans). By analyzing the relationships between these variables, using a statistical approach known as a regression analysis, they identified striking patterns that held across primate taxa.

One especially strong correlation was that adult female body weight was closely tied to their offspring’s weaning age, so much so that knowing the first would allow you to predict the second with a 91% success rate. As a result, as anthropologist Katherine A. Dettwyler has shown in her book Breastfeeding: Biocultural Perspectives (co-edited with Patricia Stuart-Macadam), it can be calculated that a young primate’s weaning age in days is equal to 2.71 times their mother’s body weight in grams to the 0.56 power. This calculation predicts, given the range of female body sizes around the world from the !Kung-San of South Africa to the Arctic Inuit, that humans should have an average weaning age of between 2.8 and 3.7 years old.

How well does this prediction hold for our species? According to data compiled by UNICEF, half of the world’s population continues breastfeeding until at least the age of two. Furthermore, weaning in these cases doesn’t mean the total cessation of breastfeeding. It simply means the introduction of solid foods, with supplemental breastfeeding continuing for months or even years. However, these statistics are all drawn from sedentary, agricultural societies that have at least some contact with modern trends in child development. What about those societies whose way of life is most like that of our Pleistocene ancestors?

To answer this question Yale University anthropologist Clellan Stearns Ford utilized the largest historical collection of anthropological data available, the Human Relations Area Files, and analyzed the weaning age of 64 non-Western “traditional” societies–small-scale horticultural and hunter-gatherer populations. His analysis (see Figure 1 below) determined that the average age of weaning is approximately three years old, just as Harvey and Clutton-Brock’s data predicted. Furthermore, because these traditional societies are dispersed throughout the globe and have no contact with one another (or often anyone except the visiting anthropologists) these societies offer a broad enough sample size to avoid the problem of confirmation bias.

A comparison of age at weaning

Figure 1. A comparison of age at weaning in the United States and in 64 traditional societies. Reproduced from Stuart-Macadam & Dettwyler (1995).

“Regardless of ecology,” write anthropologist Barry Hewlett and psychologist Michael Lamb in their book Hunter-gatherer Childhoods, “hunting and gathering groups are characterized by frequent and extended breastfeeding and extraordinarily high levels of parent-child physical contact and proximity.”

In contrast to these global trends among traditional societies and non-Western countries, U.S. government data estimates that fewer than 15% of Americans continue nursing their infants after they are just six months old (while Canadians are slightly higher with an average of about 25%). Likewise, as detailed by the Organisation for Economic Co-operation and Development Family Database [PDF], most countries in Western Europe cluster in the same 15-25% range as those in North America.

The worldwide trends therefore seem to be relatively straightforward: most humans tend to wean at a similar stage in their life history as other primates, which works out to about three years old based on our relatively large body size. This weaning age can then be adjusted based on the environment or traditions in a particular culture. However, Western nations appear to be an outlier to what is otherwise a natural behavior for our species. On this point the World Health Organization and UNICEF are in line with the predictions from primate life history. Both global health organizations recommend the following:

Initiation of breastfeeding within the first hour after the birth; exclusive breastfeeding for the first six months; and continued breastfeeding for two years or more, together with safe, nutritionally adequate, age appropriate, responsive complementary feeding starting in the sixth month.

The benefits of extended breastfeeding have been demonstrated both in the less developed and the industrialized world. For example, research carried out in Burkina Faso by epidemiologist Simon Cousins for the Bulletin of the World Health Organization and in Washington, DC by Dr. Kathleen M. Buckley for the Journal of Human Lactation, both showed that extended breastfeeding until three years old resulted in lower rates of malnutrition compared to those who were not breastfed as long.

Longer duration of breastfeeding has also been shown to significantly improve a child’s immune response to infectious disease. Writing in the British Medical Journal, Kåre Mølbak and colleagues at the University of Copenhagen analyzed the incidence of diarrhea in weaned and partially breastfed children in the West African Republic of Guinea-Bissau. They determined that not only did breastfed children get sick less often than weaned children, but those who continued partial breastfeeding up until the age of three had the lowest rate of infection. As the authors concluded:

Apart from the remarkably higher incidence of diarrhea in weaned children the clear decline in the rates of diarrhea in breast fed children in the second year of life was also surprising since the older children were breast fed irregularly and their main diet was as for adults.

Identical results were found in rural East Bhutan by Erik Bøhler and colleagues from the Department of International Health in Oslo, Norway, as reported in the journal Acta Paediatrica.

