Why Are Violent Killers Almost Always Men?

Is it our genes, hormones, or something much more insidious?

Matt J Weber 🦢
11 min readJan 16, 2019


Last year there were over 300 mass shootings in the United States. That’s almost one mass shooting for every day of the year. So they’re something of a daily occurrence now, making them about as notable as the rising or setting of the Sun — if not for the terrible loss of life.

It can be hard to find a common thread amongst all these mass shootings. Motives can be varied and sometimes nonexistent. They can happen just as easily in a nightclub as an elementary school. Their timing is random and practically unpredictable. But if we can count on anything, in almost every case the shooter will be like me — a male.

And if we expand our criteria to include all violent crime — not just mass shootings, which are comparatively rare — men are still the number one perpetrators. About 90% of all murders are committed by men. This is true across cultures, throughout history, and all over the world. We’re living on a planet where 50% of the population is liable to go nuts and kill a bunch of people at any moment.

So what’s going on here? Why do so many men become violent killers?

Well, it might be in our genes . . . or our jeans.

Or it could be something much more insidious.

But first let’s talk about the many scientific benefits of castration.

In 1927, Chemistry professor Fred C. Koch had a lot of balls. This was at the University of Chicago, and at the time, Chicago was “the hog butcher to the world.” Stockyards covered much of its south side, and in addition to hogs, they specialized in the dismemberment of all varieties of livestock. Consequently, pretty much anybody could acquire all the animal appendages, hides, bones, or viscera their heart desired — given the right motivation.

As it were, Professor Koch had a lab full of castrated roosters that — in a manner of speaking — could use a healthy dose of motivation.

So he purchased 40 pounds of bull testicles.

Koch took these testicles and extracted a mysterious substance from them, which when administered to a castrated rooster, miraculously resurrected some semblance of the fowl’s former manhood.

What Koch had distilled from all those testicles was testosterone — the primary male sex hormone. It’s the catalyst that turns a perfectly good embryo into a potentially murderous male.

Testosterone not only sparks the production of male sexual characteristics, it can affect our behavior. And it is strongly associated with increased aggression.

That’s why roosters are castrated in the first place. Besides preventing them from knocking up all the hens in the coop, it makes them more docile, less likely to start fights with other roosters, and therefore, less likely to commit the chicken-equivalent of mass murder.

Women have testosterone too — just at much lower quantities. Men can have up to 20 times more of it coursing through our bodies.

And that might make all the difference.

Homicide Rate by Age and Sex (Daly & Wilson, 2001)

If we look a graph of the homicide rate between boys and girls, the line is basically flat for both genders up until the age of 14. As it happens, not many murders are committed by elementary school children. But once we hit Junior High something changes. Homicide rates skyrocket. But only with boys. The rate hardly changes for girls.

This is a worldwide phenomenon. Boys in Canada are just as likely to become violent killers as boys in Cambodia or Connecticut. Something happens to male children around the age of 14 that makes them more likely to kill.

This also happens to be around the time both sexes reach puberty. But girls’ bodies aren’t being flooded with a magic murder potion of testosterone.

This would make violence just as much a male secondary sex characteristic as beards and deeper voices.

But the link between testosterone and increased aggression isn’t as straightforward as it might seem. Recent studies have shown that winning a competitive sporting event — even one as seemingly restrained as chess — can instigate the release of testosterone. Conversely, losing can lower testosterone levels. So in either case, the level of testosterone is largely dependent on the behaviors men are already engaged in — not causing the behavior itself. Much like how our body releases adrenaline whenever we need to deadlift a Chevy Impala, we might receive a surge of testosterone whenever we find ourselves in a situation that calls for a little aggression.

But the male proclivity for violence extends beyond our hormones. It even extends beyond being human.

Chimpanzees also exhibit male-pattern violence. They are the great apes closest to us on the hominid family tree and it’s best to keep their proximity entirely genealogical because adult male chimps are the ones most likely to tear your face off so.

In the wild, troops of male chimpanzees will band together and wage chimp-sized turf wars with competing groups. While female chimpanzees spend their days busying themselves with relatively more peaceful ventures.

