What makes mothers kill their own children?

And yet it’s the third time in a matter of weeks in the Washington region that a mother has been suspected of harming her offspring.

The act of killing one’s child is unthinkable for any parent, but owing to long-standing cultural, emotional and biological factors, a mother who kills her offspring has the power to inspire special shock and revulsion.

“Momma is the loving person, the giving person, the sacrifice person — for them to do something like that is like denying God or something,” said Bobby Hicks, former Union County deputy sheriff who was the first to interview Susan V. Smith, a South Carolina woman who killed her two children in 1994 by letting her car roll into a lake with the toddlers inside. “How could a mother do that to her children?”

Such a mother is seen as someone who is not only guilty of a crime but has violated a law of nature and rebelled against instinct.

But the motives behind maternal filicide, as it is known, are much more complex, even counterintuitive — and troubling, even for those professionals who have devoted their careers to trying to understand them.

"It's just such a delicate subject," said Sarah Blaffer Hrdy, an emeritus professor of anthropology at the University of California at Davis who has conducted pioneering research on the evolutionary, psychological and historical factors in infanticide.

The work of Hrdy [pronounced HURDY] has created controversy by demonstrating that under certain circumstances, infanticide could serve as an evolutionary adaptation, not necessarily a pa­thol­ogy, in the human struggle for existence.

In an interview and in her book "Mother Nature," Hrdy said that infanticide is extraordinarily rare among primates, whose offspring are among nature's most costly to raise because of their long path to maturity. It is more common among other mammals, such as lions, that cull their litters.

But nature, over eons, has instilled hard calculations in primates and humans, too: A mother faced with inadequate resources to ensure survival of herself, the child or other offspring might feel compelled to abandon or kill it.

Hrdy’s work also suggests, paradoxically, that those pressures may be greatest in patriarchal cultures where a woman’s role as mother is idealized and she is under intense pressure to give birth to children and nurture them with self-denying devotion.

“[I]n societies where women have a lot of social support and also have access to birth control and education about birth control and the freedom to use it — in those societies, rates of child abandonment and infanticide are going to be very low,” Hrdy said in an interview. “It is in the societies where you don’t have those ­choices where the rate goes up.”

Phillip J. Resnick, a professor of psychiatry at Case Western Reserve University’s Medical School who is considered to be an expert in the study of filicide, distinguishes between neonaticide, a term he coined to describe the killing of an infant by its parents within the first 24 hours of birth; infanticide, which involves a parent’s killing of a child less than 1 year old; and filicide, which is the killing of a child up to 18 years old by a parent, stepparent or guardian.

Resnick also categorized five basic motives. There is “altruistic filicide,” when a mother kills in the belief she is saving her child from a fate worse than death; “acutely psychotic filicide,” in which a mother obeys voices or hallucinations commanding her to do so; “fatal maltreatment filicide,” in which a child dies from abuse or neglect; and “unwanted child filicide,” in which a mother rids herself of a child perceived as a hindrance. The rarest motive involves a mother seeking revenge against her spouse — like Medea, a figure in Greek myth who killed her children to avenge herself against their father after he had abandoned her for another woman.

"In reality there are both rational and irrational reasons," Resnick said in an interview.

A statistical analysis by Brown University researchers of more than 15,000 homicide arrests over 32 years found that about 500 parental filicides occur annually, or about 2.5 percent of homicide arrests.

Just this month, Prince George's County authorities charged Sonya Spoon, 24, with killing her two toddlers — Ayden Spoon, 1, and Kayla Thompson, 3 — in their Cheverly home on Sept. 7. On Sept. 16, District police filed murder charges against Frances Lyles, 25, in the fatal beating of her son, Xavier, 3, in June. Earlier this year, Montgomery County police brought murder charges against a Germantown woman who allegedly killed her two toddlers because she believed they were possessed by demonic spirits. None of the defendants has entered a formal plea, online court records show.

Filicide goes back to the earliest days of human existence, its enduring grip on the psyche still preserved in myths and fairy tales. Even in the Bible, the stories of two key figures — Isaac and Jesus — evoke themes of sacrificial filicide.

In practice, the most common reasons for infanticide include disability, illegitimacy, lack of resources and cultural preferences for males. When twins were born among the Inuit, for example, the indigenous Arctic people sent one off on an ice floe because providing for two was too daunting, Resnik said. Hrdy cites a letter from a Roman soldier to his pregnant wife in the first century BCE with some blunt instructions: “If it is a boy keep it, if a girl discard it.”

By the 16th century, some European countries enacted laws making filicide a capital offense. But in 1922, Britain reduced penalties for maternal filicide, based on the presumption that a woman who killed her child must be imbalanced from the effects of giving birth — a stance that caused feminists to criticize lawmakers for treating childbirth as a pathology and assuming women were less able to govern their behavior than men.

“On the one hand, the mother is a nurturer and a protector, and if they violate that role the public feels they deserve the harshest punishment,” Resnick said. “And yet on the other hand, mothers unconditionally love their children, and there must be something very wrong, and so they deserve leniency.”

The British law reflected the sense that mothers who kill their children must be in the grip of a force strong enough to overpower the deep psychological, biological and cultural traits that normally bind them to their children, beginning in pregnancy. When a woman breastfeeds, her body undergoes changes that deepen the attachment further. Stress-related chemicals subside, and her body releases oxytocin, a potent hormone that promotes emotional bonding. The chemical reaction is so powerful that women who nurse their newborns are less likely to harm them in those first few days when infanticide risk is at its height.

To prevent maternal filicide, Resnick urged psychiatrists to be alert to the potential in mothers suffering from mental illness. Mothers who appear to be in the throes of severe depression, substance abuse or personality disorders should be queried about their child-rearing practices and parenting problems. Mothers who voice suicidal thoughts should be identified early, and perhaps asked about the fate of their children if the mother were to die. Resnick also urged a lower threshold for psychiatric hospitalization.

“Psychotic mothers who fear that their children may suffer a fate worse than death due to persecutory delusions should either be hospitalized or separated from their children,” Resnick wrote, in a paper he co-authored for the World Psychiatric Association’s journal.

Society’s profound ambivalence toward maternal filicide often plays out in high-profile cases, such as those of Susan Smith and Andrea Yates, a Texas woman who drowned all five of her children.

Smith initially told investigators that she had been carjacked by a black gunman who drove off with her two sons — Michael, 3, and Alexander, 14 months. Hicks, the former sheriff’s deputy, said he believed her story until the moment she confessed nine days later.

“You have to understand: This is a small town. In small towns, you know everybody,” said Hicks, who lived less than a mile from Smith and attended church with her. “And you don’t hear of mothers hurting their children.”

Prosecutors sought to impose the death sentence. But the jury that found Smith guilty elected to spare her life. She is still in prison for the October 1994 murders.

Andrea and Rusty Yates were a deeply religious couple whose lives revolved around raising and home-schooling their large family in a Houston suburb.