I agree with Katha that anyone who claims to understand human nature in any detail is dealing in speculation. But I also think Miles got it right when he asked and answered his own question:
>Is human nature malleable enough to create more peaceful societies than the
>one we're living in? Yes, the anthropological evidence on that is
>pretty clear.
This is the crucial point. Humans have the capacity for cooperation and mutually beneficial social interaction. They are also clearly capable of horrible cruelty and extreme selfishness. The form a society takes, its institutions and the incentives they present to individuals, can greatly influence which tendencies get expressed.
Capitalism rewards selfishness, so people behave selfishly. Socialism rewards cooperation and solidarity, which is how I think people would behave in a socialist setting.
Brett