As you mentioned above, it probably depends on your definition. I'm looking at altruism as being selflessness without any reward. For instance, if you died to save your child, it would be for the reward of the preservation of your genes and therefore a sort of survival. If it was for a friend it could be for the reward of not living in their absence, or going to an Elysium, or you want to be a hero - or a combination of these or other reasons that require an evaluation of risk/reward. You might even jump on a grenade for a stranger out of the principle that it is simply the right thing to do - which might be better described as martyrdom rather than altruism - but I think that all acts of a sentient being are filtered through a risk/reward evaluation. Even if the reward is being a "good person" or is posthumous. Many people believe in an afterlife, reincarnation, karma, nirvana, etc.
I think true altruism can only be mindless. Like bees sacrificing their lives for the hive without sentient thought about why they're doing it.
Like I said, I'm no scholar or authority on the subject. Just my opinion.