Biological determinism: The “natural” range of weaning

I’m not quite ready to let go of Kathy Dettwyler quite yet. Dettwyler is the author of a scholarly paper which I’ve read but cannot find, unfortunately, but a layperson’s version can be found at her site, which claims that the natural range of weaning for humans is 2.5-7 years.That is, supposedly all human children would cease nursing between those ages if our (stupid and unnatural, it’s implied) cultural biases weren’t involved. She bases this upon various extrapolations made from the animal, and particularly the primate, world – animals wean when they reach a certain weight, or grow certain teeth, and what not.

I’m sure Dettwyler is correct in the particulars of her descriptions of the animal kingdom (though I’m hard pressed to know which “pediatricians” she claims to have culled this knowledge from. I know precious few pediatricians who are also comparative zoologists), but it’s all really beside the point: the way the information was gathered about the animal kingdom was by observation, not by extrapolation. We should be applying the same standard when studying humans. And as Dettwyler herself says (and I have no reason to disbelieve her):

In societies where children are allowed to nurse “as long as they want” they usually self-wean, with no arguments or emotional trauma, between 3 and 4 years of age.

This, therefore, is what we’d expect to be the “natural” (i.e, rules-less) age of weaning. What’s more, this actually corresponds to what pre-industrial societies do: in a large study about infant feeding patterns, spanning nearly a century of anthropological research about 133 different nonindustrialized societies, the average age of weaning was 29 months, with a range of 12-66 months (1-5.5 years). The authors noted (emphasis mine):

Classical evolutionary biological approaches have focused on the adaptive aspects of weaning in relation to life history and aimed to infer a human pattern from analysis of nonhuman primate data. Cross-species comparisons have generated several models to predict ages at which primates are adapted to terminate lactation, which include: 1.5 times the length of gestation , eruption of first molar teeth, tripling or quadrupling of birth weight and attainment of one-third adult weight. Unfortunately, the range of variation in weaning patterns within and between species of nonhuman primates has yet to be fully described and explained, and no model reliably predicts age at weaning for all primate species. When applied to humans, each model yields a wide range of predicted values due to variation in birth and body weights across populations, most likely because they do not incorporate the modifying effects of diet, work activities, culture, social organization and technology. Independent characterization of species-typical values for human weaning parameters will facilitate evolutionary analyses of human life history traits.

See, what Dettwyler’s overlooking is that it’s an inseparable part of human nature to create cultural rules – about weaning and everything else. Therefore, trying to extrapolate what humans would do if they were driven solely by biological forces is about as useful as speculating what humans would breathe if they had gills.

One of Dettwyler’s assumptions especially rankles in that regard: she claims that since the human immunological system isn’t fully developed until age 6 (true to an extent), some humans breastfeed until that age and beyond. However, if breastmilk were necessary to boost children’s immune system until it became fully functional, we’d expect all human societies – especially primitive ones with no access to antibiotics or modern hygienic practices – to nurse that long. The fact that the overwhelming majority of the world’s children don’t should be evidence enough that this assertion is bogus.

I would also submit that just because certain things may be “biologically correct”, doesn’t mean they’re actually the correct thing to do. For example, most girls in the western world develop reproductive ability between the ages of 8-16 years. Would any anthropologist be so bold as to suggest that girls as young as 8 should be reproducing, just because they can? It’s also biologically correct to walk around naked – after all, nobody is born clothed – but except for a handful of nudists, nobody actually walks around without clothing. Especially not outside, in mid-January!

I’m sure Dettwyler would absolutely love this family (short video). But Is this type of absolute worship at the Shrine of the Breast really natural to humans? I really hope not.

Unlike that woman in the video, Dettwyler acknowledges that the importance of breastfeeding diminishes as the infant grows into a child. She makes an amusing analogy about how even small amounts of breastmilk matter:

“Clearly the phenomenon of diminshing returns is at work here — the first six months of breastfeeding are clearly much more important in terms of the baby’s nutrition and immunological development than the six months from 3.5 to 4.0 years. That doesn’t mean that you shouldn’t continue to provide breast milk if your baby wants and you don’t mind. It would be like saying, “Well Mabel, we don’t get very much income from that oil well anymore. Used to get $56 a month in royalties, now we’re lucky if we get $25 a year. Guess we should tell that oil company just to keep their durn money.” And Mabel says, in return “Good grief, Clyde, don’t be ridiculous. That check still buys $25 worth of food. Where has your mind gone to now?””

I don’t know about Clyde, but if I all I were getting was a measly $25 in royalties from my proverbial oil well, I’d sell the land it stands on to the highest bidder and put the money in a high-yield CD. Or in real-life terms, I’d take the body attached to those lactating breasts and go do something else with it. Because, contrary to what Dettwyler asserts, breastfed children of that age (2 and beyond) have not been proven to be significantly “healthier, smarter or happier” (where on Earth does she get that last one?!) than other children.

UPDATE Jan 27th – a bit more on the subject here.

UPDATE #2 May 1st – and yet more.

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