I once heard something said by a philosopher, though I have no idea who it was. It went like this:
Let's say a mother has twins. She breastfeeds them, one at a time, and when the first baby is completely full, it will let go of the nipple, showing that it's had enough. The mother will then go to feed the other child, and the one that has already been fed will see this and try to push the other child away and continue sucking, despite being full a moment ago.
The point of this was to show that all people are born with sin, that even babies who do not know anything know selfishness.