This is something I've given a lot of thought to including examining my own motives... which doesn't necessarily make it dead solid certain
It may sound cynical but like Michael D, I'm not sure I believe in real 100% altruism either. This may sound deceptively simple but think about it. All acts of altruism may benefit others but they benefit the person who commits the altruistic act as well. If the person has a conscience and a heart. It's something I question anyway.
I'd love to hear what Plato or Freud think about it if anyone knows