Men, Women, Infanticide
by Michael G. Heller
Published in Social Science Files, June 11, 2025
Childless Millionaire and The Poor with Children by Niko Pirosmani (1862–1918), Russia
Normal in 2000 CE, therefore normal in 200,000 BCE?
Why did men and women become monogamous couples 100k-200k years ago, and why was this an extremely positive evolutionary preparation for the building of the first primitive ‘societies’? I will argue that the vital preparatory step was a rational and negotiated division of labour between independent male and female individuals.
Before presenting that argument more fully I now discuss an alternative reason for coupling that has been proposed in evolutionary science — the dangers of infanticide.
The standard evolutionary explanation for infanticide among mammals, and especially primates, is as follows. Since a dominant male usually has only a limited number of years for optimal breeding he maximises opportunities for reproduction by killing the infants of females who are strangers to him. The elimination of the need to nurse an infant resets a mother’s menstrual and oestrus cycle. In a short space of time she becomes available for mating again. By copulating with the mother of the infant he has killed, the male shortens the waiting time for the generation of his own genetic lineage, and he will be willing to invest his energy in protecting that particular female during the gestation and infancy of his offspring. Among chimpanzees, the females not infrequently gang-up to ward off abusive males. In some cases, this may be a deliberate strategy to prevent infanticide. It is also possible that female primates mate promiscuously during their ovulation, or that they conceal their ovulation. A male will then hesitate to kill their infant in case it be his own offspring.1 There are, in other words, good evolutionary reasons for males to commit infanticide in order to spread their genes, and for females to devise strategies protect themselves against infanticide.
In his groundbreaking Human Evolution (2014), the evolutionary psychologist and anthropologist, Robin Dunbar, argues that “monogamy (and pairbonding) is likely to have arisen in the context of infanticide risk”. He rejects the other two reasons most commonly offered for male-female coupling — ‘mate guarding’ and ‘biparental care’. Unlike among other primate species whose females frequently disperse across large territories, human females stay put in groups. Therefore, human males do not need to mount guard over their chosen females by coupling up with them. Whenever she is receptive to sex, he is ready and nearby. Nor can male-female coupling be explained by the need or desire for biparental care. Dunbar’s essential insight, which we should unequivocally endorse here, is that “biparental care is more likely to be a consequence of monogamy rather than its cause”, and, among primates, “biparental care invariably evolved after pairbonding”. Instead, however, Dunbar concludes that “a female locks on to a male in order to use the male as a ‘hired gun’ or bodyguard to defend her against other males that might harass her and/or kill her offspring”. This strategy, he argues, matches the pattern among primates more generally — “the switch from polygyny to monogamy never occurs in the absence of high infanticide risk”. Rather, among primates “infanticide seems to have been the crucial factor driving the evolution of monogamous mating systems”. Dunbar also suggests that in the human context — roughly 150k-200k years ago — infanticide, harassment of females and the ‘hired gun’ strategy became ever more likely. By then large-brained hominids had lengthened the interbirth-nurturing intervals, and had begun to develop language and cognitive skills for living in the larger groups that were experiencing an influx of unknown and transient males who posed a potential risk to single females.2
Dunbar is surely correct to say that biparental care —in so far as male input was ever really routine and significant — followed rather than preceded male-female coupling. Indeed, as we will presently see, child care was incorporated in the negotiations about larger divisions of labour within the new coupling unit. However, the suggestion that protection against infanticide played a major role in causing human coupling seems to me less persuasive. I propose, instead, that the general problem of within-group insecurity and male violence was a greater reason for women to seek out a male protector than infanticide per se. Secondly, I draw on contemporary evidence of quite widespread infanticide by females. It seems possible that ever since the final stage of human brain enlargement and the consequent capacity for language learning, the main protagonists and proposers of infanticide were often the women themselves.
Presumably, females have always had a natural interest in seeking the protection of a male. This common pattern among mammals follows from the fact that males are normally physically larger, more aggressive and better skilled and equipped for violence, less selective in their partnerships, shorter-lived and slower to mature, possessed of a stronger sexual appetite, and less enthusiastic about nurturance.3
Females would naturally seek to protect themselves and their offspring by living within a defensive group of females or by seeking protection from a single male who is a close relative or with whom she thinks she might form a collaborative partnership. Among humans these risks were magnified because larger brains prolonged gestation in the womb and extended infant nursing and dependence. Long intervals between births also produced a longer exposure to violent males.4
Regardless of the biological claims about genetic-instinctive inclinations — which may be realised as subconscious primate male motivations for infanticide — what can be claimed with logic and certainty is that the danger posed by unpredictable violence from un-assessable male strangers was a strong incentive for females to couple with a known, rationally evaluated male for purposes of physical protection.