Since we share a common ancestor with chimps, there’s a good chance this shared aggressiveness was coded into both of our species sometime in the distant past via natural selection.

From an evolutionary perspective, human males have acquired a couple characteristics that make them slightly better killers than our female counterparts. For one, we’ve got a little more upper body strength in general — which comes in handy if you spend your days cracking skulls with a club.

And if you look at the numbers, men not only commit the most violence — men are most often the victims of said violence.

In this way, male humans might have evolved to fight and kill other male humans.

This is called the Male Warrior Hypothesis.

Which goes like this:

In prehistory, male humans were in constant conflict with other tribes of male humans — much like modern chimpanzees. Rival tribes fought over territory and resources so beating your neighbors to a bloody pulp could mean the difference between life and death.

So the stronger, more violent males would acquire more resources. This would in turn give their offspring a better chance of survival — upping their reproductive fitness and passing their extra-aggressive male genes onto the next generation. After a few thousand cycles of this, soon you have an entire population of crackerjack murder machines.

Hominids have been making stone weapons for tens of thousands of years. We have found fossil evidence of male humans whose untimely deaths were precipitated by these stone weapons. We’ve even excavated prehistoric mass graves where most of the victims are male.

After all, tribal violence can be observed in almost all human cultures today. Now we call it war (or if it’s your own country doing the killing it’s an “overseas contingency operation”) but it was probably standard practice in hominid communities throughout prehistory. It’s even observed in Rhesus Monkeys.

Violence and warfare are so common across the world and throughout history, it can be considered a universal human trait — like bipedal locomotion and oversharing. Yet historically wars have been almost exclusively started and fought by those afflicted with a Y chromosome.

This all kinda makes sense if you’re competing for hunting grounds with the most mammoths or the cave with the most curb appeal, but evolution might even explain instances of random, even seemingly self-destructive acts of violence, acts of violence that can even end in the perpetrators own death — like that we see in mass shootings.

Generally, evolution is all about reproductive fitness. This means the more an organism reproduces, the more of its genes will end up in the next generation.

But fitness is relative.

If an organism can somehow reduce the reproductive fitness of its competitors — decrease the chances those other genes are passed on — his own reproductive fitness will be increased even without having any more offspring.

And so you can have instances where a single lone individual goes on a killing spree, ending the reproductive abilities of anybody within club’s reach — even ending his own reproductive future — but coming out on top in an evolutionary sense if he has already passed on more of his genetic legacy than his hapless victims.

In the grand scheme, such self-destructive and diabolically evil survival strategies can be an evolutionary advantage, and therefore, propagate throughout the species.

If this hypothesis is right, violence and aggression are hardwired into human males. That makes assaults, homicide, and mass shootings a natural outcome of our evolutionary heritage. So if that’s the case, no man should ever be allowed to buy a gun. I probably shouldn’t even have a driver’s license. For the good of society, anyone unlucky enough to be born with a Y chromosome should probably be incarcerated before they reach puberty. Better yet (with the exception of a few lucky breeding studs) most males should be castrated at birth — like Koch’s roosters.

Anyhow, if our behavior is so fixed, then maybe men should be too.

I’d prefer it if we didn’t have do any of those things — especially the last bit.

So if we aren’t natural born killers, then some outside force must be transforming men into killers. There’s no evidence of a rage virus infecting the male population, so we must be the target of some insidious and underhanded influence campaign.

In other words, we are being taught to be violent.

And again we can find an example of this in our primate brethren.

Namely, baboons.

Baboons are not widely celebrated for their mild-manners. They have large, powerful jaws adorned with exceptionally pronounced canine teeth. Packs of them have been known to hunt and kill prey as large as goats and sheep. But they reserve much of their innate aggression for members of their own species.

Baboon troops have a very hierarchical structure. That hierarchy is maintained through systematic acts of intimidation and random violence. Like us, male baboons will fight with other males for dominance or territory or resources — even if those resources turn out to be rotten meat.

