How Long Should Breastfeeding Last? The Science Says…

Modern society lets cultural beliefs about breastfeeding trump biological needs. 


  • Breastfeeding has been made a contentious issue in western culture.
  • According to science, typical western breastfeeding practices do not match what children need.

Families are in a pickle. Parents in the USA are unable to provide what their children need from a biological and scientific viewpoint. Stress among American families may never have been greater, even before the pandemic (American Psychological Association, 2019). It may help to understand what has brought us to this point.

When hierarchical authorities became too demanding, feudalism began to fail across Europe by the 15th century. Instead of listening to and adopting peasant ideas of social transformation, for next centuries European aristocrats took over common lands and banned hunting and gathering, which dissolved communities and their self-governing solidarity, creating deep economic inequality, massive social dislocation, and homelessness.

Watch the Evolved Nest’s six minute film, Breaking the Cycle, and find resources and a film guide here. Based on the award-winning science of Darcia Narvaez, PhD.

All this was part of what Karl Polanyi (2001) called the ‘great transformation’ to the type of societal structures that govern western societies today and what western institutions continue to export to the rest of the world. What has been normalized are privatization of common goods and resulting forced wage labor from people no longer permitted to subsist on their own as they had for generations. Even in North America, forests were cut down so that homesteaders could not hunt and forage for subsistence but had to work in factories to earn money for food (Stoll, 2017).

Consequences of the great transformation include market and exchange as central forms of interaction rather than gift economies and relational indebtedness (Graeber, 2013; Vaughan, 2015). Polanyi’s (2001) long list of consequences includes political turmoil; hunger and poverty; migration to cities for jobs and food; ecological abuses by the non-local control of environmental resources. All these have caused great stress for generations of families.

We can be more specific about effects on children. Those in control of economies and guarding access to subsistence needs expected long work hours of the non-elites. Only work outside the home would be paid. With low wages and no other sources to meet basic needs, everyone had to work. When mothers had to leave home to work in factories (textile initially), they breastfed less, and the skulls of their children showed what happened—a shrinkage of the palate, which is related to sleep problems and orthodonture issues. It’s an ongoing problem as work for mothers is still a priority in western and westernized cultures.

What also happened over the last few hundreds of years was the denigration of anything to do with bodies, especially female bodies. The scientification of having and raising children began with the overtaking of midwifery by male doctors. Ancestral knowledge and wisdom, often carried by elder women, was thrown out as ‘old wives’ tales’ (or destroyed in the “witch” executions that took place from the 15th to the 19th centuries; Federici, 2000). Instead, opinion masquerading as proven fact started to prevail. It is still the case.

It is common to believe that artificial infant formula is about as good as breast milk. It is common to believe that breastfeeding should not go on too long because it can be psychologically damaging. Both of these beliefs are contrary to the scientific knowledge we now have available. Infant formula contains non-human ingredients missing most brain and immune-system-building ingredients breast milk has. Children in traditional societies who breastfeed in a species-typical manner are happy, healthy, and cooperative.

What is the normal length of breastfeeding for the human species? It is a complicated question because humans evolved, like all primates, in tropical areas where food sources were plentiful for the taking. When hominids, our line, moved into a variety of environments, from artic to deserts, the raising of children adjusted with the food sources. When fire allowed cooking of foods, and grinding and processing of plant foods took place, adult diets changed which impacted children’s diets. Then with settled mono-agricultural societies, a further shift occurred when the range of foods was narrowed from dozens to a handful.

But scientists can still answer the question by examining practices among primates, human societal practices that vary according to available resources, biological developmental comparisons across species that include length of gestation, child weight, and adult size. Dettwyler (1995) examined all these.

She asked, what is the ‘hominid blueprint’ that is not contaminated by cultural beliefs? “What would modern human breastfeeding and weaning patterns look like if they were not modified by beliefs?” (p. 42)

Reviewing existing research, she examined weaning age (complete cessation of breastfeeding) and several gauges that have been proposed as guides in deciding:

  • Tripling or quadrupling of birth weight
  • Reaching one-third adult weight
  • Adult female body weight
  • Length of gestation
  • Age at eruption of first permanent molar

According to analyses of good data in each of these criteria, the answer to how long breastfeeding should last is six years.

One particularly useful tool is the eruption of the first permanent tooth, which occurs around age six. The timeline is the same no matter what kind of nutritional experience a child has had. The tooth’s appearance signals various changes, including a cognitive shift in the child, hence most schooling starts around this age. Its appearance is also linked to immune system development reaching adult levels of capacity. A child’s active immune response is otherwise enhanced by the lymphokines in maternal milk, hence their value until the child’s immune response reaches full capacity around age six (Hahn-Zoric et al., 1990; Pabst & Spady, 1990).

Dettwyler (1995) notes: “It is possible that the conjunction of weaning, first permanent molar eruption, and achievement of adult immune competence is the result of several different and unrelated genetic factors affecting rates of development in primates” (p. 56).

Although some believe breastfeeding for 12 or 24 months is “long” for our species, it is not. In fact, there are many mothers who follow their instincts and the needs of the child who end up “closet nursing,” breastfeeding the child beyond the culturally acceptable age as believed by their pediatricians, families or friends (which ranges from 3-12 months). These mothers should be applauded.

Dettwyler concludes that the average age of weaning for our species is minimally 2.5 years, with 6 years recommended (though up to 15 years has been documented). Interestingly, James Prescott (1996), formerly of the National Institutes of Health, observed that peaceful societies typically carry and breastfeed their infants until at least 2.5 years of age. In an era of increasing dysregulation, turmoil and violence, perhaps this is the guidepost we should be using, and establish policies and practices that make it possible.


American Psychological Association (2019). Stress in AmericaTM2019. Washington, D.C.: American Psychological Association.

Dettwyler, K.A. (1995). A time to wean: The hominid blueprint for the natural age of weaning in modern human populations. In. P. Stuart-Macadma & K.A. Dettwyler (Eds.), Breastfeeding: Biocultural perspectives (pp. 39-74). NY: Aldine de Gruyter.

Federici, S. (2014). Caliban and the Witch: Women the body and primitive accumulation. Autonomedia.

Graeber, D.R. (2013). Debt: The first 5000 years. Brooklyn, NY: Melville House.

Hahn-Zoric, M.F. et al., (1990). Antibody responses to parenteral and oral vaccines are impaired by conventional and low protein formulas as compared to breast-feeding. Acta Paediatrica Scandinavica, 79, 1137-1142.

Pabst, H.F., & Spady, D.W. (1990). Effect of breast-feeding on antibody response to conjugate vaccine. Lancet, 336, 269-270.

Polanyi, K. (2001). The great transformation: The political and economic origins of our time, 2nd ed. Boston: Beacon Press.

Prescott J.W. (1996). The origins of human love and violence. Pre- and Perinatal Psychology Journal, 10 (3), 143-188.

Stoll, S. (2017). Ramp Hollow: The ordeal of Appalachia. Hill and Wang.

Vaughan, G. (2015). The gift in the heart of language: The maternal source of meaning. Mimesis International.

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