“Breastfeeding between 12 and 36 months of age was associated with reduced risk of diarrhea,” wrote the authors. “Breastfed children also gained significantly more weight during the monsoon season, and breastfeeding protected children against weight loss due to diarrhea.”

The unusually low level of breastfeeding in the United States therefore has public health implications rather than simply being a lifestyle choice. Ultimately, mothers–as well as fathers–need to decide for themselves how much, or how little, breastfeeding they are comfortable with. However, as a society, we can support their choices by making sure that everyone has access to reliable information and by creating a positive environment so that breastfeeding mothers aren’t subject to social stigmas or value judgements for doing what, after all, is only natural.

This piece has been corrected from how it originally appeared. Katherine A. Dettwyler calculated that human weaning should occur between 2.8 to 3.7 years old, not Harvey and Clutton-Brock.

Eric Michael Johnson About the Author: Eric Michael Johnson has a Master's degree in Evolutionary Anthropology focusing on great ape behavioral ecology. He is currently a doctoral student in the history of science at University of British Columbia looking at the interplay between evolutionary biology and politics.

Follow his work on Facebook and Google+. Follow on Twitter @primatediaries.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. blackbird79 4:15 am 05/16/2012

    I’d be considerably more worried about Fox “News”, Kirk Cameron, and other members of the persistently anti-empirical, religious extremist “base” of the Republican party offering “a prescription for psychological disaster” than I would about the extended effect of a crassly commercial magazine cover.

    Dr. K. A-Blowhard sold out what might have been his connections to actual science in order to become a political propagandist and dogmatist. His recurrent focus on the attractive blonde woman’s getting naked in front of other people — who he helpfully lists — (for money, as she’s a “model”) plainly has her placed on the predictable side of the Madonna/whore equation, depending on which Madonna we’re talking about. And places himself and Fox News, as usual, out on some irrelevant fringe of a social “debate”, such as it is.

    I might note that CNN, though “covering” the story beyond the point of tedium, also could not be bothered to reference pointedly any comprehensive, scientific, statistical profile that might help establish a reference point for judging normal human behavior.

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  2. 2. AkbarLightning 10:14 am 05/16/2012

    I am not against extended breast-feeding, but I contest your arguments.

    Any argument grounded in the presence of activity in other primates has no weight since it ties us to any practice among our less evolved cousins, like tossing feces around…and, we do that too…but my point is, that we, as a species have decided to make conscious choices about our practices, and therefore we repress many of our natural instincts. I am not advocating repression, but merely pointing to its inevitability, and how this inevitability is a glaring weakness in your argument style.

    There are so many cultural and statistical peculiarities that your argument depends upon, concerning health and its relationship to breast-feeding, that this begs further investigation. In other words, your certainty is in now way justified.

    This leads to my final point, my most confrontational one, but confrontation is a healthy part of science. As I have implied, our cultural choices can only rarely claim scientific soundness, so your argument would be WAY more powerful, if it were grounded in the psychological sciences, as this would more deeply reflect the concerns of the western parents, who actually have a choice…for many breast-feeding is a part of the natural environment within which they live. your article is a western article, and will fail to be convincing if it does not address the nature of choice, which is grounded in the psycho/social…and most of us will need more to go on than ‘the monkeys do it’…

    with respect,
    just another primate whose rage arises out of my early weening…lol

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  3. 3. kathyd 10:29 am 05/16/2012

    While I agree with everything you say in your Scientific American article, you need to know that what you have done in the way you have written the column, and cited me, constitutes serious academic dishonesty. You have obviously read my work and taken some information from it and repeated it without giving me the proper credit, and indeed, without being accurate. For example, the equation of weaning age in days = 2.71 x adult female body weight in grams is MISSING the information that it is the weight in grams to the .56 power. Also, I was the one who calculated (and published, in 1995) what size of adult females would mean for weaning age in Inuit and !Kung San populations, yet you write as though you did those calculations. Likewise, the Figure One you reproduce in your column is taken directly from my work. I’m the one who took the information from Ford and calculated the 2.8 year average. Ford himself did not do this. I would appreciate a public apology in the form of an addendum to your column on line.

    Katherine A. Dettwyler, Ph.D.

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  4. 4. EricMJohnson 10:54 am 05/16/2012

    Katherine: Thank you for your comment. I did come across your work and I can assure you that I was not trying to take credit for the calculations. This was the result of poor note taking on my part while preparing the piece so that I thought I was summarizing Harvey and Clutton-Brock’s work on the subject, not yours. I will immediately correct my oversight and give you the proper credit. Please accept my apologies for the mistake.

