How long should a mother breast-feed? Current advice is that it is very much up to what feels right for her and her baby, although anywhere between about six months and a year is considered normal.
Before the baby is three or four months old, the transfer of maternal immunity via breast milk is not thought to be complete. After 18 months, questions start to be raised about the mother's motivation for continuing to suckle her child.
These estimates of what is the "natural" length of time for breast-feeding are apparently wildly inaccurate. An American professor believes the biologically natural period for humans to nurse their children is between three and six years and she has theories to back this up.
This claim, which will surely horrify legions of mothers, not to mention those who blanche at the sight of mothers nursing in public, is based on a new evolutionary perspective.
"If you look at the length of time other large primates nurse," says Katherine Dettwyler, an associate professor in the department of anthropology at Texas A&M University, "and at what is common in nearly all hunter-gatherer societies, you arrive at a figure of anywhere between three and six years.
"From a global perspective the Western practice of cutting off babies at six months or so is the unnatural one."
The current norm has been arrived at in several ways. One rule of thumb is that nursing should stop when the baby has reached around three times its birth weight, which usually occurs by about 12 months. Another is based on the principle that mammals nurse their young for about the same amount of time that they are in the womb.
But Professor Dettwyler, co-editor of a book on breast-feeding due out later this year, says both these principles are based on a faulty interpretation of the animal studies.
"The three-times-the-birth-weight guide was re-evaluated in a study published last year. A more accurate norm for primates is four times the birth weight. For humans that gives a weaning time of two to three years," she says.
Another series of primate studies shows that they continue nursing until the offspring is about one-third of their adult weight. Applied to humans, this would result in nursing girls for five and three-quarter years and boys for seven.
Professor Dettwyler's analysis of non-human primates also shows that while some smaller mammals nurse for about the same time as the gestation period, the figure for our nearest relatives - chimpanzees and gorillas - is six times their gestation length. Applied to humans, that gives a nursing time of about four-and-a-half years.
A final biological marker that has been linked to weaning in non-human primates is the appearance of the first of the permanent teeth, which in humans happens at about six years. "This could be because the youngster is now fully equipped to deal with its own food," she says. "But there could be another reason. Around the age of six a child's immune system becomes fully mature, before that it can still be boosted by immune factors in the mother's milk."
Professor Dettwyler believes early hominids probably weaned their children at about six years. But as methods of cooking food developed, and much later, of grinding up plant material with mortars, it became possible to provide a supply of baby food. "It could be that over several hundred thousand years the natural weaning time was reduced to three or four years. Mothers who did this would have a reproductive advantage as they would be able to have more children."
This is all very well but humans are not chimpanzees. But Professor Dettwyler's point is that breast-feeding is a basic physical function and that we ignore what our bodies are designed for at our peril. She points out that a lengthy nursing period does not mean children go without solids, but that breast milk is available as well as solids, usually morning and evening.
She hopes the new observations will encourage a more tolerant and informed attitude to breast-feeding older children. "Here in the States extended nursing has been used in custody cases as evidence that the mother is unfit. There are psychologists who will say it is harmful because it infantilises the child. The truth is the opposite.
"There is considerable evidence that breast-fed babies are at less risk than bottle-fed ones for a range of conditions, including allergies, obesity, cot death, cancer, diabetes, multiple sclerosis and ear infections.
"The most recent evidence is that breast-feeding is linked to a higher IQ and better grades. And the longer nursing continues, the better the results get."
She does not believe in a specific age for weaning but says parents ought to know that the advantages of breast-feeding can continue for several years. "Ideally, it would be nice to let children wean themselves when they are ready."
One barrier to long-term nursing is the way that in the West the breast has become a sexual symbol. Professor Dettwyler takes the view that they are there to feed children. She cites in support of this opinion a study of 190 cultures which found that only 13 viewed breasts as sexually desirable.
Weaning Western males off their love affair with the breast might seem a hopeless task, but she is optimistic. "Look what has happened to public attitudes towards smoking, who would have believed cigarettes could have gone from stylish to stupid so fast. I dream of the day when breast-feeding is as normal and public as eating chips."
`Breast-feeding: Biocultural Perspectives', edited by Patricia Stuart Macadam and Katherine A Dettwyler, is due to be published by Aldine de Gruyter.Reuse content