Infanticide, the deliberate killing of an infant, is one of the most disturbing yet persistently documented behaviors across the animal kingdom and throughout human history. Far from being an aberration confined to a few species or cultures, it appears in primates, rodents, carnivores, birds, insects, and in human societies spanning thousands of years. Understanding why it occurs requires looking at biological drives in nonhuman animals, historical and economic pressures in human populations, and the psychiatric and social conditions that surround modern cases. None of these explanations excuse the act, but together they reveal a phenomenon that resists simple categorization.
Why Male Animals Kill Infants
The most widely studied form of animal infanticide involves males killing offspring they did not sire. The core logic, first proposed as a sexual selection hypothesis decades ago, is straightforward: if a female is nursing young, she typically is not fertile. A new male who kills those offspring can accelerate her return to a fertilizable state, giving him a chance to father the next litter. This hypothesis has found strong support in several groups of mammals, including primates, carnivores, rodents, and cetaceans, though not universally across all species where infanticide occurs.1PubMed Central. Infanticide as sexual conflict: coevolution of male strategies and female counterstrategies More recent research has expanded the picture, showing that in some insects, birds, and mammals, males gain other advantages from infanticide, such as acquiring breeding status or improving a female’s body condition ahead of a new reproductive cycle.
A large-scale comparative study across mammalian societies found that infanticide by males primarily evolves in species where a small number of males monopolize reproduction. When one dominant male controls mating access, any new male who displaces him stands to gain enormously by eliminating existing young. The same study found that the most effective counterbalance is not group living or changes in social structure but rather female sexual promiscuity, which dilutes paternity certainty and makes it risky for any male to kill an infant that might be his own.2PubMed. The evolution of infanticide by males in mammalian societies
Bonobos offer a striking case. Despite sharing many traits with species where male infanticide is common, male-led infanticide among bonobos is absent or extremely rare. Researchers have pointed to strong female coalitions, paternity confusion through cryptic ovulation, and relatively nonlethal male aggression as likely explanations.3PubMed Central. Human cooperation and evolutionary transitions in individuality The bonobo example is a useful reminder that even where the conditions for infanticide seem ripe, social dynamics can suppress it.
Counter-Strategies and the Arms Race
Females are not passive victims of male infanticide. Across species, evolution has produced an array of counter-strategies. The paternity dilution strategy described above, where females mate with multiple males so that none can be sure which offspring are his, is among the most effective. But some species have developed more unusual defenses. In rats, pregnant females synthesize a chemical signal of low volatility during pregnancy that appears to deter infanticidal males, essentially a chemical counter-strategy deployed before the pups are even born.4Physiology & Behavior. Infanticide in rats: Male strategy and female counter-strategy
Some researchers have also proposed that social monogamy with paternal care in certain primates evolved partly as a defense against male infanticide, because having both parents present to guard offspring, combined with shorter lactation periods enabled by biparental care, reduces vulnerability.5PubMed Central. Human cooperation and evolutionary transitions in individuality In cooperative-breeding birds, reproductive synchrony within clutches serves a similar purpose: nestlings in asynchronous clutches face higher mortality because early-hatching chicks are more likely to be killed by adult group members, while late-hatching chicks starve from competing with older nest-mates. Synchronizing hatch timing appears to be an evolved response to this dual threat.6Oxford Academic. Infanticide and within‐clutch competition select for reproductive synchrony in a cooperative bird
When Females Kill
Male infanticide gets most of the attention, but females of many mammal species also kill infants that are not their own. The motivations are different. Female infanticide tends to occur in contexts where another female’s offspring directly threatens the killer’s reproductive success by competing for critical resources like food, shelter, care, or social position.7PubMed Central. The evolution of infanticide by females in mammals In some rodent and carnivore species, a dominant female kills subordinate females’ pups to eliminate competition for her own young. Among chimpanzees, females sometimes spend much of their time alone partly because of the risk of infanticide from other females competing over foraging resources.8PubMed Central. Human cooperation and evolutionary transitions in individuality
The takeaway from the animal literature is that infanticide is not random violence. It is deeply embedded in reproductive competition, shaped by specific ecological and social conditions, and met by counter-strategies that can be remarkably creative. Whether any of this maps onto human behavior is a separate and more complicated question.
