In November, 1988, police arrived at 1426 F Street, in Sacramento, California, to search for a fifty-one-year-old man named Alvaro Montoya. The house, a two-story Victorian, was operating as an unlicensed boarding home; Montoya, who was developmentally disabled and mentally ill, had been living there, but he had recently gone missing and his case worker had grown suspicious. After digging in the yard, the officers discovered Montoya’s remains and those of six other people. Investigators came to believe that the owner and operator of the rooming house had poisoned the elderly and disabled tenants and cashed their government-benefit checks. It seemed to be an obvious case of serial killing, except that the alleged perpetrator, Dorothea Puente, was a woman. When F.B.I. agents discussed the case with Eric Hickey, a criminal psychologist and the author of “Serial Murderers and Their Victims,” a textbook now in its seventh edition, they declined to label Puente a serial killer. (She was eventually convicted in three of the deaths.) “They said, ‘Oh, that’s not serial killing,’ ” Hickey told me. “Of course, it was. They just didn’t recognize it then. Women were not considered to be predators that way.”

Female serial killers are more rare than their male counterparts, but they aren’t nonexistent; about one in six serial murderers is a woman. As a group, they are often overlooked and underestimated. “I think society is in denial that women are capable of such hideousness,” Marissa Harrison, an evolutionary psychologist at Penn State Harrisburg, said. Harrison conducted a study of female serial killers that recently appeared in The Journal of Forensic Psychiatry & Psychology._ _It is part of a small but growing body of research into the subject, and the data suggests that female serial killers are lethal in their own unique way.

Harrison, who has previously studied the evolutionary origins of mass murder, began by compiling a list of American female serial killers, which she defined as women who had killed three or more people with at least a week between each death. She and her colleagues started at murderpedia.org, an online encyclopedia of serial killers and mass murderers. They ultimately identified sixty-four female serial killers who were active between 1821 and 2008. The researchers then used reputable news sources to compile a profile of each murderer, noting her age, birthplace, ethnicity, relationship status, religion, and more. From these profiles, the researchers assembled a portrait of the average female serial killer operating in the United States; what stood out was just how ordinary she was.

“She’s likely to be in her twenties or thirties, middle-class, probably married, probably Christian, probably average intelligence,” Harrison said. “I just described, you know, your next-door neighbor.” (Something similar is true of male serial killers, who tend to possess average intelligence and work blue-collar jobs. Very few are legally insane.) Altogether, the women on the list had killed at least three hundred and thirty-one people, an average of six victims each. More than half had murdered children, and a quarter had targeted the elderly and infirm. Female serial killers also appear to have become more common over the years. Harrison’s team identified thirty-eight who were active in the United States between 1965 and 2014, compared with just fifteen during the preceding fifty years. That’s an increase of more than a hundred and fifty per cent, although, Harrison noted, it’s possible that serial killers are simply more likely to be caught in the modern era.

The details of the women’s crimes differed notably from those committed by men. Nearly all of the women in Harrison’s study had killed people whom they knew, often targeting their husbands and children. Male serial killers, in contrast, appear much more likely to kill strangers. Whereas the most common motive for male serial killers is sex, female murderers were most often driven by money; almost half of the women in Harrison’s sample killed for financial gain. And poison was by far their preferred method; male murderers are most likely to shoot, strangle, or stab their victims.

Harrison also found that many of the killers worked in caregiving roles, as nurses, Sunday-school teachers, babysitters, or stay-at-home moms. But it’s hard to know precisely what to make of this finding, Harrison said. Perhaps the murderers sought occupations that gave them easy access to potential victims. Or perhaps disturbed women with aggressive impulses are more likely to become killers if they find themselves in jobs that give them power over vulnerable people. “What comes first?” Harrison asked. “ ‘I want to kill, so I adopt that profession,’ or ‘I am in that profession, so wow, I see easy access to victims’?”

One reason that female serial killers garner so little attention may be their modus operandi—targeting relatives, the elderly, or the ill, and using relatively inconspicuous weapons. “We call these women ‘the quiet killers,’ ” Hickey, who conducted some of the first studies on female serial killers and is now the dean of the California School of Forensic Studies at Alliant International University, told me. “They’re not detected as easily.” Plus, he said, women are less likely to be seen as capable of serial murder: “Historically, they get away with it for as long as or even longer than the men do, because no one suspects them.” In contrast, Aileen Wuornos, who killed seven men in 1989 and 1990, attracted obsessive media coverage, quite possibly because her crimes—she used a gun and targeted strangers—more closely resembled those of a male killer. (Wuornos is sometimes referred to, mistakenly, as America’s first female serial killer.)

Harrison’s study wasn’t able to explain why, say, a seemingly normal homemaker might become a serial poisoner. Nearly forty per cent of the killers that Harrison identified were reported to have serious mental illnesses, and almost a quarter were said to have a history of drug or alcohol abuse. They also had suffered physical and sexual abuse, as well as severe childhood illness or trauma, at relatively high rates. (Many male murderers have also experienced abuse or trauma.) But plenty of women have similar histories, and few of them become murderers, let alone serial murderers. “We approached this paper with the hope that we could come up with some kind of predictive criteria, but I changed my mind,” Harrison said. “I’m not sure we’re there yet.”

Harrison plans to continue her line of research; she is currently working on a more detailed comparison of female serial killers with age-matched male murderers. But she admits that it can take a mental toll. “I was telling my friends, I seriously need to go look at flowers and puppies now,” she said. “I’ve been reading this for so long: Somebody who set their kids on fire, poisoned their kids, went to a veterans home—disabled veterans—and killed people. You don’t get used to it. It’s always alarming, on every pass.”