Researchers such as psychologist Marvin Zuckerman have long noted the morbid curiosity of humans; there’s just something about horror and terror that captures our attention.

Indeed, there may be nothing more horrifying—and fascinating—than murder. With my colleague Tom Bowers at Penn State Harrisburg, I’ve studied the crimes and characteristics of mass murderers for years, and still, I’m alarmed by every reread of each case.

But it wasn’t until last year when an undergraduate student, Erin Murphy, approached me about studying female serial killers (FSKs) that I realized how little scientific literature existed on this topic. We routinely hear about male serial killers (MSKs)—the Jeffrey Dahmers and Ted Bundys of true crime lore—and one can indeed find volumes of literature analyzing their killing sprees.

On the other hand, few have heard of Belle Gunness and Nannie Doss, whose crimes were no less heinous: Gunness killed more than 25 people in the late 19th century, including her children and husbands. Doss killed 11 people in the first half of the 20th century, including her own mother and grandson.

After parsing the data, we found that female serial killers do tend to possess a number of unique characteristics.

Middle- and upper-class killers of kin

Male serial killers tend to stalk and kill strangers. But FSKs tend to kill people they know.

The research that did exist on FSKs provided some good insight. Fresno State criminologist Eric Hickey—author of Serial Murderers and their Victims—interviewed 64 FSKs in the US, yielding a disturbing portrait of women who poisoned, shot and stabbed countless men, women and children.

Most were white and typically killed between seven and 10 victims. They were more likely to murder family members than strangers. And even though the most prevalent motive for murder was money, most of the murderers were middle- and upper-class.

Other research used smaller samples, but had notable findings. For example, in a 2011 study, Amanda Farrell, Robert Keppel and Victoria Titterington reviewed newspaper reports of 10 American FSKs. They found that FSKs tended to operate for a substantially longer time than did MSKs, while 80% knew their victims. Farrell and her colleagues— along with Deborah Schurman-Kauflin, who interviewed eight FSKs in a 2000 study—pointed out that, ironically, nursing is an occupation that’s overrepresented among FSKs.

Fleshing out the profile

So when we decided to study FSKs, we approached the topic with two main goals.

First, we wanted to document the means, motives and histories of these criminals with a larger, more recent sample (the larger the sample size, the more likely the findings and patterns are to reflect true life). Moreover, being psychologists, we found a relative absence of research on the psychology of FSKs.

Like Farrell and her colleagues, we used the mass media approach to study female serial killers.

We found the internet site Murderpedia.org to be a valuable consolidation of media reports of murder, and we found it verifiable 100% of the time. In the end, we collected profiles of 64 female serial killers (the same number Hickey found) who committed their crimes in the US between 1821 and 2008.

Although our findings were limited to information that newspapers chose to include about these women and their crimes, our results lend validity to the mass media approach.

Along the lines of previous studies, we found that most FSKs were middle- and upper-class.

Almost all (92%) knew their victims, almost all were white, and their most common means to kill was poison, while the primary motive for murder was money.

Most of these women had earned college degrees or had attained at least some higher education. They held a wide variety of jobs, ranging from religious teacher to prostitute. Nearly 40% worked in health-related fields as nurses or aides, and about 22% worked in direct caregiving roles (mother and babysitter).

Most FSKs were married at some point. In fact, these serial killers were serial monogamists—married, on average, twice, and as many as seven times. Where we could ascertain appearance, most were reported to be average to above-average in attractiveness; where we could ascertain religion, 100% were Christian.

Nearly two-thirds were related to their victims, nearly one-third killed their significant others and about 44% killed their own biological children. More than half the sample killed children, and about one-quarter killed those who were elderly or infirm, those who had little chance of fighting back.

An aberration of unconscious drives?

From an evolutionary perspective, two important pictures emerged.

First, our data (in line with other studies) showed that female serial killers primarily kill for money. Previous research, such as Eric Hickey’s, has shown that male serial killers primarily kill for sex.

This aligns with evolutionary psychological theory. Robert Trivers’ 1972 work pointed out that due to their limited reproductive potential (relatively few ova), women have evolved to place a premium on securing resources (likely through wise mate choices in the ancestral environment). Conversely, virtually unlimited reproductive potential (relatively unlimited sperm) has likely predisposed men to seek a vast number of sexual opportunities.

Of course, I’m not saying we evolved to be serial killers. What I am saying is that an aberration of genetically mediated unconscious drives might explain some of the reasons for these crimes. These urges could also explain some of the differences between male serial killers and female serial killers.

Second, research has shown that male serial killers tend to stalk and kill strangers. But FSKs tend to kill people they know. It seems, then, that MSKs are hunters and FSKs are gatherers. Although aberrant, this evinces the psyche operating much like the conditions of our ancestral environment.

So, do we know for sure what makes a woman turn into a serial killer? Sadly, no. Even though brain imaging studies, such as psychologist Adrian Raine’s, do point to some trends, we can’t predict this with certainty.

We’re fascinated by murder, perhaps out of an innate drive to pay attention to phenomena that can ultimately kill us. Creating an informed profile of the “typical” female serial killer will, we hope, lead to further analysis and, possibly, prevention.