My contention is that what people generally mean by "altrusim" doesn't exist and is in fact incoherent.
What I am arguing is that all rational action must be motivated by a perception of self-advantage, else it is not rational action. To do something with no hope of self-benefit means to do something with no good reason. The fact that someone/thing else might benefit is only relevant as a motivator if one cares about that person/thing - and why does one care about it? It seems to me that if this caring is rational it must be because that person/thing is somehow perceived to be of benefit to oneself.
Is it rational to walk alone on the African plane so that the lions will have a nice meal? It is certainly "altruistic". Is it rational to give your life savings to a heroin addict? Is it rational to jump in front of the body of a totalitarian dictator who is about to be targeted by an assassin?
I am suggesting that "rational motivation" is indistinguishable from "motivated by perceived self benefit" and that there is no down-side to this. The reason that people believe in "altruism" is because they have the idea that "selfishness" is naughty and so seek to produce another "disinterested" motive (such a contradiction in terms!) to assuage their feelings of "mixed-motivation".
Human beings do many sub-human irrational things. They shouldn't! I want to rescue some acts (which seem to be irrational, but which I suspect are not at all sub-human) from the accusation of irrationality. Historically, religion always did this by offering the prospect of Divine reward - either in this life or else after death. If such a message is taken seriously, the irrationality of altruism vanishes into a miasma.