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Baumann, Donald J.; Cialdini, Robert B.; Kendrick, Douglas T. . (1981). Altruism as hedonism: Helping and self-gratification as equivalent responses.. Journal of Personality and Social Psychology, 40(6), 1039–1046. doi:10.1037/0022-3514.40.6.1039 
10.1037/0022-3514.40.6.1039