What is open to criticism, however, is the conflation of male violence with an inevitable proclivity for infanticide in the contexts of increases in group size. Similarly, it cannot be assumed that lower infanticide risk is necessarily casually related with a transition to monogamous relationships and the decisions of men and women to join in a stable partnership. Sometimes, the cause of infanticide among apes and humans is attributed to evolutionary male-female biological or relational complexities when the real reason was that infants were simply the casualties of cannibalism, or collateral damage during territorial fights that run out of control.5
It is worth noting in passing that nineteenth century studies of recently colonised societies show that when males bragged about their rights of ownership over females and their offspring this could be symbolised as their prerogative to “eat” them. Nineteenth century ethnographers saw extensive evidence of female infanticide, especially when more men were needed for subsistence or for warfare. More generally, they saw evidence of more or less routine killing of both children and elders who were considered to be a burden on society because they were weak or sickly. In some societies they found strict rules prohibiting the killing of children, whereas in other cases the various circumstances in which infanticide was permissible were made explicit.6 For example, when a man married a woman who already bore children by another man, the new husband may be entitled to kill those children. Infanticide is reported in contemporary anthropological studies in much greater detail, sometimes with a rousing defence of a woman’s right to kill her child if it be ‘defective’ (below).
Substantively, there is a) good reason for supposing that women had their own rational and evolutionary reasons for committing infanticide, and b) good evidence for supposing that communities sanction or tolerate infanticide as part of their survival strategies.
The astounding contemporary fact, which casts doubt on hypotheses that male protection against infanticide was the cause of a Stone Age transitions to human monogamy, is the frequency and distribution of deliberate and socially sanctioned infanticide among contemporary hunter gatherers, when women are themselves in control of the decision. Along with abstention from sex, and abortion, infanticide is a primary method of keeping population below carrying capacity in societies that lack access to modern contraception methods. Comparative studies of twentieth century hunter gatherer groups suggest that up to 50 percent of them may practice infanticide. In other studies of forager societies the average rates seem to fluctuate between 10 and 25 percent. On the basis of such evidence some scholars7 have calculated that Stone Age infanticide rates fluctuated between 15-50 percent.
In a very thorough review of all of this evidence8 Robert L. Kelly concludes that female infanticide is probably only disproportionally significant in societies (such as those in Alaska) where hunting prey is especially dangerous and exceptionally reliant on males. Rather, the principal universal reason for infanticide is to space births a few years apart. This largely depends on a mother’s decision about whether she is able to care for her children. That decision is based on a number of variables relating to divisions of labour, including the contributions that children themselves can make to actives of foraging and childcare, and, of course, the mother’s estimation of how much help she can expect to receive from the father and from the grandparents. Indeed, in evolutionary terms it probably makes more sense for a mother to rely on her own parents for help with childcare. This9 may explain why human females are the only mammals to lose their capacity for reproduction during their midlife menopause.
A single passage from a classic anthropological text illustrates why most hunter gatherer societies do not regard infanticide as punishable malfeasance, and why, if the infant was disposed of soon after birth, the infant was not yet a real human being.10
The !Kung women consider child-birth and child-care as their sphere of responsibility and they take steps to guard their prerogatives in this area. For example, the fact that women go to the bush to give birth and insist on excluding men from the child-birth site is justified by them in terms of pollution and taboos; but the underlying explanation may be that it simplifies matters if a decision in favour of infanticide is made. Since the woman will commit a considerable amount of her energy to raising each child, she examines the newborn carefully for evidence of defects; if she finds any, the child is not allowed to live and is buried with the afterbirth. By excluding men from the childbed women can report back to the camp that the child was born dead without fear of contradiction. But if the child is healthy and wanted by the woman she accepts the major responsibility for raising it. In this way the women exercise control over their own reproduction. [Richard Lee, ‘Sex Politics’ in Eleanor Leacock, Richard Lee eds., Politics and History in Band Societies, Cambridge University Press 1982:41-2
The conclusion I draw from this discussion is that although protection against random male violence is a credible reason that will have contributed to the original decisions a woman may have made for coupling with a man whom she judged to be a potentially reliable partner, this line of reasoning does not — either specifically or inevitably — extend to the violence of infanticide. A woman had reasons of her own for resorting to infanticide as one of her methods for survival in adverse conditions. Even today women still rationalise this way in some isolated groups that live at the margins of poor societies. Contraception solved the problem in the West. What we do continue to see in some countries (in place of infanticide) is the political and religious hot potato of abortion, which might sometimes even decide election outcomes. In our prehistoric context, however, there is perhaps no topic that more decisively illustrates the piecemeal and fraught nature of evolutionary choice-making than infanticide.
