A thought-provoking new study has for the first time presented an empirical comparison between male serial killers and female serial killers. The compelling sex differences in behaviors and crimes have been framed through a lens of evolutionary psychology, suggesting our hunter-gatherer history has manifested in the differences between male and female serial killers.

Female serial killers are inarguably rare, but they do exist. Despite the potentially apocryphal story of famous FBI profiler Roy Hazelwood claiming "there are no female serial killers," it is thought that around 15 percent of all serial murderers are women. In fact, one academic suggests the very first serial killer ever chronicled in history was a woman named Locusta, a notorious serial killer/assassin active in Rome two thousand years ago and infamous for her skill with poisons.

Marissa Harrison, a psychology professor at Pennsylvania State University, never considered how little literature there was on the topic of female serial killers until approached by a curious undergraduate student in 2014. Harrison's subsequent work was to initially document the means, motives and histories of female serial killers, primarily in the United States over the past 200 years.

Her most recent study, conducted with Penn State colleague Adam Jordan Gott and Albright College's Susan Hughes, looked to specifically compare matched data from female and male serial killers and then frame those differences though an evolutionary psychological perspective.

"Historically, men hunted animals as prey and women gathered nearby resources, like grains and plants, for food," says Harrison. "As an evolutionary psychologist, I wondered if something left over from these old roles could be affecting how male and female serial killers choose their victims."

Archival data was collected on 55 male and 55 female serial killers who committed their crimes in the United States between 1856 and 2009. In this study a serial killer's crimes were defined as the "premeditated, intentional killing of three or more victims, with a cooling-off period between killings of at least one week."

The core sex differences between male and female serial killers were fascinatingly stark. The majority of female serial killers came from middle or upper class backgrounds and were college-educated, while the majority of male serial killers were less educated and from lower class backgrounds. The method of killing also dramatically differed between the sexes, with nearly half of the women using poisoning to murder their victims, while men preferred asphyxiation or shooting.

But perhaps the biggest sex difference between the two serial killer groups appeared to be in the victims that were chosen. A stunning 90 percent of female serial killer victims were someone familiar to the killer, whereas men were much more likely to target strangers. In fact, 85 percent of male serial killer victims were strangers compared to less than 15 percent of female serial killer victims. Furthering the hunter-gatherer thesis distinction, 65 percent of male serial killers were found to stalk their victims before murder, while only 3 percent of female killers did the same.

"In our sample, there were two female serial killers who engaged in stalking-like behavior during their crimes," suggests Harrison. "Interestingly, reports indicate that men were also involved in those crimes."

While the hunter-gatherer perspective on sex differences between serial killers may feel a little simplistic or reductive, it certainly offers an intriguing frame to view many of these empirical distinctions identified in the study. For example, the researchers note that female serial killers were three times more likely to be motivated by financial gain compared to male killers. It is suggested in the study that this is, "in line with the "gatherer" hypothesis, female serial killers seem to be gathering resources as a result of their killings."

On the other hand, male serial killers are 10 times more likely to have a sexual motivation underpinning their crimes. The researchers hypothesize this to be a kind of "aberrant form of mate-seeking." Of course, killing your mate certainly puts a hold on the passing on of genes, however, it is argued that this fundamental tendency towards sexual predation is an evolutionary characteristic.

Harrison is quick to note these psychological profiles are not prescriptive, or intended to suggest a person is born to commit crimes in a specific way, but it is hoped these conclusions could help investigators in the future better target time and resources in their hunt for criminals.

"Evolution doesn't mean you're predetermined to do certain things or act a certain way," says Harrison. "It means that it may be possible to make predictions about behavior based on our evolutionary past. In this case, I do believe that these behaviors are reminiscent of sex-specific behaviors or assignments in the ancestral environment. And perhaps we can understand this better through an evolutionary lens."

The new study was published in the journal Evolutionary Behavioral Sciences.