You're looking at it the wrong way I think. Human culture (yes, even including warfare) only works with instincts towards cooperation. But once that culture exists, individual actors can succeed by essentially hacking those instincts to their own benefit. Evolution doesn't work to the collective benefit of entire gene pools--it works to the benefit of an individual's own genes. Within a population, divergent strategies can both be successful.

I'm not going to rule out that psychopathy can have been a good thing in some society roles, but they do look more like shirking parasites? (An evolved strategy that is turned on sometimes, depending on environment.)

[Edit: Afaik, psychopathy is not turned on after someone reaches adulthood. An evolved strategy for catastrophes would be turned on at e.g. hunger or stress.]

I especially wonder if warfare is a good example? Most of the time when humans evolved we were in a clan society. Clan warriors aren't generally known for live and let live-attitudes to people from outside their clan anyway; a psychopath would probably not stand out much in blood thirst. (And if you are in a war party, you really really need to trust the members in the party. Manipulative people would not last long when they were caught at it.)

Historically, soldiers have generally been reluctant to use their weapons. During WWII, around 20% of soldiers actually fired their weapons in combat. Even fewer shot to kill often aiming over the heads of the enemy. This is a form of posturing both to the enemy and their comrades. Through extensive use of conditioning, that rate and lethality of fire was raised during Vietnam and subsequent wars.

Even among tribal societies, warfare is highly ritualized in a manner that does not optimize for maximum lethality. Richard Gabriel, in studies on tribal societies in New Guinea have noted that hunts occurred with accurate feathered. Tellingly, tribal warfare employed featherless arrows. Similarly "counting coup" among American Indians involves touching rather than killing the enemy.

Cold blooded killers have their uses for society, especially in warfare. An excellent book on this subject is On Killing.

Cold blooded killers aren't synonymous with psychopaths. It turns out that many of the Nazis weren't psychopaths, but rather psychologically unremarkable people whose institutions and culture allowed them to collectively perform acts of evil over and above what any of them could have done individually. This is called the "banality of evil", and it became widely understood after the Milgram experiment and the Eichmann trial.

Groups of people have always been more ruthless than individuals. Psychopaths are remarkable not in their ruthlessness but in their ability to achieve it all by themselves.

This is a point that the late Jungian psychologist James Hillman raised (and I encountered for the first time) in his book a decade ago titled "A Terrible Love of War." http://www.amazon.com/Terrible-Love-War-James-Hillman/dp/014...

It's different than the normal HN fare, but worth a read. We may be a society of killer apes, but we're not as eager to kill in warfare as you might think. Especially when the other guy was compelled at gunpoint by his government to join the military (that is, drafted) just like you were.

The point was, with the attitude to strangers in clan warfare (not modern post-clan societies), psychopaths have nothing to add regarding ruthlessness. See old Scandinavia.

For stylized cattle raids (e.g. historical Ireland, before the vikings) among old neighbors, there will of course be agreed levels below extermination (or the neighbours will be gone long before western contact).

It can just be an unfortunate synthesis of advantageous traits.

Detachment is pretty clearly a (potentially) useful trait for a leader. Same thing for stuff like charisma.

Well, communities have subgroups. Even if someone is a net drag on the larger community, they could contribute to a subgroup.

For example, a leader might be good for their country at the expense of the international community. Or a senator might be good for their state but bad for their country. Or an activist might be good for his community but bad for society at large.

Such a person probably wouldn't be popularly seen as a shirking parasite. The subgroup that benefits would probably say they were doing a super job.

I was discussing the period when we humans evolved. Not so large groups then. Evolutionary pressure creating psychopaths from a group evolutionary basis seems unlikely (these generally need quite specific circumstances to work).

(Your argument also miss that psychopaths manipulate and damage the direct environment around them most.)

Perhaps psychopathy is like being bald - no real advantages, perhaps a small disadvantage, but it doesn't disadvantage you enough to stop you having a few kids so the genes get passed on anyway.

Indeed, if psychopathy is a combination of traits ('feels no empathy' and 'good at faking empathy') perhaps the survivors had a combination of traits that helped them to survive, while those with one or the other failed to survive.

Uh, no...

There are large effects on behavior of psychopathy, people that are affected by a psychopath will mostly -- at a minimum -- make a point of warning everyone else they know. That is very detrimental for the psychopath. (Note that how we communicate really applies evolutionary pressure. Just check the number of muscles in a human face compared to most anything else. This is not a small effect.)

Also, psychopathy seems to be partly environmental and partly from genetics. And exist in most populations.

All this suggests an evolved behavioral strategy.

Researchers need to check for your hypothesis (the large different effects almost cancels each others out in many types of historic societies(!), so it is just a random genetic drift) since afaik it is a standard hypothesis, but it does seem very unlikely.