Infanticide in Human History
Archaeological and textual evidence leaves little doubt that infanticide was practiced across many ancient societies. One of the most vivid examples comes from Ashkelon, where the skeletal remains of roughly 100 neonates were found in a sewer beneath a Roman bathhouse that may have also served as a brothel. DNA-based sex identification of 19 testable specimens revealed 14 males and 5 females, a ratio that stunned researchers because Roman written sources describe the common practice of killing female infants specifically. The high proportion of males at this site suggests females were being selectively preserved, possibly to be raised as courtesans for the establishment.9Journal of Archaeological Science. Determining the Sex of Infanticide Victims from the Late Roman Era through Ancient DNA Analysis The finding illustrates how context-dependent sex-selective infanticide has always been: the dominant cultural pattern might favor killing girls, but local economic incentives could reverse it entirely.
The practice persisted well beyond antiquity. A study drawing on longitudinal records from rural Spain between 1750 and 1950 found evidence that families mortally neglected a significant fraction of their female babies. Baptism records showed exceptionally high sex ratios at birth until the late nineteenth century, and having no previous male siblings increased the probability that the next baptized child would be male. These patterns were concentrated at higher birth orders and among landless and semi-landless families, pointing to economic pressure as a driving force. The researchers ruled out under-registration of female births as an explanation.10European Review of Economic History. Death, sex, and fertility: female infanticide in rural Spain, 1750–1950
Sex-Selective Killing and Population Effects
A persistent question in demography is whether widespread female infanticide, practiced over generations, can shift a population’s sex ratio in lasting ways. Genetic modeling suggests the answer is not straightforward. The killing of infant daughters can lead to either a female-biased or a male-biased sex ratio depending on the decision rules families use to determine a child’s fate.11PubMed. Female infanticide and human sex ratio evolution In other words, the evolutionary feedback loops are complex enough that even sustained preference for sons does not always produce the demographic skew you might expect. The real-world effects depend on whether families stop reproducing after achieving a desired number of sons, whether they continue until daughters arrive, and how strongly inheritance and labor economics favor one sex over the other.
Modern sex-selective practices, particularly in parts of South and East Asia, have shifted from infanticide to prenatal sex selection through ultrasound and selective abortion. The demographic consequences have been dramatic in some regions, producing generation-level surpluses of men. But even in these contexts, the underlying drivers, son preference rooted in inheritance customs, old-age support expectations, and dowry economics, echo the same forces visible in the Spanish and Roman historical data.
Neonaticide in the Modern World
The term “neonaticide” refers specifically to the killing of an infant within its first 24 hours of life. Modern neonaticide cases have a remarkably consistent profile across countries: they are most often committed by poor, relatively young, single women who received no prenatal care.12PubMed. Neonaticide: Phenomenology and considerations for prevention In many of these cases, the pregnancy itself was unrecognized or denied. An Austrian study found that among a sample of neonaticide perpetrators, roughly 91% of pregnancies had gone unperceived. These women had substantially lower contraception use than the general population, and the few who used any contraception rarely used effective methods.13PubMed Central. Denial of reproductive potential: a predictor of unperceived pregnancy in an Austrian neonaticide sample
Denial of pregnancy is central to understanding neonaticide. It is not simply a woman choosing to hide her condition, though that happens too. In many cases, the woman herself is genuinely unaware she is pregnant, sometimes right up to labor. Identifying women at risk is difficult precisely because the psychological traits involved, including dissimulation, lack of awareness, and low likelihood of seeking help, combine with environmental factors like social isolation, low socioeconomic status, and limited education to make these cases nearly invisible to healthcare systems until it is too late.14PubMed. Denial of pregnancy and neonaticide: A historical overview and case report
Neonaticide is distinct from infanticide committed days, weeks, or months after birth, which tends to involve different perpetrator profiles and different psychological dynamics. Lumping all child killings together obscures these differences and makes prevention harder.
