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Why would this be, though? Evolutionary theory posits that most traits present in the population of a social species are there because in some way those traits benefit the species as a whole. This may not be true for all traits or for all species, but natural selection tends to work pretty well. In other words, it tends to help useful traits survive in a population, and it tends to suppress traits that don't work well. (The operative word being "tend," because there are some traits, like armpit hair, that persist for no real reason. But that's another essay.) So assuming natural selection is acting on our population still, it has found a balance. The human population tends to produce pretty nice people who are good at working together and who find plenty of other people to like and love. But it also tends to produce a few members of the society who don't think like this, who are willing to do violence when they think it's necessary, and who, it seems, are actually pretty good at it.
If you look at history, it becomes pretty clear that sometimes these sociopaths rise up to lead their tribe, their clan, their nations. Stalin, Hitler, Hussein... There is a long depressing list, and I'll stop there. Somehow these crazy nut-jobs get the healthy people to follow them down a rabbit hole of horrors. Why?
I'm no evolutionary psychologist, but I have a theory about this. (I doubt I'm the first, but whatever. It's my blog.) Most of the time, the healthy personality thrives, and helps everyone else thrive too. The healthy people work together to create a vibrant, happy, healthy community that is stable and safe for its members. This worked very well for thousands, maybe millions of years of human evolution. But sometimes it doesn't work. Sometimes there is a horrible threat from outside, a threat from other people, and sometimes the healthy people have to take up arms, go against their kinder natures, and do violence.
In these conditions, the sociopath shines. The sociopath doesn't look at the onrushing tribe with their stone axes and think about how he'd rather hide and hope for the best than kill other people. The sociopath looks at them and says to himself, "Remove the threat, permanently." Unlike the healthy people who are preoccupied with empathy and guilt, he coolly and efficiently sees the most expedient way to remove that threat. Instinctively, the healthy ones see how efficient the sociopath is under warlike conditions, and suddenly they're willing to do all kinds of things to make themselves and their kids safe again. In other words, for a time, they're willing to act like the sociopath.
And that is where we get war.
So the question arises: What is "healthy?" If you define healthy in terms of evolutionary survivability, then in some more turbulent milieu, can it be argued that the sociopath the "healthy" one?
UPDATE:
Naturally, I don't think serial killers or Hitler have a use in society. These are bad people and we need to get rid of them and control them. I'm just wondering how it's possible that, sometimes, perfectly nice people give the sociopath power. I mean, it's weird, right? So maybe in our ancient past, there was a reason for having these nasties around. In a small band of twenty-five people, the sociopath probably couldn't do that much damage. Not compared to now, when humans organize themselves into thousands and millions... Then the crafty sociopath can do all kinds of damage.