A completely rational person is altruistic, on occasion, because it's the right thing to do, because it feels good, and because a completely rational person can still do whatever they feel like doing. Some researchers say that our dna predisposes us to sacrifice for our blood kin. Coming from a small family and hating children as I do, that means all you bastards out there are my family. That's why we should all try to make the world a better place than it was when we got here.