Lots of people retain the notion that our self-interest is not hard-wired into us in the same way that our physical attributes are hard-wired into us; many people suppose that with just the right upbringing or education, or with enough self-discipline, a human being can be made non-self-interested.
This notion is bunk. It has no more validity than the belief that, say, a man can run as fast as a cheetah if he trains with sufficient diligence, or that a woman can grow an extra set of arms if only she tries hard enough to do so. Nurture plays a role in conditioning who we are, physically and mentally, but our fundamental phenotype and mental attributes are hard-wired into us.
The depth and the naturalness of self-interest is illustrated by Rebecca Stern, the adorable seven-year-old daughter of my colleague Dan Klein and his lovely wife Lotta Stern. A few weeks ago, Dan and Lotta found these ‘Ruls’ taped to Rebecca’s bedroom door.  These rules clearly indicate a child’s sense of mine vs. thine — of a person’s desire to have protected personal dominion over space and things.
(Note that Rebecca’s mother, Lotta, is Swedish and so Rebecca spends much of her time in Sweden with her mother. The ‘Swenglish’ is interesting, too!)