**TL;DR Research links certain human genes to aggressive behaviour** There is no denying that humans are violent. From a zoological perspective, [humans are the most violent species on Earth](https://www.psychologytoday.com/gb/blog/the-new-brain/201610/humans-are-genetically-predisposed-kill-each-other): as children we hit and bite each other; as individuals we brawl and murder; as groups we wage wars and commit terrorism. We have even invented weapons to help us carry out violence against others. We are distinguished from much of the animal kingdom because of our propensity to kill our own kind. In [a study researching lethal violence in mammals](https://www.livescience.com/56306-primates-including-humans-are-the-most-violent-animals.html), scientists found that on average, only 0.3% of deaths in mammals are caused by members of the same species, compared to 2.3% in humans and primates. This inclination towards killing each other is attributed to joint factors: that humans are territorial _and_ that we live in social groups. Research suggests that these animal behaviours are influenced by both [environment and genetics](https://www.nature.com/scitable/knowledge/library/both-environment-and-genetic-makeup-influence-behavior-13907840/). Some people are more genetically predisposed to violence than others. In [a study by Karolinska Institute](https://www.bbc.co.uk/news/science-environment-29760212), scientists analysed the genetics of a group of 900 violent offenders and found 2 genes associated with aggressive behaviour in a significant proportion of the most violent: monoamine oxidase A (MAOA) and a variant of cadherin 13 (CDH13). MAOA has been previously associated with aggressive behaviour and nicknamed “the warrior gene”. MAOA catalyses multiple neurotransmitters, such as dopamine and serotonin (which give us pleasure). Low MAOA activity in the brain means that fewer of these neurotransmitters get catalysed, and research suggests that people with lower MAOA activity might therefore be more prone to aggressive impulses, leading to violent behaviour. The Karolinska Institute study found that those with these genes were 13 times more likely to have a record of repeated violence. This does not mean that everyone with this gene will commit violent acts, but that they may well have to police themselves more closely than others and practice more self-restraint. This genetic predisposition does not guarantee or excuse violent behaviour: like all humans, individuals with this gene still have free will and consciousness and are therefore nevertheless accountable for their actions.

This very much depends on your point of reference. All creatures have the capability for violence. Humans especially are extremely adaptable, able to change our behavior in wildly-varying ways. But if we are going to talk about some genetic propensity for violence that is in some way unique to humans, or makes us uniquely violent beyond most creatures, it's really quite the opposite. Even our anatomy shows it. For creatures in which violence is a priority, usually the males physically fight or intimidate each other for power. This incentivizes bigger and stronger bodies, and leaves no room for becoming thinner and weaker. Yet we are the thinnest, gangliest great apes. You can't pin this on tool use, because a stronger tool user still has the advantage. Furthermore, one of the more common ways to die is by choking to death on food. For most of our relatives, the esophagus and trachea split at a different location so this is less of an issue for most animals. But humans can literally die from eating. Why? It gives us more vocal range. We can literally die from eating because being able to talk a little more expressively was more important. If you compare our behavior to our relatives, the difference between hunter-gatherer behavior and chimpanzee behavior is one of the most striking contrasts amongst the great apes. This puts us in line with the likes of bonobos, making bonobos, humans, and gorillas similar, and chimpanzees being the odd ones out for their level of bloodshed. And contrary to how things might seem when reading history, life has actually gotten less bloody for the average person ever since. The 20th century, for instance, with its nuclear bombs, numerous genocides, and world wars, was the safest and least violent century for the average human in our entire existence. The average person hasn't killed someone. In fact, the average person is appalled at the idea of someone even being slapped or bitten. Something that is a simple and standard communication method for most animals. And for many primates, killing is the only way to solidify your right to live. Do we have the capability for violence? Absolutely. A wolf will bite animals on their territory. A Gorilla can rip you limb-from-limb if you don't show submission when they threaten you. Both these animals are considered peaceful. And they ARE peaceful. Yet if humans behaved like either one, they would be considered especially violent for humans. It's easy to talk about millions of deaths when there are billions of people. But the simple fact is, your life involves less death, bloodshed, and murder than do a great number of animals. Socializing and cooperation are literally the primary reason we have the largest brains in the world. Tool use came secondary.