Student Altruism in a Large Class Setting

One of the more surprising things about my flipped class is the extent of student altruism at play.  Perhaps it's just that it is more visible to me because of piazza and the indirect reports I hear about the class Facebook group.  I've had students create study guides for every pre-recorded lecture and distribute those.  They extracted the embedded review questions, put it into a single document, and posted it on google docs for everyone to complete and view.  Others made e-flashcards and made them available to the class.  Study groups abounded.  The persistent mantra on the Facebook page was something along the lines of "let's team up and all get As."  I don't grade on a curve so there's no incentive NOT to work with others.  Still, I've been really surprised at the displays of altruism.  I mean, I can understand small groups sharing work on study guides with one another; but why with the whole class?  I'm really interested to hear about other instructors' experiences with this, especially in a large class setting where students don't know everyone.  And interested to hear thoughts on the psychology of this (and whether there are studies of this phenomenon).