Filicide by Fathers and Mothers
When the killing of a child happens beyond the neonatal period, the picture changes. Comparative studies of maternal versus paternal filicide reveal meaningful differences in perpetrator age, methods, and motivation. Roughly a quarter of fathers who killed their children in one study did so in reaction to threatened separation or divorce, suggesting the act was linked to perceived loss of control over the family. Over a third of male perpetrators and more than half of female perpetrators were mentally ill at the time.15PubMed. Filicide: a comparative study of maternal versus paternal child homicide A separate study of ten men who committed paternal filicide found that most cases were classified as pathological, with mood disorders being the most frequent diagnosis and a majority having personality disorders.16PubMed. Paternal filicide: a study of 10 men
The clinical takeaway, such as it is, suggests that increased monitoring by primary care physicians during high-conflict domestic situations or during periods of acute mental illness could have some preventive value. The challenge is that filicide remains statistically rare, making it nearly impossible to predict at an individual level even when risk factors are present.
The Cinderella Effect and Its Complications
One of the more controversial ideas in evolutionary psychology is the “Cinderella effect,” the claim that stepchildren face a dramatically elevated risk of abuse and homicide compared to biological children. The original estimates, generated by researchers Martin Daly and Robin Wilson, suggested very large risk multipliers for stepchildren. A Swedish study of paternal filicide found that stepchildren were overrepresented as victims compared to children of fathers in two-biological-parent families, and this overrepresentation was even more pronounced among young children.17PubMed Central. Paternal Filicide in Sweden: Background, Risk Factors and the Cinderella Effect
But the picture is messier than the headline version suggests. A reanalysis of UK data found that while the raw risk to young stepchildren appeared roughly 16 times higher than for biological children, several confounds eroded this figure. Stepfathers tend to be much younger than biological fathers, and younger fathers are more likely to kill their children regardless of genetic relatedness. Additionally, nearly half of perpetrators in the homicide data did not live with their victims, meaning the population of “stepfathers” in the denominator was undercounted. After accounting for these factors, the elevated risk largely disappeared for older children and was substantially reduced for younger ones.18The Conversation. The Cinderella effect: are stepfathers dangerous?
A historical demographic study added another wrinkle: while losing a mother in childhood was associated with elevated mortality risk, parental remarriage did not increase the risk of mortality among stepchildren compared to other children who had also lost a parent. In fact, stepchildren showed higher survival than their half-siblings within the same family, the opposite of what a naive evolutionary prediction would suggest.19PubMed Central. Was Cinderella just a fairy tale? Survival differences between stepchildren and their half-siblings The debate is unresolved, but the evidence is clear that the simple story of stepparents as inherently dangerous to non-biological children does not hold up without heavy qualification.
How Legal Systems Handle Infanticide
Many legal systems have grappled with whether infanticide by a mother should be treated differently from other forms of homicide. England’s Infanticide Act of 1938 is among the most influential pieces of legislation on this question. It was designed to offer mercy to women who kill their infants while the “balance of her mind was disturbed” as a result of giving birth or lactation. Despite its long history, the statute has received limited judicial interpretation, and debate continues over whether the mental disturbance must be supported by medical evidence of a bio-psychiatric illness or whether a broader reading is appropriate.20The Journal of Criminal Law. The Need for Judicial Interpretation of the Infanticide Act 1938
The combination of the earlier 1922 Infanticide Act, which first recognized the special circumstances of the postpartum period, and the 1957 Homicide Act, which introduced the broader concept of diminished responsibility, has been described as offering a legal framework worthy of consideration by other nations.21PubMed. Infanticide: forms, causes and jurisprudential solutions Many countries do not have equivalent legislation. In the United States, for example, there is no federal infanticide statute, and cases are typically prosecuted under general homicide or manslaughter laws. The result is enormous variation in outcomes: a woman in one jurisdiction may receive a lengthy prison sentence for an act that would have been treated as a mental health crisis in another.
