Hours after gunfire erupted at YouTube’s headquarters in Northern California on Tuesday, police identified the suspect as disgruntled video creator Nasim Aghdam—a woman, who died of a self-inflicted wound.

San Bruno Police Department said Tuesday there “is no evidence that the shooter knew the victims of this shooting or that individuals were specifically targeted.” Aghdam had a website on which she railed against the Google-owned platform for suppressing her videos, fueling speculation that a grudge was her motive.

Mass shootings have become seemingly commonplace in the United States, but amid the disturbing trend, female shooters are extraordinarily rare.

Numerous studies of mass shootings reveal that men carry out nearly all of the attacks. (It should be noted that Tuesday’s incident, which injured three aside from the gunwoman, may not qualify as a mass shooting, depending on the definition used.)

An FBI report published last year found that of 220 “active shooting incidents” between 2000 and 2016, just nine— or 4%—were perpetrated by a woman. A Secret Service report on 28 mass attacks in 2017 (defined as incidents in which at least three people were harmed) identifies all of the attackers as male. Research by University of Alabama criminal justice professor Adam Lankford, published in 2016, says that one woman was among 292 public mass shooters worldwide. Another FBI study revealed that of the 16,964 murderers in 2016, 7.6% were women and 60.7% were men; the gender of the rest was unknown.

Searching for the reasons behind this phenomenon will take you down two main roads—one scientific, the other societal.

Male and female brains grow differently, with women’s forebrains—home to executive functions like impulse control and reflection—developing far faster than men’s. Going without these functions for longer than women, men, who are arguably predisposed for aggression because of testosterone, have more years to potentially behave badly.

Beyond biological factors, Eric Madfis, a sociologist at the University of Washington-Tacoma who’s studied the identifies of American mass murderers, points to men’s tendency to externalize blame and frustration. Women, meanwhile, are more likely to internalize it. He notes that past mass shooters have often failed to live up to society’s masculine expectations financially or romantically. They may have resorted to violence as retribution for being denied what they think they’re owed.

“It’s time to have a close look at our culture and what is going on in terms of how masculinity is defined and characterized, which is often as something that is performed or ‘proven’ through acts of aggression and even violence,” he told Politico earlier this year.

Likewise, Candice Batton, director of the School of Criminology and Criminal Justice at the University of Nebraska at Omaha, told NPR in 2013 that “some research supports the idea that males are more likely than females to develop negative attributions of blame that are external in nature, that is: ‘The cause … of my problems is someone else or some force outside of me.’ And this translates into anger and hostility toward others.” Women, on the other hand, cast blame internally, Batton says, seeing their personal shortcomings—not trying hard enough, not being good enough—as the cause of their own failures.

It should also be noted that gun ownership in the U.S. is more common among men than women, with 39% of men saying they personally own a gun, compared to 22% of women, according to Pew Research.

Those factors hint, to some extent, at why women may not perpetrate mass shootings, but Lankford told CNN that there are just too few women shooters to pinpoint the difference between male and female offenders. “The problem, or the good news, is we don’t have enough female offenders for a statically significant sample,” he said.

The gender of a mass shooter may seem irrelevant in the wake of the loss of life, but reducing gun violence—beyond firearm regulation and personal safety measures—depends on understanding the profile and motive of who is carrying out the act.