In the United States, women receive conflicting advice about when to wean their children completely from breastfeeding. The American Academy of Pediatrics recommends one year, while WHO and UNICEF recommend at least two years. Many physicians consider six months to be "extended" breastfeeding, and some health professionals question the motives of women who nurse for more than a year. In turn, women may hide the fact that they are still nursing an older child from disapproving health care professionals or family members. From anthropological research, we know that in many non-Western cultures children are routinely nursed for three to four years. Are they eccentric, or are we? Can we look to other animals to determine what the natural age at weaning would be in modern humans if it was not modified by cultural beliefs?

Like all mammals, humans have mammary glands that function to nurture their offspring. Within the class Mammalia, humans are members of the order Primates, and have the basic primate pattern of breastfeeding and weaning activity that has been molded by more than 65 million years of natural selection to ensure the best possible survival rate of primate offspring. This basic pattern is assumed to be primarily genetically based. In addition, a number of life-history variables are also associated with age at weaning in the non-human primates. What do these variables suggest about the "natural" age of weaning in humans?

Weaning according to tripling or quadrupling of birth weight

The idea that mammals wean their offspring when they have tripled their birth weight is widely reported in the breastfeeding literature (Lawrence 1989). This rule of thumb holds true for small-bodied mammals, but not for larger ones. Recent research has looked at age at weaning and at growth among large mammals, including primates. The research shows that weaning occurs some months after quadrupling of the birth weight, rather than tripling (Lee, Majluf and Gordon 1991). When do U.S. infants typically quadruple their birth weight? For males, the average age is around 27 months, and for females, around 30 months.

Weaning according to attainment of one-third adult weight

Other studies suggest that primates are like other mammals in weaning each offspring when they reach about one-third their adult weight (Charnov and Berrigan 1993). Humans come in different sizes, but 4 to 7 years of nursing would be the weaning age for humans using this method of comparison, with boys generally being nursed longer than girls, and large-bodied populations nursing longer than small-bodied groups.

Weaning according to adult body size

Harvey and Clutton-Brock (1985) published a study of life-history variables in primates, including a formula for calculating age at weaning based on adult female body weight. The equation predicts an age at weaning for humans at between 2.8 and 3.7 years, depending on average adult female body weight, with larger-bodied populations nursing the longest.

Weaning according to gestation length

It is often reported in the literature that, among mammals in general, weaning age is approximately the same as the length of gestation (Lawrence 1989). By this criterion, weaning in humans might be expected to take place after only nine months of breastfeeding. However, this one-to-one relationship is greatly affected by the adult size of the animal. For many small-bodied primates, the duration of breastfeeding is shorter than the length of gestation. Among large-bodied primate species, the duration of breastfeeding far exceeds the average length of gestation. For humankind's closest relatives, the chimpanzee and the gorilla, the duration of breastfeeding is more than six times the length of gestation. Humans are among the largest of the primates, and share more than 98 percent of their genetic material with chimpanzees and gorillas. Based on these comparisons, an estimated natural age at weaning for humans would be a minimum of six times gestational length, or 4.5 years.

Weaning according to dental eruption

According to the research of Smith (1991), many primates wean their offspring when they are erupting their first permanent molars. First permanent molar eruption occurs around 5.5 to 6.0 years in modern humans. It is interesting to note that achievement of adult immune competence in humans also occurs at approximately six years of age, suggesting that throughout our recent evolutionary past, the active immunities provided by breast milk were normally available to the child until about this age (Fredrickson).

Our evolutionary past has produced an organism that relies on breastfeeding to provide the context for physical, cognitive, and emotional development. The human primate data suggest that human children are designed to receive all of the benefits of breast milk and breastfeeding for an absolute minimum of two and a half years, and an apparent upper limit of around seven years. Natural selection has favored those infants with a strong, genetically coded blueprint that programs them to expect nursing to continue for a number of years after birth and results in the urge to suckle remaining strong for this entire period. Many societies today are able to meet a child's nutritional needs with modified adult foods after the age of three or four years. Western, industrialized societies can compensate for some (but not all) of the immunological benefits of breastfeeding with antibiotics, vaccines and improved sanitation. But the physical, cognitive, and emotional needs of the young child persist. Health care professionals, parents, and the general public should be made aware that somewhere between three and seven years may be a reasonable and appropriate age of weaning for humans, however uncommon it may be in the United States to nurse an infant through toddlerhood and beyond.