It's Human Nature
You can through certain manipulation make people be selfless and this could be used to prove that people are 'good' inside. You can make people be self serving and this can be used to prove people are 'evil' inside. Either can be magnified to larger scales depending on the example. Both can be 'proven' because niether is 'human nature' at all. It's just how you manipulate the tribal model. If you bring it close and make the humanity of the second party full and true, the tribal model will accept them as one of thier own and the subject will act selflessly. If you bring it farther, and make the second party into a statistic or an idea the party will be selfish. any ideas of innate flaws or original sins are just misunderstandings of where the tribal model was. It doesn't help that it is usually distant in this culture. We depend upon that, or we otherwise wouldn't protect resources as we do.