Just 6% of mass murders are by women. Here’s how, when and why they kill

The world has been transfixed by the case of Erin Patterson: the Australian woman convicted of mass murder, having brutally killed three members of her family using death cap mushrooms, as well as the attempted murder of a fourth.

While Australia doesn’t have a strict definition, mass murder, also referred to as “mass killing”, is defined in United States federal law as the killing of three or more people in a single incident, excluding the perpetrator.

Cases about women who commit mass murder are rare. Very rare. This makes them all the more interesting for the public.

So why are there so few women mass murderers? How, and why, do women kill differently to men?

Just 6% of mass murders

Because there are so few female mass murderers, the majority of research has focused on men who commit mass killings.

However, a 2024 study analysed 1,715 worldwide mass killing events from 1900 to 2019, finding 105 (or just 6%) were perpetrated by women.

In fact, women rarely kill compared to men. The Australian Institute of Criminology (AIC) reports that for the period January 1 2004 and June 30 2014, women were responsible for 13% of murders.

In total, this equated to 302 incidents of homicide perpetrated by women.

The most common victims were the female’s intimate partner (115 cases, or 34%), 96 family homicide offenders (28%), and 86 (26%) cases of non-family murder. In a further 6% (20) of murders, the relationship was unknown.

Fascinating the public

Given it’s so rare for women to commit murder, when a woman is even accused of having committed multiple murders simultaneously, public interest skyrockets.

As a consequence, Erin Patterson is not the first female mass murderer to become a public fascination, or even the first Australian woman to do so. And poison has been a method of choice.

In 1953, Sydney-based Caroline Grills was tried on four counts of murder and one count of attempted murder after police accused her poisoning her in-laws and a friend of her mother using thallium – a key ingredient of rat poison.

Her use of thallium led to her receiving the nickname of “Aunty Thally” by her fellow inmates.

She was eventually sentenced to life in prison for the attempted murder of her sister-in-law.

Who are these female mass murderers?

While there were no significant differences in age and ancestral origin, there are other notable sex-specific factors that differentiate male and female mass murderers.

A 2024 study found around 75% of mass murders committed by women included at least one family member.

In contrast, only about 39% of mass murders committed by men included a family member as a victim.

Psychologically, men and women mass murderers vary too. Women are twice as likely to exhibit psychotic signs than male perpetrators (25.7% compared to 12.5%), and the occurrence of other psychiatric or neurological conditions was also higher among females (29.5% vs 17.1%) in this homicide group.

Notably, more than half of female perpetrators committed or attempted to commit suicide after the murder.

Why women kill

It is believed women commit fewer murders compared to men due to differences in the reasons men and women resort to violence in general.

Humans can, of course, behave in all sorts of ways. But generally speaking, for men, violence is often used to establish dominance and control. For women, violence commonly generally used as a last resort.

In essence, men’s violence is offensive, women’s violence is defensive.

As an example, the 2020 AIC study notes that for women who killed someone with whom they were in an established relationship, domestic violence was a component. Of the 15 incidents reported, the woman was either the primary victim of male abuse (eight cases or 53%) or the violence was reciprocal (seven cases, 47%).

In no case reported was the woman the sole aggressor and the male the sole victim.

There was also often a simultaneous trigger. In 14 cases (52%), the women that murdered their intimate partner did so in a single, spontaneous act following conflict with the victim. In 28% of incidents (24) the male had physically or sexually assaulted the woman immediately prior to the murder.

The method of murder also varies. Relative to males, female perpetrators are significantly less likely to employ firearms, using them in less than half of cases. Women prefer “cleaner” methods, such as poison, asphyxiation, drowning or drugs to kill their victims.

Why Erin Patterson will keep our attention

The rarity of murders committed by women, and the different rationales and methods used by women when they do, combine to create an event that captivates the public.

For these reasons, Erin Patterson and the female killers who will enter the headlines after her will intrigue us for years to come.

Adding to the intrigue is that Patterson does not fit the standard pattern of a female murderer: there is no evidence of a trigger event. There’s no suggestion of domestic or family violence by Simon Patterson (Erin’s estranged husband whose family was the target of her murderous lunch).

In fact, no motive has been offered at all as to why she murdered three people and attempted to murder a fourth.

So she is an outlier, a mystery. And that is also why she will be studied for years to come to help us understand how a woman with no known criminal or violent history could commit such an abhorrent act.