There is a lot of variety in humans at all times, and not all of the traits that comprise this variety have been at a large enough advantage to become the norm.

There is a lot of variety in humans at all times, and not all of the traits that comprise this variety have been at a large enough advantage to become the norm. Since psychopaths are not the norm (as far as we know) it probably means there has not been a point in our evolution in which this trait was an advantage enough to propagate.