The human individual, in the evolutionary process, considers his choices and selects that of which will be to his advantage. This does not mean that every choice is to his exclusive advantage but at least as much. The conduct of man is not determined by his will, but is established by his object of desire. (Adam Smith, The Wealth of Nations) Since it is only natural for man to act in self-interest for survival means, I conclude that the nature of man is neither inherently good nor evil, but a product of the environment that molds his perceptions of self-interest.
A person normally proceeds to take steps which he thinks would best promote his advantage, and since humans are naturally social animals, this often entails "scratching another person's back" It is widely accepted truism that every man, in whatever he does, is aiming at something in which he considered will promote his happiness.
Adam Smith, in his writings on the foundations of political economy, clearly eliminates every other motive. Man generally neither intends to promote the public interest, nor knows how much he is promoting it, but he is often led by an invisible hand to promote an end which was no part of his intention. (Adam Smith, The Wealth of Nations) By pursuing his own interest man frequently that of the society more effectually than when he really intends to promote it.
There is of course may appear to be exceptions to this truism regarding human nature, such as people that seemingly do things which are not in their interest or even against their interest. However, there is no limit to the roles that self-interest will play, even the role of impartiality. (Maurice Hewlett, "The Crystal Vase") This is where environment comes into play, molding one's perception of self...