lean seasons of marginal food supply, when it becomes difficult for the small number of productive adults to provide food for the larger number of non-producing children and old people. An additional consuming but non-productive mouth is then a burden that the society can ill afford.
Another circumstance associated with infanticide is a short birth interval: i.e., an infant born within only two years of the birth of the mother’s previous child that is still nursing and being carried. It is difficult or impossible for a woman to produce enough milk for a two-year-old and also for a newborn, and to carry not just one but two children while shifting camp. For the same reason, twin births by hunter-gatherer women may result in the killing or neglect of at least one of the twins. Here is an interview with an Ache Indian named Kuchingi reported by Kim Hill and A. Magdalena Hurtado: “The one [the sibling] who followed me [in birth order] was killed. It was a short birth spacing. My mother killed him because I was small. ‘You won’t have enough milk for the older one [i.e., Kuchingi],’ she was told. ‘You must feed the older one.’ Then she killed my brother, the one who was born after me.”
Still another factor predisposing towards infanticide at childbirth is if the father is absent or dead, and thus unable to help feed the mother and protect the child. For a single mother, life is hard even today. It was harder in the past, especially in societies in which lack of a father tended to result in a higher probability of a child dying, e.g., because fathers provided most of the calories or protected their children against violence by other men.
Finally, in some traditional societies the ratio of boys to girls increases from birth to adolescence, as a result of female infants dying through passive neglect, or (in exceptional cases) even being intentionally killed by strangling, exposure, or burying alive—because many societies value boys over girls. For example, among the Ache Indians, 14% of boys but 23% of girls have been killed by the age of 10. The absence of either the father or the mother increases by four-fold the chance that an Ache child will be killed by homicide, but the risk is higher for girls than for boys. In modern China and India that widespread valuing of boys over girls results in an excess of infant boys by a new mechanism: pre-natal sex determination permitting the selective abortion of female fetuses.
The !Kung consider it a mother’s obligation to evaluate the case for or against infanticide at the time of childbirth. The sociologist Nancy Howell wrote, “The custom that women should or can give birth alone gives the mother the unquestioned right to control infanticide. At the scene of the birth, usually before the baby is named and certainly before bringing the baby back to the village, it is the mother’s responsibility to examine the baby carefully for birth defects. If it is deformed, it is the mother’s duty to smother it. Many !Kung informants told me that this examination and decision is a regular and necessary part of the process of giving birth. !Kung infanticide is not equivalent to murder in their eyes, since they do not consider birth to be the beginning of life of a zun/wa [a !Kung person]. Life begins with giving a name and the acceptance of the baby as a social person back in the village after the birth. Before that time, infanticide is part of the mother’s prerogatives and responsibility, culturally prescribed for birth defects and for one of each set of twins born. There are no pairs of twins surviving in the population….”
However, infanticide is certainly not universal in traditional societies and is less common than infant death due to “benign neglect.” (That euphemism means that an infant is not actively killed but instead dies through neglect, e.g., due to the mother stopping nursing, or nursing the infant less often, or rarely cleaning or washing the infant.) For example, when Allan Holmberg lived among a group of Siriono Indians in Bolivia, he found that infanticide and abortion were unknown. Even though 15% of Siriono children were born with club feet, and only one out of five of those children survived to adulthood and raised a family, those children received normal love and feeding.
Weaning and birth interval
In the U.S. the proportion of infants who were nursed at all by their mothers, and the age at which those nursed infants were weaned, decreased through much of the 20th century. For example, by the 1970s only 5% of American children were being nursed at the age of six months. In contrast, among hunter-gatherers not in contact with farmers and without access to farmed foods, infants are nursed far beyond six months, because the only suitable infant food available to them is mother’s milk: they have no access to cow’s milk, baby formula, or soft food replacements. The age of weaning averaged over seven hunter-gatherer groups is about three years old, an age at which children finally become capable of fully nourishing themselves by chewing enough firm food. While some solid pre-chewed foods may be introduced around the age of six months, a hunter-gatherer child may not be fully weaned off its mother’s milk until the mother is pregnant with the next child. Individual !Kung children continue to nurse beyond the age of four if a next sibling has not yet been born. Studies show that, the older the age of a !Kung child when it is weaned, the more likely is the child to survive to adulthood. But in settled agricultural populations and among hunter-gatherers trading with farmers, those weaning ages and birth intervals of two and a half to four years for nomadic hunter-gatherers decrease to an average age of two years, because farmers do have livestock milk and soft cereal gruels onto which to wean a small child. For instance, when the !Kung themselves settle down to become farmers, as has been happening increasingly in recent decades, their birth interval quickly drops from three and a half years to the two years typical of farmers.
The ultimate evolutionary causes and the proximate physiological mechanisms responsible for those long birth intervals of nomadic hunter-gatherers have been the subject of much discussion. It appears that the ultimate reasons are two-fold. First, a mother without access to cow’s milk or cereal gruel, and hence likely to nurse a child until the age of three or more years, cannot produce enough milk to nurse both a newborn and a not-yet-weaned older child. If she tried, one of those children would be likely to starve for lack of milk.
The other reason is that only when a child is around four years old or more does it become capable of walking fast enough to keep up with its parents when they are shifting camp. Younger children have to be carried then. While walking, a 90-pound !Kung woman has to carry an under-four-year-old child of up to 28 pounds, a load of wild vegetables weighing from 15 to 40 pounds or more, and several pounds of water, plus utensils. That’s already a large burden, and it would be even heavier if a younger infant were added to the load. We thus have a second ultimate evolutionary factor contributing to the rapid decrease in birth interval when nomadic hunter- gatherers settle down to become farmers: most farmers live in permanent villages and don’t face the problem of having to carry all children less than four years old whenever they shift camp.
That late weaning age means that, for a hunter-gatherer mother, much physical and emotional energy goes into the rearing of one child. Western observers have the impression that a !Kung child’s very close relationship with the mother, and the exclusive attention that it enjoys for several years without younger siblings, provide an emotional security in childhood that translates itself into the emotional security of !Kung adults. But when a hunter-gatherer child finally does become weaned, the result can be traumatic. Within a short time, the child receives much less maternal attention, becomes hungry without mother’s milk, has to cede to the next infant its sleeping place at night next to the mother, and may be increasingly expected to enter the adult world. !Kung children being weaned are miserable and have tantrums. !Kung who survive to become old adults still look back on weaning 70 years earlier as a painful experience. In camps of Piraha Indians at night, one often hears children screaming, almost always because they are being weaned. Nevertheless, while traditional societies do wean at a later age than do modern Americans, the specific patterns vary among societies. For example, Bofi and Aka Pygmy children wean gradually rather than abruptly, tantrums are rare, and weaning is often initiated by the child rather than by the mother.
On-demand nursing
Those two ultimate causes responsible for the long birth intervals of hunter-gatherers leave open the question of the proximate physiological mechanism ensuring that there are not two children less than several years old to be cared for simultaneously. One mechanism is the resort to neglect or (less often) infanticide, as we have already mentioned: if a hunter-gatherer mother becomes pregnant when her previous child is still less than two and a half years old, then she may neglect or even kill the newborn, knowing that she cannot take care of it as well as of the older child. The other proximate factor is that physiological mechanisms operating in a mother nursing according to the on-demand schedule of frequent feedings characteristic of hunter-gatherer babies (as opposed to