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  5. 5. AkbarLightning 11:54 am 05/16/2012

    a great example of the territorial instinct above…

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  6. 6. kclancy 12:07 pm 05/16/2012

    Thanks for this piece Eric! What a great perspective and I’m glad you placed humans in a broader primate context. My daughter self weaned at just over two and a half years (she is 4 now), though sometimes I think she thinks it was a mistake and wishes she had gone longer!

    If I missed this in your post, please excuse me, but I’d also like to point out that there are good reasons we primates breastfeed so long, related to immune function, brain development, maternal/infant closeness, and social and cognitive development. I wonder if the relationship between breastfeeding length and body size has something to do with the fact that larger bodied animals tend to “live slow,” and we are a particularly strong example of this with our extended learning period through childhood and adolescence.

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  7. 7. kclancy 12:10 pm 05/16/2012

    Also, Kathy: as we are all academic colleagues (Eric has a MA in ev anth and is getting his PhD in history of science, I’m an assistant professor at Illinois), giving Eric the benefit of the doubt in the tone of your comment, or emailing him privately, would have resolved your issue with his post just as quickly. If you read Eric’s work regularly you’d see he’s a pretty stand-up guy, and the error sounded like a simple mistake that he was clearly happy to correct.

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  8. 8. Pharmaguy 12:21 pm 05/16/2012

    I don’t mean to pile on, but there is also an apparent transcription error in the quotation from the BMJ article which completely changes its meaning: the sentence in the original article reads, “Apart from the remarkably higher incidence of diarrhea in weaned children”, NOT “breast-fed children”.

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  9. 9. EricMJohnson 12:42 pm 05/16/2012

    Thank you Pharmaguy. I always appreciate being corrected. Well, not ALWAYS, but I definitely prefer it to allowing a bonehead mistake to linger.

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  10. 10. drafter 2:44 pm 05/16/2012

    It seems clear to me that it’s not the breast feeding itself that diminishs diarrhea but the rate of change in types of food consumed and probably the type of food consumed. So you could start the weaning at a much younger age in fact it was jus a couple of generations ago that our society became so puritan that breast feeding was considered a no no, period at any age. I for example was born in the sixties and neither me or my sisters were breast feed, my father still though it absurd that my sister breast feed her children. I have no problem with breast feeding children and as is typical all children are different and need their own time frame however I believe there are some people who probably breast feed longer than the child needs to or even wants to.

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  11. 11. SiaraDelyn 3:26 pm 05/16/2012

    No way.

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  12. 12. eurotimbr 5:23 pm 05/16/2012

    Extended breast feeding is also a partially successful birth control technique. The presence of other options may be another reason for shorter periods of breast feeding in our society.

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  13. 13. KathyO 7:46 pm 05/16/2012

    In a recent study (http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0032452), the authors make the case that time to weaning is much more closely correlated to infant brain development than to maternal body size. The only reason I know about this is because I happen to be writing a blog post about it!

    In any case, extended breastfeeding (more than two years) is certainly a normal human experience.

    Link to this
  14. 14. Kim Williams 1:59 am 05/17/2012

    My daughter weaned just before her 3rd birthday. I wasn’t following advice in nursing that long, it just seemed the right time for us. When weaning happened we were both ready and it wasn’t stressful in the slightest. It’s nice to hear these natural instincts are supported by science!

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  15. 15. Mammals Suck... Milk! 10:46 am 05/17/2012

    Eric,
    Thanks for your awesome post! I just wanted to to note that the really tricky thing about investigating “weaning age” is that it implies that its an event, when we know that its an incredibly complex process in primates. There is no universal agreement about the definition of weaning: is it the timepoint when infants first consume solid foods? is it timepoint when they are no longer allowed any nipple contact? is it the point when <50% of their calories come from mother's milk? For the last definition, there is almost no way to know when that occurs without using doubly labeled water techniques to measure energy transfer, something that can be logistically and ethically problematic in most research contexts with primates. The takeaway is that the phylogenetic analyses about weaning age in primates, and implications for humans, are constrained by the murkiness of the operational definitions in the empirical literature. Moreover humans are unique in that not only do we engage in extensive alloparenting (tip of the hat to Kate Clancy's awesome post yesterday! http://tinyurl.com/7rcnumt), but we provide specially processed weaning/transitional foods. These practices may allow for weaning in humans to diverge importantly from other non-human primates in ways that the data in hand do not allow us to systematically tease apart.