20 years ago, an especially vicious troop of male baboons took control of a garbage dump in Kenya. Dubbed Forest Troop by researchers, these baboons sported a gang of particularly nasty alpha males — even by baboon standards. They maintained their dominance not only by quarreling with each other but by regularly attacking and bullying the younger, weaker, and lower-ranking baboons in the troop. They were real jerks. If a lower-ranking baboon was motivated enough, he could fight and bully his way up the social ladder — but he’d have to be just as mean as all the other alpha jerks. So Forest Troop was perpetually in the grip of a rotating cast of abusive male thugs. That was their system and it had been that way ever since researchers had begun studying them.

Their exceptionally aggressive behavior eventually won them control of a cornucopia of free food — a human garbage dump. Of course, the alpha males got first dibs on anything they found. So when they came across a trove of leftover meat, they gorged themselves, leaving nothing for the rest of the troop. Unfortunately (or fortunately, depending on your place on the social ladder), that meat had been infected with tuberculosis. Thus, each and everyone of those alpha jerks became sick and died.

Without these uber-aggressive baboons in power anymore, a new social order was established in Forest Troop. Baboons are inherently aggressive animals so it wasn’t like they stopped beating up on each other, but when members of the troop did engage in fights it was between individuals of equal standing. The strong did not prey on the weak. Bullies weren’t allowed anymore. This new code of conduct had been transmitted throughout the troop and maintained to this day. Even baboons who didn’t grow up in Forest Troop, and were admitted into the hierarchy later in life, adopted these comparatively hospitable customs.

In other words, they were being taught. They just needed the right role models.

This all suggests that male aggression and violence might be at least as much a product of our culture as our biology.

Because in the right circumstances, women can be just as violent as men.

Female athletes exhibit just as much competitive and aggressive behavior as their male colleagues. Judges with two X chromosomes don’t dole out sentences less harsh than their male peers — in fact, they might be harsher. Female leaders will still go to war against each other and female generals will still draw up battle plans and female soldiers will still kill the enemy.

And even though they are rare, women can be mass shooters too.

In more egalitarian societies, where there are more female role models at the top rungs of the social ladder in positions historically associated with aggressive, typically masculine behaviors — in other words, societies with less gender inequality — there tends to be less of a disparity in the rate of violent crimes committed by men and women.

So it’s not just 50% of the population that’s liable to go nuts and murder you. It’s everybody — male and female. All of humanity has the potential to be a violent killer — given the right motivation.

Men just might need a little less motivation.

But that’s exactly what we’re doing.

Strength, courage, and aggression are traditionally masculine traits. Our culture actively fosters these characteristics in men and downplays them in women. Emotional vulnerability, compassion, and thoughtfulness are devalued — even systematically discouraged. Look at the toys marketed to boys and you will find a veritable arsenal of weaponry and battalions of muscle-bound action figures. We want big, strong men with guts, who go with their gut. Men who speak softly and carry a big stick and shoot first and ask questions later. These are the kind of men who become presidents and CEOs. They are the rich and successful. They are the alphas who occupy the upper echelons of the human hierarchy. This makes aggression, violence, and masculinity all intricately tied to social status and power.

At least that’s what we’ve been taught.

So when women begin to occupy positions of power historically reserved for men, they often adopt the macho behavioral posturing we culturally associate with masculinity.

We all do.

And when men are fed violence and aggression at the expense of all other qualities, masculinity can become just as toxic as that rotten meat that killed all those baboons.

If violence is built in to us — if it’s an innate feature of the male persona — then rewarding aggression in the very segment of society most likely to take it to the extreme might not be the greatest idea.

This is a little like throwing burning matches at a stick of dynamite. You shouldn’t be surprised when it explodes.

But unlike our genes, gender stereotypes don’t take millions of years of natural selection to evolve. They aren’t fixed. We can change what it means to be a man any time we want.

At any rate, masculinity isn’t just about being a bloodthirsty warrior. It includes being a protector, provider, and part-time pickle jar opener. So it isn’t all bad. In the perpetual turf war of what it means to be a man, we can pick and choose which areas of masculinity we will fight for and which we will abandon on the garbage heap of history.

This won’t be accomplished overnight. After all, it involves overturning centuries of ingrained social habituation and eons of ruthless evolutionary conditioning.

But baboons did it.

So I like to think humans can do it too.

Otherwise, there’s always castration.

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