“What Clancy did has little precedent in nature.” (Greg Derr/Pool/AFP/Getty)
The trial of Lindsay Clancy, the Massachusetts mother who strangled her three children to death, has gripped and divided America. For now, the question of whether she was responsible for her actions — she claims she was suffering from postpartum psychosis at the time — remains unresolved after her case ended in a mistrial earlier this month. Yet like Andrea Yates, the Houston mother who was acquitted in 2001 of drowning her five children in a bathtub by reason of insanity, Clancy has become the focus of an intense debate on postpartum mental health.
At the time of Yates’s acquittal, one Wall Street Journal headline read: “The Newest Feminist Icon: A Killer Mom”. And this, it seems, gets to the heart of the national response to both mothers, with much of the public’s morbid fascination stemming from the violation of the mother-child bond. It is the bond that a child depends on most, and is often described as a relationship of unconditional love. It is celebrated across cultures and religions to the point that breaching it is thought to be unnatural. Indeed, the WSJ writer described Yates’s behavior as “not a mere variant of maternal behavior” — but as “nature’s aberration”.
Would nature agree? From an evolutionary perspective (and others), few relationships matter more than that between parent and child. Children are the primary means by which we pass on our genes, and so our psychology has evolved to prioritize this relationship. That is why infanticide by mothers and fathers is rare throughout the animal kingdom, even though infanticide itself is not (male gorillas and lions for instance kill infants that aren’t their own). Among the great apes, our closest living relatives, maternal infanticide is close to unheard of. Yet among humans, it has been documented with quite striking regularity.
Kim Hill and Magdalena Hurtado open their 1996 ethnography of the Aché, a Paraguayan hunter-gatherer group, with a story told to them several times. A jaguar is stalking a group of Aché hunters through the forest. When the hunters return to camp, they notice that one of their number is missing. The lost hunter’s 17-year-old wife starts sobbing; she is heavily pregnant with his child. A week later the baby is born. “No woman volunteered to cradle the baby… no man stepped forward to cut the umbilical cord,” Hill and Hurtado wrote. “The signs were clear, and it took only Kuchingi’s verbal suggestion to settle the point. ‘Bury the child,’ he said, ‘it is defective, it has no hair. Besides, it has no father’.” An old woman silently digs a hole. The child is placed in it and covered with red soil. A few minutes later the Aché have packed up and moved on.
The Aché are not an isolated case: hundreds of examples of parental infanticide exist, written down in ethnographies of hunter gatherers or traditional farming populations spanning from the 1800s to the late 1900s. Among the !Kung of the Kalahari, abandonment has been estimated at about one in 100 live births. Among the pre-missionized Eipo of highland New Guinea, it was reported at exceptionally high levels; almost 41% of live babies born were abandoned, and they were overwhelmingly girls.
The reason for infanticide tends to be similar wherever you look: parents believe they cannot care for the child. Infanticide often occurs, for instance, when a mother is still breastfeeding an older infant and cannot support another. Another recurring occasion for infanticide is the birth of twins. Ethnographers working with the Mbuti of the Congo rainforest reported that “no woman would feed a child at each breast”. If twins were born one would be taken away and buried. Diamond Jenness, an anthropologist working with the Copper Inuit, wondered about the origin of one woman’s name — “almost two”. Could she have been a twin? Jenness had never encountered twins during his time there. When he asked a local man, Hikhik, the reply was matter-of-fact: “Little man, there is room only for one baby in the mother’s hood. So the other one has to go. Sometimes one is adopted, but the spirits don’t like that.”
Sex often enters into the calculation. Female infanticide and neglect recur in societies where sons are valued more. India and China are obvious contemporary examples. In 2020, there were an estimated 45.8 million “missing women” in India and 72.3 million in China: women who would have been alive in the absence of prenatal sex selection and excess female mortality, such as that caused by infanticide or neglect. This happens in many traditional societies too. Ethnographers have documented higher rates of female infanticide in many other traditional societies beyond the Eipo. One reason often given is that it is boys who will look after parents when they are old, whereas daughters marry into other households.
In all these cases, these are evidently not decisions taken easily and are events that no woman wants to recall. Most ethnographers tiptoe around the subject, broaching it indirectly by asking people if they know about others that it has happened to.
Nor is this a uniquely human calculation. Though parental infanticide is rare in the animal kingdom, examples of it do exist. Take white storks. When a parent judges that it cannot raise its whole brood — because food is short, or a chick is weak — they may kill the weakest, or toss it from the nest, usually within the first few days or weeks after hatching. Then there’s the blue-footed booby, a large seabird that lays its eggs a number of days apart so that the first chick to hatch has a permanent size advantage over its younger siblings. In periods of scarcity, the elder chick pecks the younger one to death or drags it from the nest, while the parents watch on without intervening. There are even a handful of exceptions to the primate rule. Tamarin mothers, for instance, abandon their offspring. They don’t pick them up when they fall to the ground and have also been seen forcing clinging infants off their backs.
Together these examples tell us something about how maternal psychology works. Natural selection did not equip us with an unconditional drive to protect every child at all costs. It conditioned us to balance that drive against our perceived environmental constraints. That sensitivity evolved over hundreds of thousands of years in a world of real scarcity that has only very recently changed.
