Are religious people more altruistic, or altruistic people more religious?

It appears to be perceived wisdom that religious people tend to give more to charity, they get more involved in helping those less able, and they have more of a sense of serving the community.

Let's assume that this is all true - (and I have no reason to doubt it).  Is it their religion that makes them more altruistic, or is the fact that they are inherently predisposed towards altruism that means that they find a natural home in religion?  Are we confusing which is cause and which is effect? 

If it is true that altruistic people are naturally drawn to a group where they can express this, then there is a future for humanity without religion.  As religion continues to wane, perhaps we need to find a way to replace religious groups with equivalent secular groups.

If, however, it is belief in a supernatural God is what drives people to be altruistic, then perhaps we need to continue to accept that religion is a necessary civilising influence, even if most of us are no longer believers..