Character  &  Context

The Science of Who We Are and How We Relate
Editors: Mark Leary, Shira Gabriel, Brett Pelham
Oct 30, 2019

What Can Serial Killers Like Amy Archer-Gilligan Tell Us about Human Evolution?

by Marissa A. Harrison
Woman silhouette on background of the night city in fog

The play Arsenic and Old Lace was said to be inspired by the story of Amy Archer-Gilligan, a Connecticut nursing home owner and proprietor in the early 1900s.  Patients would pay a large amount of money for residence and Archer-Gilligan’s care, often leaving their estates to her when they died.  And dying they were—soon after arriving—at a highly improbable rate.  With increased suspicion from the victim’s families and townspeople, police investigated and found arsenic in Archer-Gilligan’s pantry.  “Sister” Amy Archer-Gilligan claimed that she was using the arsenic to tackle a rat problem in the nursing home, but there must have been an enormous number of rats – she had enough arsenic to kill over 100 people.  Sure enough, autopsies of some of the victims revealed arsenic in their systems.  Upon discovery of her crimes, one newspaper called her nursing home “a murder factory” and noted that 60 people had died in her care in about five years.

Postcard of Sister Amy’s “Murder Factory” from the Windsor Historical Society

Postcard of Sister Amy’s “Murder Factory” from the Windsor Historical Society (Connecticut, United States).

Archer-Gilligan’s means and motive were typical of a female serial killer (FSK).  By scouring all the mass media stories we could locate, we identified a sample of 64 FSKs.  Consistent with previous findings, we found that these female American serial killers typically poisoned their victims, usually knew their victims, and were often related to their victims.  In fact, most were official caretakers of their victims, such as nurses and stay-at-home mothers.  We also found that a large number—nearly 40%—had some form of mental illness when they committed their crimes.

FSKs’ most common motive was financial gain.  FSKs most commonly kill for profit.  Before doing this study, we already knew that male serial killers (MSKs) more typically kill for sex.  As an evolutionary psychologist, I found this intriguing. This difference in male-female kill motives follows evolutionary predictions based on parental investment theory

Parental investment theory examines the role of sex in reproduction and caregiving.  In the ancestral environment in which human evolution occurred, females were more physically vulnerable than were males.  In addition, females have far less reproductive capability than men do—that is, they have the potential to have fewer children—as ova are scarce.  Women thus needed resources to ensure the survival of their offspring and themselves.  Of course, this does not justify killing others for financial gain.  Nonetheless, amassing physical resources (if you are female) does follow evolutionary predictions. 

In contrast, males produce millions of sperm daily, and they pursue sex more than women do.  Worldwide, men seek sex and have sex more frequently than do women, and women seek partners with resources more than do men.  Again, of course, this does not justify stalking women, sexually assaulting them, strangling them, and keeping their intimate body parts as souvenirs.  But, just as patterns of helping, parenting, and mating differ by gender, patterns of serial killing appear to do the same.

A deeper dive into our data supported evolutionarily predictable differences in how and why male and female serial killers operate.  For example, MSKs kill people in ways that are reminiscent of hunting. In fact, MSKs are even labeled “hunters” by some investigators. Think of Jeffrey Dahmer. In contrast, FSKs kill people in ways that are more reminiscent of gathering.  FSKs more often kill nearby victims in ways that allow them to gain resources, mostly money. As an example, “The Giggling Granny” Nanny Doss is believed to have murdered 11 people by poisoning them, killing her kids, mother, sister, grandson, and four husbands, and collecting their insurance money. 

Along the same lines, our analysis showed that MSKs were 10 times more likely to have a sexual motive for murder. This was particularly true for MSKs in their prime reproductive years.  On the other hand, we also found that a large portion of both male and female serial killers showed evidence of having one or more mental illnesses.    

Do our findings suggest that killing is “in our genes”?  Yes, and no.  A large study of people and animals showed high rates of killing other members of one’s own species.  And in fact, human beings killed each other much more often in ancient times than we do today.  Having said this, it is important to acknowledge that serial murder is extremely rare.  Experts estimate that the lifetime probability of being murdered by a serial killer in North America is about .0004%.  You are more likely to get hit by a car while crossing the street or be struck by lightning.  Still, maintaining a protective vigilance in the world around you can’t hurt.  Moreover, a better understanding of murderer psychology may lead to intervention and prevention. 

For Further Reading

Harrison, M. A., Hughes, S. M., & Gott, A. J. (2019). Sex differences in serial killers. Evolutionary Behavioral Sciences, 13(4), 295-310.

Rodgers, G. (2015, December 3). How to avoid being murdered by a serial killer. HuffPost. Retrieved from

Trivers, R. (1972). Paternal investment and sexual selection. In B. Campbell (Ed.), Sexual selection and the descent of man (pp. 136–179). Chicago, IL: Aldine-Atherton.

Windsor Historical Society. (2019).  Amy Archer-Gilligan: Entrepreneurism gone wrong in Windsor.  Retrieved from


Marissa Harrison is an associate professor of psychology at Penn State Harrisburg. She studies human sexuality and violence through an evolutionary lens.


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Why is this blog called Character & Context?

Everything that people think, feel, and do is affected by some combination of their personal characteristics and features of the social context they are in at the time. Character & Context explores the latest insights about human behavior from research in personality and social psychology, the scientific field that studies the causes of everyday behaviors.  

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