What links the breeding systems of white storks, blue-footed boobys, tamarins, and humans? The answer is that, in each case, a mother does not usually raise her young alone. Monogamous birds depend on a father who incubates and feeds alongside the mother. Tamarins are cooperative breeders, meaning they are reliant not just on fathers but also on older siblings and other group members to carry and feed their infants. The likelihood that a tamarin mother abandons her infant rises when she already has multiple young and is short on helpers.
Humans are cooperative breeders too. We have always depended on others to help raise our children, and it is precisely this that allows a human mother to raise several dependent children at once. This is something a chimpanzee mother cannot do: she has to raise her children by herself, and so she will space out her births so that each infant is independent before the next one arrives.
In species where a mother can raise her offspring single-handedly, as most great apes can, infanticide by mothers is almost never observed. Where she cannot, her commitment to any one infant becomes contingent on whether that help arrives. The anthropologist and primatologist Sarah Hrdy calls it “cooperative breeding’s dark side”: the very system that gave humans their deep instinct for cooperation and altruism is double-edged, with higher rates of child abandonment and infanticide as its corollary.
And while infanticide has grown increasingly rare in high-income countries, where most of the old dangers of motherhood and raising children have been softened, it has not vanished. In England and Wales, between 2014 and 2025, an average of 26 children under the age of 16 were killed by their parents each year.
It should go without saying that this does not mean that parents who kill their children in modern Britain are making a rational response to hardship. Nor does any of this imply that these decisions are consciously calculated. It suggests only why human psychology might contain a capacity for parental commitment to weaken in response to certain cues like hardship or too many competing dependents. In a modern society with a welfare state, those cues no longer mean that a child should not survive, but our evolved psychology does not know that.
The risk is also usually concentrated in the very youngest. When Daly and Wilson looked at Canadian data from 1974-1983, they found that children killed by their parents tended to be extremely young — most were not even a year old. In the ethnographic accounts of infanticide, babies are often killed within days of birth, and very rarely alongside other children. There is a cruel logic to this: if infanticide is, at root, a grim answer to the question of whether a parent can afford to invest in a child, then the decision is made quickly, before much has been invested, and often to protect the children a parent already has, who would otherwise go without.
This is partly why the Clancy case is so unusual. Her three children — Cora, five, Dawson, three, and Callan, eight months — were young, but two had long passed infancy, and all suffered the same fate. This was not anything resembling the cold evolutionary arithmetic of investment. It was a total collapse of the machinery that produces maternal devotion.
The period following childbirth does, however, carry an elevated risk of mental illness. And that risk has long been recognized in law. In England and Wales, the Infanticide Act 1938 charges a woman who kills her child before its first birthday with infanticide rather than murder where childbirth has disturbed the “balance of her mind”. The offense is punished as manslaughter, avoiding murder’s mandatory life sentence and gives judges the option of psychiatric treatment rather than prison. Canada has a similar offense, carrying a maximum sentence of five years.
Evolutionary theory could tell us something about postpartum depression. One account, proposed by Ed Hagen, treats it as a response to raising a child without sufficient help. Withdrawal may reduce a mother’s investment while signaling to partners and relatives that more help is needed.
A related mismatch theory emphasizes the gap between the environment in which human maternal psychology evolved and the one in which many women now give birth. We evolved within networks of helpers, yet contemporary mothers often find themselves alone with their children for much of the day. Falling fertility rates compound the problem: many first-time mothers now have little experience of caring for babies before they have their own. This might explain why rates of postpartum depression are rising. In England, one estimate shows symptoms rising from 10.3% in 2014 to 23.9% in 2020. A sample from California saw a similar rise from 9.4% in 2010 to 19% in 2021.
But postpartum depression is not just a disease of modernity. Among the Shuar, a population of small-scale farmers in Ecuador, Clark Barrett found that of the 21 women interviewed, seven of them reported having postpartum sadness. This type of sadness was well known by women in the community and described as a prolonged state that might last weeks or months. Their own explanations often involved a lack of support, such as an absent husband, or poor health following a difficult birth.
However postpartum psychosis, which Clancy claims she was suffering from, is different. It is far rarer, affecting around one or two women in every 1,000 births. It can involve mania or delusions and is associated with bipolar disorder. It is a psychiatric emergency, not an extreme form of depression.
Whether Clancy was suffering from it remains disputed. Her defense said she was obeying a voice telling her to kill her children and then herself, and was on a cocktail of drugs; prosecution experts questioned whether her symptoms fit our understanding of psychosis. No theory of maternal investment or postpartum depression can settle that disagreement. Her situation was not one of a woman abandoned to cope alone. She had a husband and family around her. She repeatedly sought medical help and admitted herself to a psychiatric hospital. Even so, multiple individuals failed to recognize what was happening to her. She was let down, her husband was let down, and so most of all were her children.
What Clancy did has little precedent in nature. The writer in The Wall Street Journal was right when she described the “Killer Mom” as “not a mere variant of maternal behavior”. And though evolutionary theory cannot shine much light on the Clancy case, it can tell us something more general. Human motherhood did not evolve as a solitary undertaking. Our unusual dependence on grandparents, siblings, and neighbors made it possible to raise several dependent children at once. It also left mothers acutely sensitive to the absence of support. If this sorry affair spurs greater research and channels more resources into protecting women and their children during these incredibly vulnerable periods, then that can only be a good thing.