FOOTNOTES
These arguments appear in various forms in — Terence W. Deacon, The Symbolic Species: The Co-evolution of Language and the Brain, W. W. Norton & Company 1998; Geoffrey Miller ‘Protean primates: The evolution of adaptive unpredictability in competition and courtship’ in Machiavellian Intelligence II: Extensions and Evaluations, edited by A. Whiten and R. W. Byrne, Cambridge University Press 1997:324; Robin Dunbar, Human Evolution, Penguin Books 2014; Frans de Waal, Chimpanzee Politics: Power and Sex among Apes, Johns Hopkins University Press 2007; Jane Goodall, The Chimpanzees of Gombe: Patterns of Behaviour, Belknap Press 1986:522
Robin Dunbar, Human Evolution, Pelican 2014:52-53, 55-56, 332-33, 335-337. Very similar arguments (citing Dunbar among others) can be found in David C. Geary, Male, Female: The Evolution of Human Sex Differences, American Psychological Association 2021:108, 120-121, 134, 141
Steve Stewart-Williams, The Ape That Understood the Universe, Cambridge University Press 2020 (Chapter 3 and passim)
As Ian Morris writes in Foragers, Farmers, and Fossil Fuels: How Human Values Evolve, Princeton 2015:235 — “Because we have such big brains, we need big skulls, but if we stayed in our mothers’ wombs until we were as ready to face the world as the babies of most other species of mammals, our heads would be too big to get down the maternal birth canal. Human mothers have dealt with this by in effect evolving to give birth prematurely, at the cost of greatly increasing the parental care that we need as babies.” For complementary accounts see — Jane Goodall, The Chimpanzees of Gombe: Patterns of Behaviour, Belknap Press 1986:522; Robin Dunbar, Human Evolution, Pelican 2014; Frans de Waal, Chimpanzee Politics: Power and Sex among Apes, Johns Hopkins University Press 2007; Jonathan H. Turner, On Human Nature: The Biology and Sociology of What Made Us Human, Routledge 2020
Examples of cannibalism and collateral deaths can be found in Jane Goodall, The Chimpanzees of Gombe: Patterns of Behavior, Belknap Press 1986:522, and in Frans de Waal, Chimpanzee Politics: Power and Sex among Apes, Johns Hopkins University Press 2007
There are many examples in Herbert Spencer, The Principles of Sociology: Vol. 1, Vol. 2 and Vol. 3, Full and Fine Edition 1895 (e.g. Part III Domestic Institutions, and passim)
Birdsell, J. 1968 ‘Some Predictions for the Pleistocene Based on Equilibrium Systems for Recent Hunter-Gatherers’ in Man the Hunter, edited by R. B. Lee and I. DeVore, pp. 229–40. Chicago: Aldine; Hassan, F. 1981 Demographic Archaeology, New York: Academic Press
Robert L. Kelly, The Lifeways of Hunter-Gatherers: The Foraging Spectrum, Cambridge University Press 2013 (Chapter 7: Group Size and Demography)
Robin Dunbar, Human Evolution, Penguin Books, 2014:335
In addition to the quote below, see Christopher Boehm, Moral Origins: The Evolution of Virtue, Altruism, and Shame, Basic Books 2012:231, 253; and Esther N. Goody, ‘Social Intelligence and Language: Another Rubicon’, in Machiavellian Intelligence II: Extensions and Evaluations, edited by A. Whiten and R. W. Byrne, Cambridge University Press 1997