    Katie Hinde
    http://mammalssuck.blogspot.com/

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  16. 16. daniellerigg 12:31 pm 05/17/2012

    Mr. Johnson, Best for Babes is in your debt for cutting to the core of Time Cover issue — our opinions and reactions are not based on facts but on feelings without much fact. You have hit the nail on the proverbial head and served humanity with this masterfully written, unbiased account of NORMAL HUMAN BEHAVIOR. We posted it on our Facebook page and its been shared 40 times in a few minutes! All our best, Co-Founders Danielle Rigg and Bettina Forbes

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  17. 17. Steve926 3:08 am 05/19/2012

    Why do western primates wean their off spring so soon? So Gerber can make a dollar.

    Anyways, is anybody engineering goats or cows to produce human milk?

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  18. 18. Denise Sumpter 10:44 am 05/19/2012

    I think that you will find that Kathy Dettwyler states the weaning age to be from approximatelt 2.5 to 7 years of age!! Serious error, there! Have you not read her article? it is easy to find by just googling!

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  19. 19. Mammals Suck... Milk! 3:40 pm 05/19/2012

    @Steve926 Teams of scientists in Europe and China are engineering transgenic dairy cows to synthesize human-type lactoferrin and lysozyme (immunofactors in higher concentrations in human milk thank in cow’s milk). Since commercial infant formula is usually manufactured from cow’s milk, it marks a step toward producing infant formula that features more of the factors that are found in human milk. But these are just two of the hundreds of constituents that differ in presence and abundance between human and cow’s milk so there remains a long, long way to go.

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  20. 20. EricMJohnson 5:31 pm 05/19/2012

    @Denise Sumpter: Dettwyler used a different diagnostic criteria to extend the range to 7 years. It wasn’t based on the 0.91 regression coefficient for adult female body size that Harvey and Clutton-Brock calculated.

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  21. 21. Old Geezer 6:20 pm 05/19/2012

    Eric, how much of the increased incidence of diarrhea in weaned children in the cited countries might be the result of their substituting tainted water as a source of hydration rather than any dietary insufficiency? Has the same analysis been made where a trustworthy source of water is readily available?

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  22. 22. CWagenet 1:03 am 05/21/2012

    Thank you for the fascinating article. I would like to make a correction, however. You say that, “U.S. government data estimates that fewer than 15% of Americans continue nursing their infants after they are just six months old.” I checked the link, and that statistic is for exclusive breastfeeding at 6 months. The percentage of women in the US breastfeeding at 6 months is 44.3% and at 12 months, 23.8%. Thus, the percentage of American women still breastfeeding after 6 months is between 23-44%.

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  23. 23. kbo569@gmail.com 9:23 am 05/21/2012

    Dear Eric-other than the little mishap with the KD reference I think you did a marvelous job in researching the anthropology literature on primate breastfeeding practices over the course of deep evolutionary time. I am a pediatric dentist and an anthropologist in-training. A couple of things I noticed in your column that might be of interest for further discussion in this area: 1.) the term “wean” has only one ‘anthropologically-correct’ definition (you seem to imply otherwise when describing North American and Western European weaning traditions vs. aboriginal weaning traditions); ‘weaning’ is simply the period of time/age-range within which infants cease exclusive (and on-demand) breast-feeding, until when they start being completely independent from breast-feeding; and 2.) while you accurately cite the UNICEF/WHO statement of ‘complementary feeding’ (“….together with safe, nutritionally adequate, age appropriate, responsive complementary feeding starting in the sixth month.”), there needs to be clarification as to what ‘nutritionally adequate’ is relative to food texture. My own research is centered around the hypothesis that infant and early childhood feeding (IECF) regimens impact development of the jaws, teeth and face. Virtually no child needed an orthodontist, otolaryngologist, sleep medicine specialist or the stimulant drug ritalin before women, the first skilled factory workers, entered the textile mills during the Industrial Revolution in the mid/late 18th-Century. That’s when the millions of years of an ancestral pattern of IECF first changed; women mill workers had to leave their children at home to be either wet-nursed or artificially fed with inferior processed formulas, manufactured rubber nipples and highly processed (soft) baby foods. Now we have an epidemic of crooked teeth, pediatric sleep disorders and attention deficit disorders…..I don’t think this is mere coincidence. And please note, I am not suggesting that women stay home from their jobs for 3+ years so they can ancestrally feed their babies, rather, we need to be more supportive of women who do indeed wish to feed their children according to a pattern that their instincts tell them is ‘correct’. And for children whose moms choose a more modern form of IECF regimen, we will need to find alternative ways to physically challenge their jaws and teeth during the formative years of development…maybe (very) early orthodontic expansion, and/or, maybe a pacifier and/or baby bottle nipple that physically works like a real breast?

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  24. 24. hartson 1:02 pm 05/22/2012

    My mother weaned went she got bit.

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