The San Bernardino shooting at the Inland Regional Center, which claimed the lives of 14 victims on 2 December, not only came a shock because of the callous nature in which it was carried out, but also because one of the two perpetrators was a woman – Tashfeen Malik. Mass killings carried out by women, especially in the west, are extremely rare – so rare in fact that there hasn't been a study carried out on it due to the lack of cases.

Violent tendencies in general are more common among men. According to the US Department of Justice, 90.5% of homicides in the US between 1980 and 2008 were committed by men.

The mass shooting at San Bernardino brought to light another statistic: just 8% of homicides by firearms are committed by women, according to an article on Live Science. This is particularly noteworthy because of the sadistic nature of the crime.

Sadism is also a feature that is mainly adopted by men. There are many theories as to why this is but experts argue that this could be an evolutionary trait where men have been rewarded in the past for their violent tendencies, particularly in the animal kingdom. For example, male chimpanzees – the human's closest primate relative – use violence to increase their social status and bolster their chances of finding a mate.

Mass shootings in the US are typically carried out by men who have grown sexually frustrated – which fits into the theory that they commit their crime to attract sexual partners, with notes often left exclaiming their hatred for women. Adam Lankford, a criminal justice professor, told Live Science: "By contrast, I'm not aware of any female attackers, even though we have a small sample, I don't know that any of them complained about not being able to have sex."

For women however, violence has not been associated with improving social standing, despite the rise of strong female characters in action movies. "Whatever the culture offers in the way of justification of righteous violence, males are more likely to implement that," James Garbarino, a psychologist at Loyola University Chicago, told Live Science.

However, this particular attack was carried out in the name of religion. Martyrdom in the name of Islam was seen as a righteous passage for these killers, but the experts theorise that mass killings in the name of religion could be seen as a loophole in a suicide bid, which is prohibited in Islam. "I wouldn't dismiss the idea that some sort of mental health problems or suicidal tendencies could be at play in this case," Lankford continued.