Serial killers are loners who stalk strangers at night to later snatch, torture, and sexually assault them before finishing the deed. They have terrifying nicknames, like “Jack the Ripper” or “BTK” for “Bind, Torture, Kill.” So goes the narrative of the archetypal monster our society loves to dissect and analyze in TV shows, books, and podcasts. And, generally speaking, statistical evidence supports that stereotype—but only for men.
Consider “Jolly” Jane Toppan, a young nurse who lived in the northeastern US in the second half of the 19th century. She killed at least 31 people, many of whom were in her care. She used poison, and relished watching them die, reportedly even lying down by their side as they were entering the netherworld.
Toppan’s modus operandi fits the patterns identified by researchers in a 2015 paper as typical of female serial killers. Women tend to kill acquaintances, people who surround them. They are often caregivers, and are well-educated.
A new paper from the same lead author, Marissa Harrison, associate psychology professor at Penn State Harrisburg, argues that these differences can be explained through evolutionary psychology—a controversial field that contends that human brains were hardwired by the way our ancestors lived in the prehistoric era, and still show signs of those deeply ingrained behaviors. Male serial killers, according to the researchers’ theory, are “hunters,” who follow their prey as they did in nomadic communities. Female serial killers, meanwhile, are “gatherers,” fitting in with their role in these societies, according to the article published in the journal Evolutionary Behavioral Sciences.
The paper clearly shows that, for the most part, men and women serial murderers kill differently. And it offers one interesting theory about why that is. But it’s also just that—a theory, and one that offers a window into some of evolutionary psychology’s limitations.
Using media reports going back to 1856, the researchers compared 55 male and 55 female serial killers in the United States on everything from the age of their victims to their education levels. It’s important to consider that media reports may be biased toward more sensationalistic murders, and any generalizations in the study’s findings are based on only several dozen people—both caveats that the researchers acknowledge. But that’s also inevitable. Documented serial killings are rare, even more so when they are perpetrated by women.
Harrison thinks there are probably more female serial killers than have been recorded, or even caught. Just as society tends to underestimate women, it also tends to be dismissive of female serial killers, giving them silly nicknames like “Jolly Jane” or “The Giggling Grandma.” Media accounts focus on their looks. “Society tends to believe that women can’t do such heinous things. Women are thought of as caregivers and kind,” she said.
Men serial killers tend to follow their victims, often going from town to town, the research shows. They wait for the perfect time to attack, like a predator hunting for their prey. They will often butcher their victims, much like hunted game, and keep trophies from their escapades.
While men largely attack strangers, previous studies have shown that about 80% of women serial killers know their victims. They “gather” them around, Harrison and her colleagues argue—although while men’s “hunting” is essentially literal, the researcher told Quartz that women’s “gathering” is more figurative. They also take care of others: 72% killed at least one person in their care, and nearly half of them have killed their children.
The 2015 paper showed that 39% of female serial killers worked in health care. They largely stayed in one place—like women in many prehistoric societies, according to the researchers—and used methods less outwardly violent, like poison. (Again, these are generalizations. As Harrison pointed out, America’s most famous female serial killer, Aileen Wuornos, portrayed in the 2003 movie Monster, had a more of “male” way of killing).
Then there’s the question of motive—and here’s where the paper is potentially the most contentious.
The researchers found that men usually hunt women to gain some sort of sexual satisfaction (49% killed exclusively women in the researchers’ sample, and 75% killed for sexual gratification). Women don’t discriminate as much in terms of gender, but they will kill for financial gain (16.7% of the men and 51.9% for women murdered for money).
The researchers theorize that the differences in the killings have to do with differences in reproductive potential. While men can procreate nearly endlessly, women have a limited supply of eggs. “In the ancestral environment, to maximize genetic fitness, women would benefit from seeking long-term, stable partners with resources to invest in them and their offspring, and men would benefit more from seeking multiple mating opportunities,” they write.
They say that the way serial killers act—men “hunting” for potential mates, and women “gathering” resources to provide for offspring—is in line with these tendencies, but it becomes an aberration, their pathological version.
There are a number of alternative explanations for the gendered differences among serial killers. Historically, for one thing, it’s been harder for women to be stalkers. Women wouldn’t travel alone, because it wasn’t customary—or safe—to do so. They may have used poison because it required less physical strength, not because of an aversion to other tactics. They may have killed for money because it was harder for them to gain it on their own. (Harrison noted that many of the women in the study had above-average education, and perhaps decent earning potential—although women have only started to catch up to men in recent decades, and the gender pay gap still persists.)
Harrison acknowledges there are more factors to consider. In addition to biological differences, there are also psychological and social forces that determine human behavior, she said. Her research found that there was a number of developmental factors in the lives of women serial killers that may have contributed to their pathologies: abuse, alcoholic parents, becoming pregnant at a very young age, and childhood illness or injury.
“I would never say that the evolutionary explanation is the only explanation for any given behavior, but I think it really provides a valuable lens of trying to understand [it],” she said.
But the field of evolutionary psychology is controversial in itself.
As Harrison describes it, the argument in evolutionary psychology is that because people lived and evolved for millions of years in certain constant conditions, the human brain became accustomed to that ancestral environment. It is likely, she said, that “those forces still influence us, albeit unconsciously.”
The notion that the way we think continues to be affected by how early humans lived has been repeatedly questioned over the years. Critics like philosopher of science David Buller say that we don’t know how human brains worked back then, or exactly how our ancestors led their lives. What’s more, he and other researchers say, our brains are actually quite flexible and can quickly adapt to changes in the outside environment.
Harrison says that she “firmly” believes that there was a sex-specific division of labor in the ancestral environment, based on anthropological research. But if we look at hunter-gatherer societies that exist today, which some researchers use as proxies to see how our ancestors may have lived, this division isn’t as clear-cut. In some communities, men participate in childcare; in others, women are the hunters. “Which groups are representative of our ancestors?” asks science writer Amanda Schaffer in a 2005 piece for Slate about Buller’s arguments.
“There is no biological commandment that says women are natural homemakers and unnatural hunters, or that hands-on fathers are breaking some eternal code of the sexes,” argues science journalist Angela Saini in her book Inferior, giving examples of nomadic societies that don’t follow what we see as traditional gender roles.
And that’s the underlying worry in using evolutionary psychology to explain the differences between men and women. While Harrison emphasized that evolutionary psychology “is not suggesting that everything is a biological absolute,” and that it doesn’t prescribe any behaviors in today’s world, the field has been has been used in the past (paywall) to rationalize traditional gender roles and elevate men’s innate abilities and downplay women’s.
At least in this case, the paper’s authors make no value judgement about which method of serial killing is “better.” Whatever the killer’s gender, murder is just as odious.