There have been studies on altruism. The fact of the matter is there probably isn't any true altruism in the world. The reward gained could be as simple as not having to feel guilty later. I think there are so many reasons why people do and do not do things and they are almost impossible to quantify.
Guy jumps on grenade! Explain!
It makes him feel good that he's going to sacrifice his life to save others. Seems pretty obvious, no?
If I'm understanding the argument in this thread, then the problem I'm left with is that altruism has been explained away so well that all we're left with is degrees of selfishness. If offering your life in place of another person's can be described as somehow stimulating the pleasure centre of the brain (however briefly), and thus becomes an example of non-altruistic behaviour, then I'm afraid both selfishness and altruism becomes pretty much meaningless.
Perhaps it might be more productive to look at the matter in terms of choice only. What distinctions can/should we make between a person who performs charitable acts, and one who doesn't? In the kind of society we'd like to live, which behaviour should be encouraged and why?
Then you can side-step what seems to me an ultimately fruitless argument over whether people can ever act altruistically.