<p>This question has been looked at over the years through several lenses:
[ul][li]Evolution: [The</a> Selfish Gene](<a href=“http://en.wikipedia.org/wiki/The_Selfish_Gene]The”>The Selfish Gene - Wikipedia)[/li][li]Anthropology/Psychology: [The</a> Science of Good and Evil: Why People Cheat, Gossip, Care, Share, and Follow the Golden Rule](<a href=“http://www.amazon.com/Science-Good-Evil-People-Gossip/dp/0805075208]The”>http://www.amazon.com/Science-Good-Evil-People-Gossip/dp/0805075208)[/li][li]Economics: [/li][quote]
How selfish soever man may be supposed, there are evidently some principles in his nature which interest him in the fortune of others and render their happiness necessary to him though he derives nothing from it except the pleasure of seeing it…</p>
<p>…It is not from the benevolence of the butcher, the brewer, or the baker, that we can expect our dinner, but from their regard to their own interest. —[Adam</a> Smith](<a href=“http://www.econlib.org/library/Enc/bios/Smith.html]Adam”>Adam Smith - Econlib)
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<p>If you’re really interested in studying the human dynamics of self-interest, studying [url=<a href=“http://oyc.yale.edu/economics/game-theory/contents/sessions.html]game-theory[/url”>http://oyc.yale.edu/economics/game-theory/contents/sessions.html]game-theory[/url</a>] would be a good start.</p>