What makes a human aggressive? You may be surprised to learn there isn't a thing -- a switch, a gland, or a little rager spot in your brain -- that controls it all. Aggression is a nuanced, contextual response that cannot be pinpointed and explained away. People want science to work that way; they want a gay gene and a fat gene and an aggression gene, but at best what happens is neuroscience may discover a section of some people's brains that in certain instances seems to promote certain reactions that make it more likely for you to be obese, or an alcoholic, or violent. But it's never 100 percent. It's just some people. Sometimes.

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"He didn't tell me they charged extra for guac. I regret nothing."

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Chimpanzees are pretty closely related to humans, and they can be extremely violent as well. But why? Studies of violent behavior in chimps come up as empty-handed as studies of violent behavior in humans. Violent chimps don't have an advantage in mating, or food gathering, or anything. Just like humans. Do violent dudes get their pick of the ladies for mating? Quite the opposite, sometimes.

And yet we do violence. We cheer action heroes. Hell, we call them heroes. Being heroic involves overcoming a great adversity that often in fiction literally means killing something. The hero slays a dragon or some terrible beast. Iron Man dukes it out with The Hulk and gets his ass handed to him, but triumphs in the end. The hero has violence done unto him and then doles out even more to balance the scales for the side of justice. Justice is being killed the right way, in this instance. Maaaaaybe being imprisoned, but only after the bad guy was beaten so badly he couldn't run away if he tried.