Safe haven laws, which allow parents to relinquish newborns at designated locations without criminal liability, represent a different legal approach. All 50 US states have some version. However, the details vary widely and those details matter. A study found that several elements of safe haven laws were associated with higher rather than lower rates of infant mortality, including provisions stating that only the mother may relinquish the child, laws that do not protect parents from criminal liability, and laws requiring providers to give legal information and referrals. The implication is that poorly designed safe haven laws can inadvertently discourage use by the very people they are meant to help.22medRxiv. The State-Level Association Between Safe Haven Laws and Rates of Infant Mortality in the United States
Forensic Challenges in Distinguishing Infanticide from Other Causes
One of the most difficult tasks in forensic pathology is distinguishing infanticide from sudden infant death syndrome (SIDS) or other natural causes. In cases of suspected smothering, a key diagnostic marker is the distribution of petechiae, tiny pinpoint hemorrhages, on the infant’s skin. When petechiae extend over the entire drainage area of the superior vena cava in the absence of a bleeding disorder, this pattern can indicate that the infant’s airways were obstructed while the chest was simultaneously compressed.23Journal of Forensic Sciences. Petechiae of the Baby’s Skin as Differentiation Symptom of Infanticide Versus SIDS But even this marker is not definitive on its own, and the overlap between findings in SIDS and suffocation cases has led to wrongful convictions. The forensic literature is full of cautionary tales about over-interpreting ambiguous physical evidence, and many jurisdictions now require a multidisciplinary review combining pathology, scene investigation, and clinical history before concluding that an infant’s death was a homicide.
Poverty, Neighborhoods, and Prevention
Research on child maltreatment more broadly, which includes but is not limited to infanticide, consistently identifies poverty as a powerful risk factor at both the individual and neighborhood level. A study examining community and individual risk factors found that neighborhood-level disadvantage was especially harmful for families already in poverty, compounding individual risk. Interestingly, neighborhood-level social protective processes, the kind of informal support networks that buffer families in higher-income areas, did not provide the same protection for impoverished families.24PubMed Central. Community and Individual Risk Factors for Physical Child Abuse and Child Neglect: Variations by Poverty Status This finding suggests that community-level interventions alone are insufficient when families lack basic material resources.
Home-visiting programs, in which nurses or trained professionals make regular visits to at-risk families during pregnancy and early childhood, represent one of the best-studied prevention approaches. A review of the evidence concluded that overall, there is limited proof that home visits directly prevent child abuse and neglect. However, home visits consistently improve maternal parenting practices, the quality of the child’s home environment, and children’s developmental outcomes, all of which are plausibly linked to reduced maltreatment over time.25PubMed. The role of home-visiting programs in preventing child abuse and neglect One landmark randomized trial tracked outcomes over two decades and found that nurse-visited families had significantly lower preventable-cause child mortality compared to families receiving standard care.26JAMA Pediatrics. Effect of Home Visiting by Nurses on Maternal and Child Mortality: Results of a 2-Decade Follow-up of a Randomized Clinical Trial The effect took years to become statistically visible, which is part of why these programs sometimes struggle to secure sustained funding despite delivering real results.
The Ethics of Even Discussing It
In 2012, a paper in the Journal of Medical Ethics ignited a public firestorm by raising philosophical questions about the moral status of newborns compared to fetuses. The authors’ stated intention was to explore normative consequences arising from existing debates in philosophy about personhood, not to advocate for killing infants.27Journal of Medical Ethics. Clarifications on the moral status of newborns and the normative implications The backlash was immediate and intense, including death threats. The episode revealed just how raw the topic remains: even an academic discussion framed entirely within established philosophical traditions about consciousness and moral status can provoke responses that make productive conversation nearly impossible. It also highlighted the gap between how bioethicists talk about personhood in seminar rooms and how the public understands those discussions. For anyone researching infanticide, the tension between clinical detachment and moral gravity is always present, and probably should be.