motivation, defined as the relative value an individual places on group versus individual welfare. Individuals are predicted to … higher contributors more, and allocated greater altruism to them in a Dictator game. Study 2 addressed an exchange … outside the group. Study 3 more directly tested the mediating role of group motivation. Contributors who sacrificed a greater …

, despite the absence of free-riding incentives. In a laboratory experiment, contributions indeed fall short of the equilibrium …