http://econ.ucsb.edu/~grossman/teaching/Econ594BT_Spring2009/Distributional_Preferences.pdf

                                                                                                                                                                                                Self-interest is the main human motive in many (most?)economic contexts


                                                However

in some (important) situations self-interest is not the mainmotive
                                                Evidence to follow
                                                even a small departure from pure self-interest can dramaticallyinfluence economic outcomes
                                                E.g. if cost of helping others is small