V. R. Lakshminarayanan and L. R. Santos, “Capuchin Monkeys Are Sensitive to Others’ Welfare,” Curr Biol 17 (2008): 21; J. M. Burkart et al., “Other-Regarding Preferences in a Non-human Primate: Common Marmosets Provision Food Altruistically,” PNAS 104 (2007): 19762; J. B. Silk et al., “Chimpanzees Are Indifferent to the Welfare of Unrelated Group Members,” Nat 437 (2005); 1357; K. Jensen et al., “What’s in It for Me? Self-Regard Precludes Altruism and Spite in Chimpanzees,” Proc Royal Soc B 273 (2006): 1013; J. Vonk et al., “Chimpanzees Do Not Take Advantage of Very Low Cost Opportunities to Deliver Food to Unrelated Group Members,” Animal Behav 75 (2008): 1757.