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Approximately 500 children in the U.S. are murdered by their parents annually. The morbid acts have accounted for 2.5 percent of total homicides in the country between 1976 and 2007.*

There is not much research on the topic—perhaps because academics are hesitant to burrow down such a dark, esoteric hole—and what does exist is often limited by small samples or a case-study approach. In what researchers are calling "the first comprehensive analysis of U.S. filicide," Timothy Y. Mariano, a third-year psychiatry medical resident at Brown University's Alpert Medical School, and his colleagues crunched FBI data on 15,691 such cases during a 32-year period to provide a clearer picture of the crime.**

"As far as death by homicide goes, you’re more likely to be killed on the day you are born than on any other day of your life."

According to the study, recently published in Forensic Science International, victims are frequently unable to put up a fight: 33 percent are less than a year old, and more than 38 percent of them are between one and six. As filicide researcher Phillip Resnick once toldTime: "As far as death by homicide goes, you’re more likely to be killed on the day you are born than on any other day of your life." Murdering sons was more common (58.3 percent of the cases) than murdering daughters (41.7 percent), and almost 90 percent of the victims were biological children.

As for the perpetrators, both fathers and mothers were nearly equally as likely to murder their own infants. But fathers were much more likely than mothers to murder their adult children (in those cases, 78.3 percent of the offenders were male). Moms were more likely to be younger (on average, around the age of 27), and fathers were older (nearly 35). "Most common killing methods included using hands and feet, strangulation, beating, asphyxiation, drowning, and defenestration," the researchers write.

But why? Theories span from mental health to evolution ("Darwin suggested that infanticide was a check on human population growth"). The most common motive cited among perpetrators is altruism: because of "real or imagined suffering," the act is "in the child’s best interests."

What kind of theories do the data support?

Mental health and serotonin levels appear to be key. Animal research shows that decreases in serotonin can lead to the behavior, and increases can decrease it. Most of the perpetrators were "between 18 and 30 years old," a stage of life associated with neurotransmitter disturbances like depression and schizophrenia.

Sex hormones can drive the behavior, and the level of violence. Animal research shows that higher testosterone levels correlate positively with killing offspring. "In the dataset, men comprised the preponderance of offenders, were older, and were more likely to use firearms, whereas women tended to use means that excluded wounding violence."

Evolution. Eliminating the youngest of the brood, the researchers write, "can confer certain evolutionary advantages" by allowing a parent to direct resources to healthier or more capable young. "The dataset showed that infants are at greatest risk of filicide, which tended to be perpetrated by biological mothers. Relatively few resources have been invested in an infant relative to an older child, so this type of filicide may reflect resource competition."

*UPDATE — July 21, 2014: The original version of this post cited 3,000 annual cases, accounting for 15 percent of total homicides. These numbers were the result of inaccurate data filtering by the scientists, and the new numbers reflect the correct figures.

**UPDATE — July 21, 2014: The original version of this post cited 94,146 such cases. As above, this number was due to inaccurate data filtering by the scientists.