Very interesting results!
Arthur Koestler's
Ghost in the Machine (after which the recording by the pop-band The Police was titled) deals directly with this issue and goes a little further to focus on human beings by asking, "Why are human beings so insanely violent toward their own species?"
His conclusions were that the likely explanation was brain morphology:
1) Human beings have a singularly large cerebral cortex the purpose of which is solely to assign symbols to the real world (abstract thoughts) which may then be manipulated to achieve greater understanding of the dynamics of the world and to therefore gain more control over it than would be available to less intellectually capable creatures.
Koestler establishes in his research that there is evidence of a singular disconnect between the functioning of the cerebral cortex (which is the location of the ego - the sense that one is separate from one's environment and from other beings) and the functioning of the so-called "lower brains" to wit, the mantle or mid-brain (dealing with emotions) and limbic (reptilian brain dealing with instinctive, autonomic and primal reactions).
He noted that the human cerebral cortex has only tenuous neural connections to the "lower" brains and therefore, the activity of the cortex is somewhat dissociated in both a physical and psychological sense, from both emotion and instinct.
Hence the proclivity of human beings to override naturally much stronger instincts in other species, even predatory ones, to refrain from harming or killing members of their own species.
Wolves fighting for pack dominance, a mate or over territory rarely seriously injure each other and even more-rarely kill each other during combat. Once one wolf gets the best of another, the loser normally shows its belly in an act of capitulation and a sign that it will no longer challenge the dominance of the winner, and the contest ends.
Koestler attributes dissociated human brain structure (ego sense) for the proclivity of human beings to commit acts of mass murder/war, which are largely unique to the human species. Wars of aggression are almost exclusively acts triggered by impulses which emanate from the cortex - ideology, religion or other abstractions in which the principle aggressor is not engaging in war for self gain alone, so much as for the purpose of bringing others into conformity to behavior they believe to be necessary or superior or both.
Utopianism which is a form of (misguided) altruism, has been the primary vehicle of global wars in the modern age. The Nazis wanted to conquer the world because they wanted to improve humanity and establish a master race for the good of Humanity, not only themselves. Same for the Communists - they were acting for what they claimed was the "best interests of the world". Same now for the muzz - who want to convert the world to Islam to fulfill the promise of their Q'ran for a perfect (Muslims dominated) Utopian world.
Not engaging in mass slaughter for self interest alone, but also for everyone's good. That is the power of a malfunctioning cerebral cortex.
There is a direct correlation between larger brain size and especially cerebral cortex size, and the proclivity to engage in sadistic injury of and wanton, casual and generally unnecessary killing of a species' own members.
