Not mentioned in the cited document is that in psychology it is called "psychological egoism"
"Psychological egoism is the view that humans are always motivated by self-interest, even in what seem to be acts of altruism".
http://en.wikipedia.org/wiki/Psychological_egoismHowever, I believe in the concept. I think the brain seeks satisfaction, and our "self-interests" are efforts to satisfy the brain. For example, someone gives money to a stranger, because it affects part of the brain to generate a satisfying signal of feeling good for being a charitable person.
The Daniel Baston studies suggest that the reason someone give money to a stranger is because he care about the stranger.