I can think of a good evolutionary reason why altruism rather than selfishness are genetically determined traits in humans. Throughout 1 million or so years of human history, we have been social animals, i.e. our society has been an importnant and formative part of our environment. Virtually every society (until the emergence of the United States, which, as someone commented, progrssed from barbarism to degeneration without the usual interlude of civilisation) actively inhibited anti-social behavior - by expelling, killing or imprisonment of the offenders. Each of these acts substantially reduced the chance of mating and thus passing on their genes.
By contracts, pro-social individuals had a greater chance of mating (females tend to prefer these over selfish assholes) and thuse passing on their genes. Ergo: social environment acted as a negative selection process to weed out anti-social individuals while favouring pro-social ones.
This, btw is true "social darwinism" because it focuses on the adaptation od the species to natural selection; whereas the crap commonly refered to by that name is a misnomer - it should be called "social lamarckism" (as it was Lamarck and not Darwin who talked about individual adaptation).
wojtek